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circle/ at grasp · seanbaxter/circle · GitHub

circle/ at grasp · seanbaxter/circle · GitHub

2023-01-22 16:00:40

2022 has been dubbed Year of the C++ Successor Languages. Everyone seems to be speaking about successor languages. What’s a successor language?

  • A language that has fewer defects/fallacious defaults in comparison with C++.
  • A language that has pleasant options in comparison with C++.
  • A language that tries to be safer, clearer and extra productive than C++.
  • A language that’s broadly appropriate with present C++ code (someway).

The successor language objectives are good objectives. However is it productive to invent all new compilers to realize them? This mission delivers the advantages of successor languages by evolving an present C++ toolchain. For language designers this technique has many benefits:

  • The language is totally appropriate with present C++ code, by building.
  • There’s at all times a working toolchain.
  • You possibly can deal with high-value adjustments, slightly than reinventing every part.
  • Builders can work on totally different options in parallel, as a result of they don’t seem to be backed up ready on the completion of important subsystems (like code era, interoperability, overloading, and so forth).

Many C++ detractors declare that the language is inherently damaged. That it is inherently unsafe or inherently sluggish to compile or inherently exhausting to repair defects. These are content-free statements. There’s nothing inherent about software program. Software program does what it does, and with effort, could be made to do one thing new.

The strategies documented right here lengthen C++ toolchains to repair language defects and make the language safer and extra productive whereas sustaining 100% compatibility with present code property.

carbon1.cxx(Compiler Explorer)

#pragma function edition_carbon_2023
#embrace <string>
#embrace <iostream>

utilizing String = std::string;

alternative IntResult {

fn ParseAsInt(s: String) -> IntResult {
  var r : int32_t = 0;
  for(var c in s) {
    if(not isdigit(c)) {
      return .Failure("Invalid character");

    // Accumulate the digits as they arrive in.
    r = 10 * r + c - '0';

  return .Success(r);

fn TryIt(s: String) {
  var end result := ParseAsInt(s);
  match(end result) {
    .Success(var x)   => std::cout<< "Learn integer "<< x<< "n";
    .Failure(var err) => std::cout<< "Failure '"<< err<< "'n";
    .Cancelled        => std::terminate();

fn principal() -> int {
  return 0;

Which programming language is that this? Components appear to be C++: it contains acquainted header information and makes use of C++’s normal output. Components appear to be Rust or Carbon: it makes use of fn and var declarations, that are fully totally different from C++’s declarations, and it has alternative varieties and sample matching.

This instance code is sort of an ideal 1:1 copy of a sample in the Carbon design document. It compiles with the Circle C++ toolchain, with 24 new features enabled. These new options are on the coronary heart of the successor language objectives. They make the language:

This mixture of options permits the toolchain to meet many selections of the the Carbon language design, not with a brand new compiler distinct from C++, however by evolving an present C++ toolchain. By building, the ensuing edition is totally appropriate with present C++ code.

The design tradeoffs of the Carbon mission signify only one level on the Pareto entrance of the language design house. What’s the proper design on your group and your mission? Your language ought to embody your imaginative and prescient of finest observe. Institutional customers will form the language in order that it delivers the perfect expertise for his or her engineers. The C++ toolchain can develop to serve totally different sorts of customers with totally different necessities.

The C++ of the longer term is not only a language. It is a place to begin for evolving programming in the direction of higher security, simplicity and productiveness, whereas staying interoperable with present C++ property.

Desk of contents

  1. Versioning with feature pragmas
    1. To err is human, to fix divine
    2. pragma.feature file
    3. [edition_2023]
    4. Editions in Rust
    5. __future__ in Python
    6. [edition_carbon_2023]
    7. Pragma.feature philosophy
    8. The case for evolver languages
  2. Feature catalog
    1. [as]
    2. [choice]
    3. [default_value_initialization]
    4. [forward]
    5. [interface]
    6. [new_decl_syntax]
    7. [no_function_overloading]
    8. [no_implicit_ctor_conversions]
    9. [no_implicit_enum_to_underlying]
    10. [no_implicit_floating_narrowing]
    11. [no_implicit_integral_narrowing]
    12. [no_implicit_pointer_to_bool]
    13. [no_implicit_signed_to_unsigned]
    14. [no_implicit_user_conversions]
    15. [no_implicit_widening]
    16. [no_integral_promotions]
    17. [no_multiple_inheritance]
    18. [no_signed_overflow_ub]
    19. [no_user_defined_ctors]
    20. [no_virtual_inheritance]
    21. [no_zero_nullptr]
    22. [placeholder_keyword]
    23. [require_control_flow_braces]
    24. [safer_initializer_list]
    25. [self]
    26. [simpler_precedence]
    27. [switch_break]
    28. [template_brackets]
    29. [tuple]
  3. Research catalog
    1. [borrow_checker]
    2. [context_free_grammar]
    3. [cyclone_pointers]
    4. [generic]
    5. [meta]
    6. [parameter_directives]
  4. Core extensions
    1. Template parameter kinds
    2. Overload sets as arguments
    3. String constant operators
    4. Backtick identifiers
  5. Metaprogramming
    1. Pack subscripts and slices
    2. Tuple subscripts and slices
    3. Integer packs
    4. Member pack declarations
    5. Multi-conditional operator
    6. Circle Imperative Arguments
    7. Extending the language with traits and metafunctions
    8. Reflection traits
    9. Pack traits
    10. Type traits
    11. Non-type traits
    12. Universal traits

If we had an alternate C++ syntax, it could give us a “bubble of recent code that does not exist at the moment” the place we may make arbitrary enhancements (e.g., change defaults, take away unsafe elements, make the language context-free and order-independent, and usually apply 30 years’ value of learnings), freed from backward supply compatibility constraints.

— Herb Sutter, Cppfront Goals & History

We have to create “bubbles of recent code” so as to add options and repair defects. However we do not really need a brand new language to do that. I contend we do not need a brand new language to do that. This part describes the best way to evolve C++ into no matter we think about, with out creating any discrete breaks. It may be an incremental, evolutionary course of.

Per-file function scoping permits language modification with out placing necessities on the mission’s dependencies. We wish every bubble of recent code to be as small as doable, capturing just one impartial function.

C++ shouldn’t be versioned with command-line choices, as these apply to the entire translation unit, together with system and library dependencies not owned by the programmer. Command-line model permits us to solely evolve the language thus far. New syntax should fill the gaps in present syntax. Defects can’t be fastened, as a result of doing so could change the which means of present code.

This part describes Circle’s file-scope versioning. Throughout lexing, every file is given its personal lively function masks, initially cleared.

  • #pragma function – set fields within the lively function masks.
  • #pragma feature_off – clear fields within the lively function masks.

Setting or clearing options solely results the present file. The lively masks of all different information within the translation unit are unaffected.

features.cxx(Compiler Explorer)

// Allow 4 options:
// [interface] - Permits the dyn, interface, impl and make_dyn keywordcs.
// [tuple] - Permits new syntax for tuple expressions and kinds.
// [choice] - Permits the selection and match key phrases.
// [self] - Retires 'this' and replaces it with the lvalue 'self'.
#pragma function interface tuple alternative self

// These information are included after the options have been activated, however are
// unaffected by them. Each file on disk will get its personal function masks.
#embrace <iostream>
#embrace <tuple>

struct Obj {
  void Inc(int inc) {
    // 'self' is new.
    self.x += inc;

    // Error, 'this' is disabled. That is new.
    // this->x -= inc;
  int x;

// Selection varieties are new.
template<typename T, typename U>
alternative Outcome {

void ProcessChoice(Outcome<Obj, std::string> obj) {
  // Sample matching is new.
  match(obj) {
    .Okay(auto obj)  => std::cout<< "Obtained the worth "<< obj.x<< "n";
    .Err(auto err) => std::cout<< "Obtained the error '"<< err<< "'n";

// Interfaces are new.
interface IPrint { 
  void print() const;
  std::string to_string() const;

// Impls are new.
template<typename T> requires (T~is_arithmetic)
impl T : IPrint {
  void print() const {
    std::cout<< T~string + ": "<< self<< "n";
  std::string to_string() const {
    return T~string + ": " + std::to_string(self);

int principal() {
  // Create alternative objects by naming the lively different.
  ProcessChoice(.Okay({ 5 }));
  ProcessChoice(.Err("An error string"));

  // Convey impl<int, IPrint> and impl<double, IPrint> into scope. This implies
  // we will use unqualified member lookup to seek out the print and to_string 
  // interface strategies.
  utilizing impl int, double : IPrint;

  // Name interface strategies on arithmetic varieties! That is new.
  int x = 100;

  double y = 200.2;
  std::cout<< y.to_string()<< "n";

  // Dynamic sort erasure! That is new.
  dyn<IPrint>* p1 = make_dyn<IPrint>(new int { 300 });
  dyn<IPrint>* p2 = make_dyn<IPrint>(new double { 400.4 });
  std::cout<< p2->to_string()<< "n";
  delete p1;
  delete p2;

  // Tuple expressions are new.
  auto tup = (1, "Two", 3.3);

  // Tuple varieties are new.
  utilizing Kind = (int, const char*, double);
  static_assert(Kind == decltype(tup));
$ circle options.cxx
$ ./options
Obtained the worth 5
Obtained the error 'An error string'
int: 100
double: 200.200000
int: 300
double: 400.400000

One miraculous line on the high of features.cxx,

#pragma function interface tuple alternative self

allows a brand new world of performance.

  1. [interface] – Permits the dyn, interface, impl and make_dyn key phrases.
  2. [tuple] – Permits new syntax for tuple expressions and kinds.
  3. [choice] – Permits the alternative and match key phrases.
  4. [self] – Retires this and replaces it with the lvalue self.

These adjustments will not battle with any of this system’s dependencies. The supply file function.cxx has its personal function masks, which controls:

  • reserved phrases acknowledged
  • new options enabled
  • outdated options disabled
  • language semantics modified

The #pragma function directive adjusts a function masks which extends to the top of the options.cxx information. It solely results that file. All different information included within the translation unit, both straight or not directly from options.cxx, are unaffected by options.cxx‘s function masks, they usually every have their very own function masks. Each file is versioned independently of each different file.

We are able to lastly innovate with out restriction and be safe within the information that we aren’t creating incompatibilities with our present code.

Does the existence of per-file options imply every file is translated with a special “model” of the compiler? Positively not. There’s one compiler. The function pragmas merely management if options are picked up in a area of textual content. That is basic monolithic compiler improvement. Now we have one sort system and one AST. The purpose is to incrementally evolve the language in the direction of productiveness, improved security, readibility, easier tooling, and so forth, with out introducing incompatibilities with mission dependencies or requiring in depth coaching for builders.

Compiler warnings could detect loads of situations, however they’re typically turned off or ignored, as a result of they detect issues in strains of code that you do not personal, and subsequently do not care about. Per-file options let engineers choose contracts that error when violated, scoped to their very own supply information. For instance, by erroring on implicit narrowing, implicit widening, C-style downcasts, uninitialized computerized variable declarations, and so forth. Packages, and even simply information, that cope with a public community or with person knowledge, have a bigger assault floor and will choose into extra strict settings than processes that run solely internally.

Importantly, we will use per-file options to repair defects within the language. For instance, [simpler_precedence] adopts Carbon-style operator silos to cut back bugs attributable to programmers not contemplating operator priority. Does || or && bind with larger priority? I do know the reply, however not each engineer does. Not solely does this error when operators from totally different silos are used inside unparenthesized expressions, it successfully adjustments the priority of operators like bitwise AND, OR and XOR by making them bind extra tightly than ==, a peculiar mistake that has been with C/C++ for 50 years.

It is even in scope to interchange your entire C++ grammar with a contemporary, name-on-the-left context-free syntax, which is parsable from a easy PEG grammar. The purpose is, we do not have to go all-in without delay. We are able to begin work on options after they’re wanted and deploy them after they’re prepared. Not like Herb Sutter’s Cpp2 design, we do not want an all-new syntax, fully distinct from Normal C++, to introduce these bubbles of recent code.

Proposals that goal to repair defects are normally rebuffed with feedback like “we won’t, as a result of it could break or change the which means of present code.” That is not true with function pragmas. Solely your supply information are effected by your options pragmas.

How are the options really utilized by the compiler?

The interpretation unit is mapped to supply areas, with one supply location per token. When tokens are emitted from information into the interpretation unit, the lively function masks for that file is recorded at that token’s supply location. As a result of lengthy runs of tokens will all share the identical function masks, the per-token function masks are saved compactly with a sparse knowledge set.

The compiler responds in numerous methods relying on the presence of options:

  • When parsing the grammar for first-class tuple types or for [simpler_precedence] operator silos, the compiler checks the function masks for the present supply location earlier than matching both the Normal C++ grammar or the function’s grammar.
  • Throughout tokenization, some identifiers are promoted to key phrases. For instance, [interface] causes the promotion of interface, impl, dyn and make_dyn.
  • When implementing semantic adjustments, the compiler checks for options at essential supply areas. For instance, [no_integral_promotions] disables the standard arithmetic promotions. The compiler merely checks for that function on the supply location the place the integral promotion would usually be carried out.

What kinds of issues could be developed and deployed as options?

  • [parameter_directives] – Parameter passing directives like [in], [out], [move] and so forth. These are extra declarative and hopefully extra comprehensible than reference varieties.
  • [default_value_initialization] – Default initialization of computerized variables. int x should not be uninitialized, except escaped like int x = void; That is a simple repair.
  • [no_integral_promotions] – Disabling implicit promotion of integer varieties to int earlier than performing arithmetic.
  • [no_signed_overflow_ub] – Disabling undefined conduct for signed arithmetic overflow. This UB gives a poison state for compiler optimizations, but it surely’s additionally a identified supply of program misbehavior. Turning it off, whereas additionally turning on options associated to detecting overflow phenomena, can cut back bugs.
  • [generic] – A generic entity which parallels template however performs early type-checking. It disables partial and specific specializations and requires full sort annotation on its generic parameters. You possibly can envision class generics, operate generics, alternative generics, interface generics and so forth. This function depends on the presence of [interface].
  • [borrow_checker]ref and refmut varieties for invasive borrow checking. A radical addition to C++, one thing that most individuals suppose is inconceivable. However what’s actually inconceivable? Extending C++ with reminiscence security semantics, or someway bridging C++ and Rust sort techniques and ASTs to permit near-seamless interop? With a monolithic compiler, there may be one sort system and one AST. It is simpler to increase one language than it’s to bridge two very totally different toolchains.

The record can go on and on. Envision extending the productive lifetime of C++ by a long time utilizing this method. New cities are constructed on the ruins of outdated cities. You possibly can incrementally modernize infrastructure with out displacing everybody working and dwelling there. The worry of introducing incompatibilities has killed creativity in C++ language design. However this worry of incompatibility is sort of solely the product of versioning translation items all of sudden. If a person or group is able to choose into an extension, they need to have the ability to try this. It will not break their dependencies.

To err is human, to repair divine

C++ has a variety of “fallacious defaults,” design selections both inherited from C or particular to C++ which many programmers contemplate errors. They might be counter-intuitive, go in opposition to anticipated observe in different languages, depart knowledge in undefined states, or typically be susceptible to misuse.

Here is a non-exhaustive record of C++ “fallacious defaults”:

  1. Uninitialized automatic variables.
  2. Integral promotions.
  3. Implicit narrowing conversions.
  4. Switches should break rather than fallthrough.
  5. Operator precedence is complicated and wrong.
  6. Hard-to-parse declarations and the most vexing parse.
  7. Template brackets < > are a nightmare to parse.
  8. Forwarding parameters and std::forward are error prone.
  9. Braced initializers can choose the wrong constructor.
  10. 0 shouldn’t be a null pointer constant.
  11. this shouldn’t be a pointer.

We should always repair all these “fallacious defaults.” A system that can’t repair its errors is a damaged system. The function pragma mechanism permits us to patch the language for brand new code, and hold the present syntax and semantics of the present language for code already written.

Circle already has function pragmas that focus on every of those defects. By holding the scope of our options slender, we make them simple to doc and to consider. What they do is clear from their names:

  1. [default_value_initialization]Uninitialized automatic variables.
  2. [no_integral_promotions]Integral promotions.
  3. [no_implicit_integral_narrowing]Implicit narrowing conversions.
  4. [switch_break]Switches should break rather than fallthrough.
  5. [simpler_precedence]Operator precedence is complicated and wrong.
  6. [new_decl_syntax]Hard-to-parse declarations and the most vexing parse.
  7. [template_brackets]Template brackets < > are a nightmare to parse.
  8. [forward]Forwarding parameters and std::forward are error prone.
  9. [safer_initializer_list]Braced initializers can choose the wrong constructor.
  10. [no_zero_nullptr]0 shouldn’t be a null pointer constant.
  11. [self]this shouldn’t be a pointer.

Not solely can we repair damaged features of the language, we will fuse off entry to options that are not wished anymore, for any purpose. [edition_carbon_2023] fuses off operate overloading, which is probably probably the most advanced a part of C++. The compiler nonetheless is aware of the best way to carry out operate overloading, and it must know to compile your mission’s dependencies, however the capability to declare overloads whereas the function is lively is denieds. There’s concern about C++ changing into an ever-increasing ball of complexity. Considered pruning of options that are not wished or have been superceded is feasible in new editions.

pragma.function file

I do not need you to mark information with pragmas. The textual pragma is for demonstration functions and to offer curious programmers a simple technique to probe the results of every function. Marking up each supply file with function pragmas creates its personal form of dependency subject: generally you’d desire a technique to hold all of them in sync. There is a resolution:

  • For every supply file opened, Circle appears in that file’s folder for a file named pragma.function. Every line of this file signifies one function. The function masks of every file is initialized to the options listed in pragma.function.

When a mission lead says it is time to improve to a brand new function (and this can be a frequent factor, as fine-grained safety-related options get deployed on a fast schedule), a construct engineer can merely insert a brand new line in every pragma.function file within the folders of curiosity, and undergo the compile/check/edit cycle till the problems are resolved. Push these adjustments and you have up to date your mission to fulfill the brand new set of necessities. This can be a path to strengthen your confidence in present code by growing strictness within the language.

What is the migration imaginative and prescient for Carbon or Cpp2? Rewrite all of your code in Carbon or Cpp2! I do not need folks to should rewrite something. As a substitute, allow one function at a time and hold at your compile/check cycle till you have resolved any conflicts created by these new “bubbles of code.”


To ship shared C++ experiences, customers ought to proceed to count on options bundled in massive packages, as C++11, 14, 17 and 20 have been. I am calling these bundles editions.

#pragma function edition_2023 – Allow all version 2023 options.

An version is collection of low-controversy options that language builders contemplate to signify “finest observe.” The primary Circle version is edition_2023. It contains options which enhance the richness and consistency of the language:

  1. [as] – Permits the as-expression for casting.
  2. [choice] – Selection varieties and sample matching.
  3. [forward]ahead parameter directive and forward-expression.
  4. [interface] – Interfaces and sort erasure.
  5. [placeholder_keyword] – Reserve _ as a placeholder object identify.
  6. [template_brackets] – Substitute < > template-argument-list with the context-insensitive braced set !< >.
  7. [tuple] – Language syntax for tuple varieties and tuple expressions.
  8. [self] – Substitute this pointer with self lvalue.

It contains options which enhance the security of the language:

  1. [default_value_initialization] – Do not enable uninitialized objects; as an alternative, value-initialize them.
  2. [no_implicit_floating_narrowing] – Disallow potentially-narrowing implicit floating-point conversions.
  3. [no_implicit_integral_narrowing] – Disallow potentially-narrowing implicit integral conversions.
  4. [no_implicit_pointer_to_bool] – Disallow implicit pointer to bool conversions.
  5. [no_implicit_signed_to_unsigned] – Disallow signed to unsigned implicit arithmetic conversions.
  6. [no_integral_promotions] – Do not promote integral varieties to int print to arithmetic. This has the impact of enabling arithmetic on sub-int varieties.
  7. [no_zero_nullptr] – The 0 numeric literal can’t be used to point nullptr. The person should write out nullptr.
  8. [safer_initializer_list] – Repair the initializer-list-ctor ambiguity defect.
  9. [simpler_precedence] – Reorganize binary operators from priority tiers to operator silos.
  10. [switch_break] – Change circumstances break by default slightly than falling via to the following case.

Packages with massive assault surfaces can choose into further safety-related options to flush out bugs.

Editions in Rust

Rust has supported a similar edition system since model 1.0 was launched in 2015.

A very powerful rule for editions is that crates in a single version can interoperate seamlessly with crates compiled in different editions. This ensures that the choice emigrate to a more moderen version is a “personal one” that the crate could make with out affecting others.

Editions do not split the ecosystem

Even within the absence of crates, Circle’s function pragmas ship the identical worth. The headers and translation unit information (.cpp/.cxx information) in a folder are conceptually like a crate, and they’re typically linked collectively. The pragma.feature file specifies the version for that folder’s supply code. As with Rust’s editions, the choice emigrate to a brand new version doesn’t have an effect on the supply in different folders.

__future__ in Python

Python has the same mechanism for relieving modules into utilizing new options. It might import options from the __future__ module.

The long run assertion is meant to ease migration to future variations of Python that introduce incompatible adjustments to the language. It permits use of the brand new options on a per-module foundation earlier than the discharge wherein the function turns into normal.

A future assertion is acknowledged and handled specifically at compile time: Modifications to the semantics of core constructs are sometimes carried out by producing totally different code. It could even be the case {that a} new function introduces new incompatible syntax (akin to a brand new reserved phrase), wherein case the compiler could have to parse the module in another way. Such selections can’t be pushed off till runtime.

Future statements

$ python3
Python 3.8.10 (default, Mar 15 2022, 12:22:08) 
[GCC 9.4.0] on linux
Kind "assist", "copyright", "credit" or "license" for extra info.
>>> from pprint import pprint
>>> import __future__
>>> pprint(__future__.all_feature_names)

Like Circle, Python makes use of fine-grained versioning to bundle small, self-contained adjustments from the bottom language. For instance, the future division statement, from __future__ import division, adjustments the semantics of the x/y operator to return the “true division,” introducing a brand new operator, x//y, that gives “ground division.” Simply making this variation with out versioning via __future__ would change the which means of present code, which expects the x/y operator to carry out “ground division” when given integral operands.


Circle implements a set of function pragmas that enable it to look and function just like the language described within the Carbon design documents. This isn’t an actual recreation of that mission. That is an try to cowl the most important design factors in a single C++ toolchain, slightly than writing a totally new compiler.

[edition_carbon_2023] contains all options from [edition_2023]. It builds on these by activating:

  1. [new_decl_syntax]Carbon uses var to declare objects.
  2. [require_control_flow_braces]Carbon requires braces after control flow statements.
  3. [no_function_overloading]Carbon does not support funciton overloading. As a substitute, it depends on interfaces to dispatch features primarily based on a receiver sort.
  4. [no_multiple_inheritance]Carbon does not support multiple inheritance.
  5. [no_user_defined_ctors]Carbon does not support C++ constructors.
  6. [no_virtual_inheritance]Carbon does not support virtual inheritance.

This isn’t an exhaustive catalog of variations between Carbon’s capabilities and C++’s. Nonetheless, the function pragma design permits a C++ toolchain to in a short time remodel to accommodate design concepts as they roll out.

Pragma.function philosophy

This creates incentives for stakeholders to contribute to the C++ ecosystem

Massive firms have software program architects, principal scientists and administrators of analysis who opine about software program engineering and manage meetups and attend conferences and submit standardization proposals, all with the distant hope that a world committee adopts their preferences, so {that a} PDF will get amended, and compilers comply with swimsuit, they usually can see their engineering preferences lastly practiced again in their very own organizations. As a substitute, why do not these firms simply go construct the options they need and use them inside their very own organizations?

Organizations and their software program architects can management their very own future. There’s a roadmap for exercising creativity in language know-how:

  1. Conceptualize the function
  2. Construct the function
  3. Check the function
  4. Deploy the function
  5. Upstream the function

The C++ month-to-month proposal mailing can stay intact. However now, contributors have a path for implementing and deploying options inside their organizations, independently of the proposal course of. Options could be submitted for ISO’s blessing, an imprimatur which can imply a lot or little to totally different customers.

Institutional customers make use of language opiners, sponsor and ship staff to conferences and contribute monetary and human sources to tooling tasks. Why do they do that? As a result of they consider there’s an financial curiosity in sustaining and advancing language know-how. There’s worth at stake, but by being regularly thwarted by a committee that may’t get something via, firms are leaving cash on the desk. Empowering institutional customers to manage their very own future will enable them to reap the bounty of their funding.

If C++ is to proceed to thrive far into the longer term, it must be attentive to financial calls for.

Will this trigger an explosion of options, and can the C++ neighborhood crumble just like the Tower of Babel?

No, as a result of price/profit is an actual factor. In case your mission has adopted so many extensions {that a} new rent takes three months to rise up to hurry, you are paying three months wage per head for these options. In case your mission is tremendous particular and the advantages are definitely worth the prices, then nice. If not, economics will scale down the options you are utilizing. The function system provides choices when earlier than there have been none:

  1. Use Normal C++, or
  2. Use solely the most recent version set of options, or
  3. Use the most recent version plus enhancements.

Since price/profit differs for each mission, let’s depart these selections to mission leads. They signify the stakeholders within the mission and needs to be empowered to make an knowledgeable alternative.

A language designer is not the arbiter of proper and fallacious. It’s fruitless to argue which language obtained which defaults “proper” or “fallacious.” Totally different tasks have totally different necessities. The mature place is to go away selections open to your customers, to allow them to choose options which have the suitable trade-offs for his or her scenario.

The case for evolver languages

Enhancing C++ to ship the form of expertise builders count on from a programming language at the moment is tough partly as a result of C++ has a long time of technical debt amassed within the design of the language. It inherited the legacy of C, together with textual preprocessing and inclusion. On the time, this was important to C++’s success by giving it prompt and prime quality entry to a big C ecosystem. Nonetheless, over time this has resulted in important technical debt starting from integer promotion guidelines to advanced syntax with “probably the most vexing parse”.

Difficulties improving C++

The declare that the historical past of design selections in C++ signify an insurmountable technical debt is what’s pushing curiosity in successor languages. However I do not suppose it is true. The accountability for supporting historical and unusual behaviors mustn’t burden bizarre builders. It solely needs to be a difficulty for toolchain builders. Software program is versatile. Software program is plastic. I do not see technical debt right here. I see a continually-improving language that yr by yr fixes its outdated errors, provides desired options, and improves security, readability and expresiveness.

Be conscious of legacy. Globally, there could also be as many as 50 billion strains of C++ code. Any evolution of Carbon that fails to account for human funding/coaching and legacy code, representing important capital, is doomed from the beginning.

Software and language evolution

How do you honor the 50 billion strains of revenue-generating C++ code that exists now?

  • First, assure supply compatibility, one thing not doable with separate toolchain successor languages.
  • Second, present an inexpensive plan for strengthening present code with options that signify finest observe: replace pragma.feature, one function at a time, and resolve construct points till all information compile and all exams cross.

As a language developer, I at all times need to have a working toolchain. Equally, as an utility developer, you at all times need to have a working utility. A feature-by-feature migration retains you near dwelling. Rewriting your program in Cppfront, Val or Carbon pushes you out to terra incognita.

Language Strains of code Has compiler?
Cppfront 12,000
Val 27,000
Carbon 42,000
Circle 282,000 ✔️

It is good to leverage an present toolchain. It is higher to have a working compiler than to not have one. By constructing on Circle, I loved an enormous headstart over the successor language tasks, which began from scratch. Day by day I had a working toolchain. Day by day I may consider the deserves of options, check how they interacted with normal library code, and discard ones that felt uninspiring in observe.


Cppfront is carried out as a preprocessor cross. Perform and object declarations that conform to the Cpp2 grammar are acknowledged by the preprocessor, parsed, and lowered to C++. All Normal C++ is passed-through with out modification. The concept is to translate Cpp2 to C++, marking up features with constraints which might be appropriate with Normal C++.

One of many massive issues with this design is that it does not enable new language entities. Circle introduces choice varieties and interfaces. Cppfront cannot do something like that. Since it isn’t a compiler, it does not management template instantiation, so would not have the ability to instantiate new language entities inside dependent contexts.

One other massive drawback is that it does not perceive overload resolution or implicit conversion sequences, so it will probably’t reliably modify language semantics, like Circle does with [no_implicit_user_conversions], [no_implicit_pointer_to_bool], and [as].

Because it does not look at its C++ dependencies, it is inconceivable to introspect varieties with Cppfront. Circle gives refleciton traits, which yield packs of entities describing enum varieties, class varieties, features varieties, class specializations, and so forth.

Investing in a compiler frontend is important if you need the flexibleness to appreciate your language design.


Val is a language all about mutable value semantics. I like that this analysis mission claims to be about one factor. If it will probably display that its taste of mutable worth semantics is each expressive and protected, that will be helpful. Even a unfavorable end result can be helpful.

The Val compiler, then again, does not deal with mutable worth semantics. It chooses to reinvent every part: a special lexer/preprocessor, a special syntax, totally different type of operator overloading, a special type of generics. None of these items have something to do with mutable worth semantics. This stuff are a distraction. All of them put extra burden on the compiler’s builders.

Altering every part hurts the scientific proposition. If Val’s authors adopted C++ and simply added a [mutable_value_semantics] function (in the way in which a [borrow_checker] function might be launched), programmers may experiment with the possession mannequin with out studying a wholly new language. The Val plan raises the barrier of entry so excessive that solely true believers will make investments the hassle to be taught to make use of.

Deploying Val as a separate toolchain versus a C++ extension creates a excessive bar for the language’s personal authors. There is no such thing as a working Val compiler. There isn’t any path for ingesting C++ dependencies. There isn’t any Val normal library. There isn’t any system-level language services for writing an ordinary library. And there is not any code generator.


The authors began work on LLVM code era, solely to take away it three months later. LLVM has a steep studying curve. You could be defeated by it. You could be defeated by many calls for of compiler engineering. What’s their possibility now? Val will attempt emitting C++ code for transpilation as an alternative of emitting LLVM IR. That is simpler within the quick run, however a lot tougher in the long term. LLVM exists for a purpose.

It is exhausting to pursue analysis once you’re battling the technical calls for of tooling. If the authors had determined to increase one other toolchain (not essentially a C++ toolchain, there’s Java, C#, Swift, Go, and so forth), they would not be combating all battles without delay. They’d on a quicker path to implementing mutable worth semantics and deploying a compiler to customers for suggestions.


Carbon is the successor language effort with loads of staffing, expertise and funding behind it. Its design ideas motivated a lot of Circle’s new function pragmas, bundled collectively in [edition_carbon_2023].

Carbon has caught down a few of C++’s most intricate features:

With a lot of C++’s basis gone, how will Carbon obtain a excessive stage of interoperability? As a separate toolchain, I do not suppose it is doable.

Overload decision is a vital a part of C++. You possibly can’t use any Normal Library varieties or algorithms with out it. The whole lot is overloaded. Carbon does not do overload decision. How does a Carbon code name into C++ code? That overload decision information has reside someplace.

Suppose the Carbon compiler interfaces with an present toolchain like Clang. It might depend on Clang to seek out the perfect viable candidate for a operate name. However now Clang must know in regards to the template and performance arguments offered to the decision, which suggests it must learn about Carbon. If a name to a Carbon operate is instantiated from a C++ template compiled by Clang, Clang has to learn about that Carbon operate. So that you’d “educate” Clang about Carbon by extending Clang’s sort system and AST to help the options in Carbon that are not in C++. You’d have to show it about late-checked generics and interfaces and choice types and pattern matching.

In impact, though you begin with a separate toolchain, you find yourself with a single toolchain. Both evolve the Carbon compiler right into a C++ compiler, or evolve the C++ compiler right into a Carbon compiler.

Is there a technique to make a separate toolchain actually work? That hasn’t been demonstrated. Carbon’s C++ interoperability goals are imprecise. There isn’t any classification as to what elements of C++ are supported and what elements aren’t. There isn’t any Carbon compiler, so there is not any technique to see it ingesting C++ dependencies and emitting C++-compatible binaries. There isn’t any Carbon normal library and there is not any system-level language services to construct one. If the Carbon workforce needs to deploy a compiler, I anticipate that they’re going to do what I did with Circle: lengthen a C++ compiler with Carbon-like options and name that Carbon.


For my part, the true worth in Carbon is the massive folder of proposals submitted by workforce members. These are numbered like C++ proposals. A lot of them describe fine-grained, self-contained options just like Circle’s function pragmas. For instance, P0845 describes as-expressions, already in Circle as [as]. These proposals could be instantly carried out in an present C++ toolchain and begin gathering utilization expertise. Carbon’s lacking compiler is obstructing the deployment of Carbon’s concepts.

In comparison with C++, Carbon is a really restrictive language. Can C++ programmers stay productive after shifting to Carbon? No one is aware of. We have to collect expertise. And the way in which to do this is to launch a compiler and get developres to attempt it. Circle could be amended rapidly. The fine-grained remedy of options means it is simple to discover a continuum of language candidates. Constructing on an present platform will get your know-how into the arms of customers rapidly and inexpensively.

I consider one of the simplest ways to evolve C++ is to hold options off a C++ compiler. It is easy for folks to reject this manner as inelegant. They’re going to say they need one thing “designed from the bottom up.” That is an irrelevant philosophical objection. People advanced from synapsids, which advanced from bony fish, which advanced from luggage of goo that exhibited bilateral symmetry, which advanced from single-celled prokaryotes. We’re constructed on a platform billions of years outdated. The C/C++ platform is just fifty years outdated. It has a few years of productive use forward of it.


The as key phrase allows the as-expression operator with the priority of postfix-expression. The as-expression has two makes use of:

  1. expr as type-id – That is shorthand for static_cast<type-id>(expr).

  2. expr as _ – This allows implicit conversions which have been disabled by different function pragmas. This which means is borrowed from Rust’s as keyword:

    as can be used with the _ placeholder when the vacation spot sort could be inferred. Observe that this may trigger inference breakage and normally such code ought to use an specific sort for each readability and stability.

    This type interacts with these restrictions:

as.cxx(Compiler Explorer)

#pragma function as        // Allow any implicit conversion with "x as _".
void f_short(quick x);
void f_int(int x);
void f_unsigned(unsigned x);
void f_long(lengthy x);
void f_float(float x);
void f_double(double x);
void f_bool(bool x);

int principal() {
  #pragma function no_implicit_integral_narrowing
  int x_int = 100;
  f_short(x_int);              // Error
  f_short(x_int as _);         // OK
  #pragma feature_off no_implicit_integral_narrowing

  #pragma function no_implicit_floating_narrowing
  double x_double = 100;
  f_float(x_double);           // Error
  f_float(x_double as _);      // Okay
  #pragma feature_off no_implicit_floating_narrowing

  #pragma function no_implicit_signed_to_unsigned
  f_unsigned(x_int);           // Error
  f_unsigned(x_int as _);      // OK
  f_unsigned(x_double);        // Error
  f_unsigned(x_double as _);   // OK
  #pragma feature_off no_implicit_signed_to_unsigned

  #pragma function no_implicit_widening
  char x_char = 'x';
  f_short(x_char);             // Error
  f_short(x_char as _);        // OK
  f_long(x_int);               // Error
  f_long(x_int as _);          // OK
  float x_float = 1;  
  f_double(x_float);           // Error
  f_double(x_float as _);      // OK
  #pragma feature_off no_implicit_widening

  #pragma function as no_implicit_enum_to_underlying
  enum numbers_t : int {
    Zero, One, Two, Three,

  f_int(Zero);                 // Error
  f_int(Zero as _);            // OK

  f_int(numbers_t::Zero);      // Error
  f_int(numbers_t::Zero as _); // OK 
  #pragma feature_off no_implicit_enum_to_underlying 

  // Use as _ to permit implicit narrowing conversions inside 
  // braced-initializer.
  quick s1 { x_int };           // Error
  quick s2 { x_int as _ };      // OK
  f_short({ x_int });           // Error
  f_short({ x_int as _});       // OK
  #pragma feature_off no_implicit_enum_to_underlying

  // Implicit conversions from tips that could bools are permitted by C++.
  const char* p = nullptr;
  f_bool(p);                    // OK
  #pragma function no_implicit_pointer_to_bool
  // They're disabled by [no_implicit_pointer_to_bool]
  f_bool(p);                    // Error
  f_bool(p as bool);            // OK
  f_bool(p as _);               // OK

as.cxx exhibits the best way to use the as-expression to allow implicit arithmetic and enum-to-int conversions. This can be a type that stands out in textual content, and could be simply looked for.

user_conversions.cxx(Compiler Explorer)

#pragma function as    // Allow as-expression.

struct S {
  // [no_implicit_user_conversion] solely applies to non-explicit 
  // conversions.
  // specific conversions should already be known as explicitly or from a 
  // contextual conversion context.
  operator int() const noexcept;
  operator const char*() const noexcept;
  specific operator bool() const noexcept;

void f1(int);
void f2(const char*);

int principal() {
  S s;
  int x1 = s;
  const char* pc1 = s;

  #pragma function no_implicit_user_conversions

  // Contextual conversions are permitted to make use of user-defined conversions.
  if(s) { }

  // Implicit user-defined conversions exterior contextual conversions are
  // prohibited.
  int x2 = s;                         // Error
  const char* pc2 = s;                // Error
  f1(s);                              // Error
  f2(s);                              // Error
  // You might use as-expression to forged to a sort.
  int x3 = s as int;                  // Okay
  const char* pc3 = s as const char*; // Okay
  f1(s as int);                       // Okay
  f2(s as const char*);               // Okay

  // You might use as-expression to allow implicit conversions.
  int x4 = s as _;                    // Okay
  const char* pc4 = s as _;           // Okay
  f1(s as _);                         // Okay
  f2(s as _);                         // Okay

The [no_implicit_user_conversions] function disables implicit use of user-defined conversion operators. The as-expression will re-enable these implicit conversions.

ctor_conversions.cxx(Compiler Explorer)

struct S {
  // [no_implicit_ctor_conversions] solely applies to non-explicit
  // constructors.
  S(int i);

void func(S);

int principal() {
  #pragma function as no_implicit_ctor_conversions

  // Applies to implicit conversion sequences.
  func(1);       // Error
  func(1 as S);  // Okay
  func(1 as _);  // Okay  

  // Additionally applies to copy-initalization.
  S s1 = 1;      // Error
  S s2 = 1 as S; // Okay
  S s3 = 1 as _; // Okay
  S s4 = S(1);   // Okay
  S s5 { 1 };    // Okay

The [no_implicit_ctor_conversions] function disables use of changing constructors as a part of implicit conversion sequences. This mirrors [no_implicit_user_conversions], which disables conversion operators as a part of implicit conversion sequences. The as-expression will re-enable these implicit conversions.


Allow alternative varieties and sample matching.

Circle alternative varieties are like Rust enums, Swift enums and Carbon choice varieties. The are type-safe discriminated unions, the place every different has an elective related sort that defines its knowledge payload. Not like the C++ std::variant class template, alternative is a first-class language function. It does not have any header dependencies, it compiles rapidly, and it has user-friendly ergonomics.

template<typename T>
alternative Choice {

template<typename T, typename E>
alternative Outcome {

Selection varieties enable C++ to implement Rust’s Option and Result generics.

Accessing alternative alternate options is most cleanly achieved with match statements and expressions. That is just like the Rust match, the Carbon match, the Python match and the C# switch. The match assertion has further performance in comparison with bizarre if-else cascades. It might succinctly destructure courses and aggregates and carry out exams on their components.

Sample matching

choice1.cxx(Compiler Explorer)

#pragma function alternative
#embrace <iostream>

alternative IntResult {

template<typename T>
void func(T obj) {
  match(obj) {
    .Success(auto x) => std::cout<< "Success: "<< x<< "n";
    .Failure(auto x) => std::cout<< "Failure: "<< x<< "n";
    .Cancelled       => std::terminate();

int principal() {
  IntResult r1 = .Success(12345);
  IntResult r2 = .Failure("Good day");
  IntResult r3 = .Cancelled();
$ circle choice1.cxx
$ ./choice1
Success: 12345
Failure: Good day
terminate known as with out an lively exception
Aborted (core dumped)

The Circle alternative sort is similar to the selection sort within the Carbon design. The choice-specifier is just like an enum-specifier, however every different could have a payload type-id. The selection sort is carried out with an implicit discriminator member, which is the smallest integer sort that may enumerate all the selection alternate options. The payload knowledge are variant members of an implicit union member. The copy and transfer constructors and task operators are implicitly outlined to carry out type-safe operations. For many makes use of, alternative varieties are a safer and extra feature-rich knowledge sort than unions.

Observe the initializer for a alternative sort. You possibly can write it long-hand:

  IntResult r1 = IntResult::Success(12345);
  IntResult r2 = IntResult::Failure("Good day");
  IntResult r3 = IntResult::Cancelled();

However Circle gives a choice-name-initializer syntax, a welcome comfort:

  IntResult r1 = .Success(12345);
  IntResult r2 = .Failure("Good day");
  IntResult r3 = .Cancelled();

Write the choice identify after a dot ., after which a parenthesis initializer, a braced initializer, or a delegated initializer. By itself the choice-name_initializer has no which means. In that method it’s very like a braced-initializer. The left-hand aspect should be a alternative object declaration or operate parameter taking a alternative sort.

On this instance we specialize func in order that the match-statement takes IntResult operands with totally different lively variant members. match-statements are assortment of clauses, and every clause consists of a sample, a double arrow =>, and a end result assertion or expression. The good succinctness and adaptability of sample matching comes from the recursive building of the patterns. Every match-clause begins with a choice-pattern:

clause:         choice-pattern => statement-or-expression;
                choice-pattern if-guard => statement-or-expression;

choice-pattern: .identify
                .(name1, name2, name3)
                .(name1, name2, name3)(sample)

A number one dot . places us in a choice-pattern. The following token should be an identifier of a alternative different or enumerator identify, or a parenthesized comma-separated record of them. The compiler performs identify lookup into the kind of the present operand for the identify or names within the sample. If a reputation shouldn’t be discovered, this system is ill-formed. After the record of names, the person can optionally recurse into a brand new parenthesized sample.

sample:             declaration-pattern

declaration-pattern: auto binding-pattern
                     var binding-pattern  (with [`)

binding-pattern:     name 
                     ... name
                     [binding-pattern, binding-pattern, etc]
                     [name1: binding-pattern, name2: binding-pattern, etc]

test-pattern-or:     test-pattern-and
                     test-pattern-or || test-pattern-and

test-pattern-and:    test-pattern
                     test-pattern-and && test-pattern

                     < shift-expression
                     <= shift-expression
                     > shift-expression
                     >= shift-expression
                     shift-expression ... shift-expression
                     ! shift-expression ... shift-expression

In choice1.cxx, the .Success and .Failure choice-patterns recurse to declaration-patterns which bind a reputation x to the lively variant member. That’s, if the selection operand has an lively .Success member, then x is sure to that, and the assertion after the primary => is executed. Then the assertion returns.

Structured binding patterns

choice2.cxx(Compiler Explorer)

#pragma function alternative new_decl_syntax
#embrace <string>
#embrace <tuple>
#embrace <iostream>

alternative MyChoice {
  MyTuple(int, std::string),   // The payload is a tuple.
  MyArray(double[3]),          // The payload is an array.
  MyScalar(quick),             // The payload is a scalar.

fn check(obj : MyChoice) {
  // You possibly can sample match on tuples and arrays.
  match(obj) {
    .MyTuple([1, var b])    => std::cout<< "The tuple int is 1n";
    .MyArray([var a, a, a]) => std::cout<< "All array components are "<< a<< "n";
    .MyArray(var [a, b, c]) => std::cout<< "Another arrayn";
    .MyScalar(> 10)         => std::cout<< "A scalar higher than 10n";
    .MyScalar(var x)        => std::cout<< "The scalar is "<< x<< "n";
    _                       => std::cout<< "One thing elsen";

fn principal() -> int {
  check(.MyTuple{1, "Good day alternative tuple"});
  check(.MyArray{10, 20, 30});
  check(.MyArray{50, 50, 50});
  check(.MyTuple{2, "Foo"});
$ circle choice2.cxx
$ ./choice2
The tuple int is 1
Another array
All array components are 50
A scalar higher than 10
The scalar is 6
One thing else

This instance illustrates a pair extra options. There’s syntax sugar for declaring tuple payload varieties:

  MyTuple(int, std::string)              // That is equal to:
  MyTuple(std::tuple<int, std::string>)

The [tuple] function provides related syntax for tuple help in additional contexts.

The match-statement demonstrates some recursive sample definitions. First, there is a structured-binding-pattern within the .MyTuple choice-pattern. This sample carefully follows the [dcl.struct.bind] grammar for declarations. The sample’s operand is destructured, and every component is recursively dealt with by one other sample. Within the first clause, we match and extract the .MyTuple different from the selection (which has sort std::tuple<int, std::string>), then destructure its two elements, check that the primary element is 1, after which bind the declaration b to the second element.

The second clause destructures the three components of the array operand. The primary component is sure to the declaration a. The second and third components check their operands in opposition to the earlier declaration a. Remember the fact that declarations begin with the auto key phrase, or when the [new_decl_syntax] function is enabled, the var key phrase.

  .MyArray([var a, a, a])                             => // equal to:
  .MyArray([var a, var b, var c]) if a == b && a == c => 

It is as much as you in the event you select to combine declarations and exams inside a sample. You need to use a trailing match-guard, however that is normally far more verbose.

The sample within the third clause, MyArray(var [a, b, c]), exhibits the best way to distribute the auto/var key phrase over the binding. Writing it earlier than a structured-binding-pattern makes all patterns throughout the structured binding declarations themselves. a, b and c are new variables, sure to the primary, second and third components of the array operand. They don’t seem to be exams.

The sample within the fourth clause, .MyScalar(> 10), comprises a relational check. Write !, <, <=, > or >=, adopted by a shift-expression, to check the operand to the expression on the fitting of the operator. These could be chained along with || and &&. C#’s pattern matching gives the identical service with relational operators.

The sample within the final clause is simply _. Underscore is the scalar wildcard, and it matches any operand. By inserting a wildcard sample on the finish of your match assemble, you make sure that it is exhaustive–that is, each doable operand worth will get matched.

Check patterns

choice3.cxx(Compiler Explorer)

#pragma function alternative new_decl_syntax
#embrace <iostream>
#embrace <ideas>

fn even(x : std::integral auto) noexcept -> bool {
  return 0 == x % 2;

fn func(arg : auto) {
  match(arg) ;

fn principal() -> int {
$ circle choice3.cxx -std=c++20
$ ./choice3
The arg is 5.
The arg is between 10 and 20.
The arg is even.
The arg is particular.
The arg shouldn't be particular.

Sample matching is beneficial even when you do not use alternative sort operands. It is simply extra concise than if-else chains. In choice3.cxx, we check some integral arguments in opposition to 5 totally different clauses:

  1. 5 – Check in opposition to 5. That is equal to arg == 5.
  2. 10 ... 20 – Check that the argument is within the half-open vary 10 … 20. That is equal to 10 <= arg && arg < 20.
  3. even – Carry out unqualified lookup. Since we discovered a operate or overload set, carry out an ADL name on even, passing the operand as its argument. This additionally works if identify lookup finds nothing, so long as the operate is in an related namespace of the operand’s sort.
  4. 1 || 3 || 7 || 9 – Check in opposition to the values 1, 3, 7 and 9.
  5. _ – The wildcard sample matches all operands. It ensures the match assemble is exhaustive.

You possibly can freely mix scalar exams, relational exams, ranges and performance calls with the || and && logical operators.

Designated binding patterns

choice4.cxx(Compiler Explorer)

#pragma function alternative tuple new_decl_syntax
#embrace <tuple>
#embrace <iostream>

struct obj_t {
  var a : (int, int);                  // A 2-tuple.
  var b : (std::string, double, int);  // A 3-tuple.

fn func(arg : auto) {
  match(arg) {
    // Destructure the a member and check if it is (10, 20)
    [a: (10, 20)]             => std::cout<< "The 'a' member is (10, 20).n";

    // Examine the vary of the double tuple component.
    [_, [_,  1...100, _] ]   => std::cout<< "The double is between 1 and 100n";

    // a's 2nd component matches b's third component.
    [ [_, var x], [_, _, x] ] => std::cout<< "A magical coincidence.n";

    // The whole lot else goes right here.
    _                         => std::cout<< "A garbage struct.n";

fn principal() -> int {
  func(obj_t { { 10, 20 }, { "Good day", 3, 4     } });
  func(obj_t { { 2, 4 },   { "Good day", 3, 4     } });
  func(obj_t { { 2, 5 },   { "Good day", 19.0, 4  } });
  func(obj_t { { 2, 5 },   { "Good day", 101.0, 5 } });
  func(obj_t { { 2, 5 },   { "Good day", 101.0, 6 } });
$ circle choice4.cxx -std=c++20
$ ./choice4
The 'a' member is (10, 20).
The double is between 1 and 100
The double is between 1 and 100
A magical coincidence.
A garbage struct.

The designated-binding-pattern is just like the structured-binding-pattern, however makes use of the names of knowledge members slightly than positions to destructure a sort. The operand should be a category object.

designated-binding-pattern: [name1: pattern, name2: pattern, ...]

You possibly can recursively use a designated-binding-pattern inside a structured-binding-pattern, and vice versa. All of the sample entities compose.

The sample of the primary match clause, [a: (10, 20)], makes use of a designated-binding-pattern to entry the a member of the operand sort obj_t. The sample for this member comes after the :. (10, 20) is a tuple-expression, actually the end result object of std::make_tuple(10, 20). That is particular syntax enabled by [tuple], which is activated on the primary line of the file.

[a: (10, 20)]   // Examine the a member operand with the tuple (10, 20)
[a: [10, 20]]   // Destructure the a member operand and examine it element-wise with 10 and 20.

These two patterns look the identical, however aren’t the identical. The previous does ADL lookup for operator== on std::tuple<int, int> arguments, finds the operate template offered by <tuple>, and invokes that for the comparability. The latter recursively destructures the tuple components of a and does element-wise comparability with the integers 10 and 20. The latter type is far more highly effective, as a result of it permits recursive nesting of relational exams, ADL exams, bindings, and so forth. However that is not essentially what you need.

The sample of the second match clause, [_, [_, 1...100, _] ], makes use of wildcards to match destructured components. The worth of the a member at all times matches the wildcard. The primary and third tuple components of the b member at all times match. The second tuple component of b is examined in opposition to the half-open interval 1…100.

The sample of the third match clause, [ [_, var x], [_, _, x] ], binds a tuple component to the identify x, after which makes use of that declaration to check the worth of a special tuple component. That is equal to writing:

if(std::get<1>(arg.a) == std::get<2>(arg.b))

Different methods to entry alternative objects

Provisionally, all alternative varieties implicitly declare an enum member known as alternate options. This can be a scoped enum with a hard and fast underlying sort that matches the underlying sort of the implicit discriminator member. The constants on this enum match the names of the alternate options. Certified lookup for a alternative different really returns an alternate options enumerator. There’s an implicit knowledge member for all alternative varieties known as lively, which holds the enumerator for the presently lively different.

choice5.cxx(Compiler Explorer)

#pragma function alternative
#embrace <type_traits>
#embrace <iostream>

alternative Foo {
  z(const char*),

template<typename T> requires (T~is_enum)
const char* enum_to_string(T e) noexcept {
  return T~enum_values == e ...? 
    T~enum_names :
    "unknown enum of sort {}".format(T~string);

int principal() {
  // "alternate options" is an enum member of Foo.
  static_assert(Foo::alternate options~is_enum);

  // It has enumerators for every alternative different.
  std::cout<< "alternate options enumerators:n";
  std::cout<< "  {} = {}n".format(Foo::alternate options~enum_names,
    Foo::alternate options~enum_values~to_underlying) ...;

  // Naming a alternative different provides you again an enumerator.
  static_assert(Foo::alternate options == decltype(Foo::x));

  // Foo::y is an enumerator of sort Foo::alternate options. However it's additionally 
  // the way you assemble alternative varieties! The enumerator has been overloaded to 
  // work as an initializer. 

  // Foo::y is sort Foo::alternate options.
  static_assert(Foo::alternate options == decltype(Foo::y));

  // Foo::y() is an initializer, so Foo::y() is sort Foo. 
  static_assert(Foo == decltype(Foo::y()));

  // Initialize a Foo object.
  Foo obj = .y(3.14);

  // .lively is an implicit knowledge member set to the lively different.
  // The sort is Foo::alternate options.
  std::cout<< "obj.lively = "<< enum_to_string(obj.lively)<< "n";

  // Examine an enumerator with the .lively member to see what's lively.
  if(Foo::x == obj.lively)
    std::cout<< "x is livelyn";
  else if(Foo::y == obj.lively)
    std::cout<< "y is livelyn";
  else if(Foo::z == obj.lively)
    std::cout<< "z is livelyn";
$ circle choice5.cxx 
$ ./choice5
alternate options enumerators:
  x = 0
  y = 1
  z = 2
obj.lively = y
y is lively

It is handy to have a technique to check and extract the lively alternative different with out opting into pattern matching, which is a reasonably heavy-weight function. Use the alternate options enumerators and lively knowledge member exterior of match statements to succinctly check the lively alternative different.

gdb image

Defining the lively different as a knowledge member of sort alternate options has the good thing about making alternative objects simple to examine in unmodified debuggers. The lively fields exhibits the precise identify of the sphere, Foo::y, not simply an index. The union members comply with.

Fairly often we would like not simply to check the lively different, however to extract it right into a declaration within an if-statement situation object.

TODO: if-let syntax.


Selection sort necessities

A usability defect with std::variant is that it may be put right into a valueless-by-exception state. This requires the person to test if the variant is worthless earlier than doing any operations with it.

The valueless-by-exception happens throughout a type-changing task. If the variant begins with sort A, and also you assign a variant with sort B, this sequence of operations is executed:

  1. The left-hand aspect has sort A and the right-hand aspect has sort B.
  2. The destructor of A is named on the left-hand aspect’s knowledge.
  3. The left-hand aspect is now valueless-by-exception.
  4. The copy- or move-constructor of B is named on the left-hand aspect.
  5. The left-hand aspect now has sort B.

What occurs when step 4 throws? Then the left-hand aspect is left in a valueless-by-exception state.

Circle’s alternative sort prevents this state by deleting task operators that would doubtlessly result in the valueless-by-exception state.

If the selection sort has just one different, then valueless-by-exception can not happen, as a result of there aren’t any type-changing operations as soon as the the selection object has been initialized.

In any other case,

  • if any different sort has a potentially-throwing copy constructor, then the selection sort’s copy task operator is deleted, and
  • if any different sort has a potentially-throwing transfer constructor, then the selection sort’s transfer task operator is deleted.

There are further structural necessities:

  • if any different sort has a deleted copy task operator, then the selection sort’s copy task operator is deleted, and
  • if any different sort has a deleted or lacking transfer task operator (see [class.copy.assign]/4), then the selection sort’s transfer task operator is deleted.

Observe that the task operators of the payload varieties are solely known as when the left- and right-hand alternative objects have the identical lively member, and subsequently the potentially-throwing standing of the task operators can not result in a valueless-by-exception state.

Many sorts have potentially-throwing copy constructors. Does that imply we won’t assign alternative varieties that embrace them as payloads? After all not! We are able to assign them by breaking one step into two steps:

  1. Copy-construct the right-hand aspect into a brief. This may throw, however that is okay, as a result of it will not depart any objects in a valueless-by-exception state.
  2. Transfer-assign the short-term into the left-hand aspect. Internally this invokes eithier the transfer constructor or transfer task operator of the left-hand aspect. However these should not throw! (In the event that they do throw, your sort may be very unusual.)

The short-term creation shifts the purpose of the exception exterior of the compiler-generated task operator. As most transfer constructors and transfer task operators are compiler-generated, they’re going to be emitted inline and really possible optimized away, resulting in code that’s aggressive with a duplicate task operator that will have the sad aspect impact of leaving the thing valueless-by-exception.

choice7.cxx(Compiler Explorer)

#pragma function alternative
#embrace <type_traits>

struct A {
  // Declare a non-trivial destructor to maintain issues fascinating.
  ~A() { }

  // Declare potentially-throwing copy constructor.
  A(const A&) noexcept(false);

  // We should outline a non-throwing transfer constructor.
  A(A&&) noexcept;

  // Outline a transfer task operator, as a result of [class.copy.assign]/4 
  // prevents is era.
  A& operator=(A&&) noexcept;

alternative my_choice_t {
  // We'd like a alternative sort with not less than two alternate options to get into
  // a code path that calls copy constructors throughout alternative task.

// The selection sort is *not* copy-assignable, as a result of that would depart it in
// a valueles-by-exception state.

// Nonetheless, it *is* move-assignable.

void copy_assign(my_choice_t& lhs, const my_choice_t& rhs) {
  // Simulate copy-assignment in 2 steps:
  // 1. Copy-construct the rhs.
  // 2. Transfer-assign that short-term into the lhs.
  // lhs = rhs;            // ERROR!
  lhs = my_choice_t(rhs);  // OK!

For the transfer task operator to be generated, make certain to have a non-throwing transfer constructor and transfer task operator on every of your payload varieties. These aren’t mechanically generated for varieties just like the copy constructor and replica task operators are. See [class.copy.ctor]/8 and [class.copy.assign]/4 for situations that can suppress implicit declarations.


  • Editions: [edition_2023]
  • Interactions: Modifications trivial initialization for computerized period objects.

The default-initializer for builtin varieties and sophistication varieties with trivial default constructors are uninitialized when given computerized storage period. These uninitialized objects are a serious driver of bugs. P2723: Zero-initialize objects of automatic storage duration proposes to zero-initialize these objects. That proposal estimates the bug-squashing impression this might have on the software program trade.

The [default_value_initialization] function implements this propsal, however scoped in line with the function pragma. I’m calling it value-initialization slightly than zero-initialization, as a result of not all builtin varieties are cleared to 0 for his or her default state. Particularly, pointers-to-data-members needs to be set to -1! I feel this was a foul ABI alternative, but it surely’s one we now have to cope with. The C++ Normal is fairly unclear what zero-initialization actually means, however a beneficiant studying of it may embrace setting the bits of builtin varieties slightly than clearing them. (In observe, “zero-initialization” does set pointers-to-data-members to -1.)

default_value_initialization.cxx(Compiler Explorer)

#pragma function default_value_initialization

// Has a trivial default constructor, so it will get value-initialized.
struct foo_t { int x, y, z, w; };

// Has a non-trivial default constructor, in order that will get known as as an alternative.
struct bar_t { bar_t() { } int x, y; };

int principal() {
  int a;                     // Worth-initialized to 0.
  int foo_t::*b;             // Worth-initialized to -1.
  int (foo_t::*c)();         // Worth-initialized to 0.
  foo_t d;                   // Worth-initialized to 0.
  bar_t e;                   // bar_t::bar_t is executed.
  int f           = void;    // Uninitialized.
  int foo_t::*g   = void;    // Uninitialized.
  int (foo_t::*h) = void;    // Uninitialized.
  foo_t i         = void;    // Uninitialized.    
  // bar_t j         = void; // Error! bar_t will need to have a trivial default constructor.

Objects with arithmetic varieties, pointers-to-member-functions and sophistication varieties with trivial default constructors get zero initialized. Pointer-to-data-member objects get initialized with -1, as a result of that is the null worth in line with the ABI. Class varieties with non-trivial default constructors get initialized as regular: their default constructor is named.

To show off default worth initialization, assign void into it. Observe that you would be able to solely do that for objects that will in any other case be value-initialized beneath this function pragma. We won’t void-initialize a bar_t object, as a result of that has a non-trivial default constructor, and the traditional factor is to run that, slightly than depart the thing uninitialized. Likewise, you possibly can’t void-initialize an object that has static (or thread_local) storage period.


The [forward] function makes ahead a reserved phrase. Do not use std::forward, as a result of it is simple to misuse. Do not write a forwarding parameter declaration–that’s an rvalue reference now.

std::ahead is dangerous

The way in which C++ presently helps parameter forwarding is unacceptable. It will get misused on a regular basis. It is burdensome. It is exhausting to show. Even consultants get it fallacious, quite a bit.

void func2(auto&& x);

void func1(auto&& x) {
  // How will we ahead x to func2?

How will we ahead x to func2? CppFront makes use of this macro to do it:

#outline CPP2_FORWARD(x) std::ahead<decltype(x)>(x)

Now we will use the macro to extract the decltype of the forwarding parameter.

forward-old1.cxx(Compiler Explorer)

#embrace <useful>
#embrace <iostream>

#outline CPP2_FORWARD(x) std::ahead<decltype(x)>(x)

void func2(auto&& x) {
  std::cout<< decltype(x)~string + "n";

void func1(auto&& x) {

int principal() {
  int x = 1;
$ circle forward-old1.cxx 
$ ./forward-old1

However that is really totally different from naming the invented template parameter (the auto parameter) and forwarding that. With the macro, decltype yields both an lvalue reference or rvalue reference. When utilizing a named template parameter, you’d cross an lvalue reference or a non-reference sort! We’re counting on reference collapsing to make them equal.

Thus far it really works, however there is a entice ready for us.

forward-old2.cxx(Compiler Explorer)

#embrace <iostream>

#outline CPP2_FORWARD(x) std::ahead<decltype(x)>(x)

struct pair_t {
  int first;
  double second;

void print(auto&&... args) {
  std::cout<< "  " + decltype(args)~string ...;
  std::cout<< "n";

void func(auto&& obj) {
  print(CPP2_FORWARD(obj.first), CPP2_FORWARD(obj.second));

int principal() {
  std::cout<< "Move by lvalue:n";
  pair_t obj { 1, 2.2 };

  std::cout<< "Move by rvalue:n";
  func(pair_t { 3, 4.4 });
$ circle forward-old2.cxx
$ ./forward-old2
Move by lvalue:
  int&&  double&&
Move by rvalue:
  int&&  double&&

Disaster! We cross an lvalue pair_t, but the .first and .second components of that pair get moved to print. These components needs to be int& and double&, not int&& and double&&. What on the planet occurred? We used the macro!

forward-old3.cxx(Compiler Explorer)

#embrace <iostream>

struct pair_t {
  int first;
  double second;

void func(auto&& obj) {
  std::cout<< decltype(obj)~string + "n";
  std::cout<< decltype(obj.first)~string + "n";

int principal() {
  pair_t pair { 1, 2 };
$ circle forward-old3.cxx 
$ ./forward-old3

The entice is that decltype is so refined, mortal beings mustn’t depend on it. Why is decltype(obj) an lvalue reference, whereas decltype(obj.first) is a non-reference sort?

If the expression E in in decltype is an unparenthesized id-expression that names a operate parameter, the result’s the kind of that operate parameter. On this case, obj is sort pair_t&, which is what we would like.

obj.first, then again, is an unparenthesized expression naming a member. For this case, the foundations for decltype say to disregard the worth class of the expression, and simply return the kind of the info member, which is int. So by accessing a subobject, we have inadvertently allowed the lvalue to slip off the subexpression, which then ends in reference collapsing including an rvalue-reference to int, leading to an possession bug.

forward-old4.cxx(Compiler Explorer)

#embrace <iostream>

#outline CPP2_FORWARD(x) std::ahead<decltype(x)>(x)

struct pair_t {
  int first;
  double second;

void print(auto&&... args) {
  std::cout<< "  " + decltype(args)~string ...;
  std::cout<< "n";

void func(auto&& obj) {
  print(CPP2_FORWARD(obj.first), CPP2_FORWARD(obj.second));   // BAD!
  print(CPP2_FORWARD(obj).first, CPP2_FORWARD(obj).second);   // GOOD!

int principal() {
  std::cout<< "Move by lvalue:n";
  pair_t obj { 1, 2.2 };

  std::cout<< "Move by rvalue:n";
  func(pair_t { 3, 4.4 });
$ circle forward-old4.cxx
$ ./forward-old4
Move by lvalue:
  int&&  double&&
  int&  double&
Move by rvalue:
  int&&  double&&
  int&&  double&&

This pattern exhibits the repair: apply the CPP2_FORWARD macro solely to the operate parameter, after which entry its subobject. Placing the member-access contained in the macro does the fallacious factor.

Flag a operate that takes a TP&& parameter (the place TP is a template sort parameter identify) and does something with it apart from std::forwarding it precisely as soon as on each static path.


This can be a widespread bug, as a result of even the C++ Core Pointers specify the fallacious factor! We needs to be forwarding obj not simply as soon as, however as soon as for each subobject entry. That is a type of conditions the place C++ is completely underdeveloped, and by making a library function, it opens an enormous entice beneath customers’ toes.

First-class forwarding

The ahead key phrase is powerful. It is an operator with the very best priority. It binds tightly. ahead pair.first is parsed like (ahead pair).first. That’s, it applies to the id-expression on the left, after which you possibly can carry out member-access to get subobjects, that hold the worth class of the forwarded parameter.

forward1.cxx(Compiler Explorer)

#pragma function ahead
#embrace <iostream>

void devour(ahead auto... args) {
  std::cout<< "  " + decltype(ahead args)~string ...;
  std::cout<< "n";

void func(ahead auto pair) {
  // Use the forward-operator on a forwarding parameter to get the fitting
  // worth class. This can be a primary-expression, although it comes on
  // the left. It applies to the parameter, not the subobject. Member-access
  // does the fitting factor right here, propagating the worth class of the parameter
  // to its subobjects.
  devour(ahead pair);
  devour(ahead pair.first, ahead pair.second);

template<typename T1, typename T2>
struct pair_t {
  T1 first;
  T2 second;

int principal() {
  std::cout<< "Move by lvalue:n";
  pair_t pair { 100, 200.2 };

  std::cout<< "Move by rvalue:n";
  func(pair_t { 1i8, 2ui16 });
$ circle /forward1.cxx
$ ./forward1
Move by lvalue:
  pair_t<int, double>&
  int&  double&
Move by rvalue:
  pair_t<char, unsigned quick>&&
  char&&  unsigned quick&&

ahead is a directive that declares a forwarding parameter. This can be a break from Normal C++, the place a forwarding parameter is an unqualified rvalue-reference to a template parameter declared on that very same operate template. That is specific, that was implicit.

You possibly can solely identify ahead parameters in a forward-expression. Naming every other entity leaves this system ill-formed.

It is value noting that the invented template parameter ‘auto’ is deduced as both an lvalue reference or an rvalue reference. It’s by no means deduced as a non-reference sort. Reference collapsing shouldn’t be concerned in argument deduction for a ahead parameter.

forward2.cxx(Compiler Explorer)

// T&& is now freed as much as imply rvalue reference.
#pragma function ahead

void f1(ahead auto x);  // This can be a forwarding parameter.
void f2(auto&& x);        // That is an rvalue reference parameter.

int principal() {
  int x = 1;

  f1(1);   // Move an xvalue to the ahead parameter.
  f1(x);   // Move an lvalue to the ahead parameter.

  f2(1);   // Move an xvalue to rvalue reference parameter.
  f2(x);   // Error: can not cross an lvalue to the rvalue reference parameter.
$ circle forward2.cxx
error: forward2.cxx:14:6
can not convert lvalue int to int&&
  f2(x);   // Error: can not cross an lvalue to the rvalue reference parameter. 

Normal C++ makes it very tough to declare rvalue reference operate parameters, as a result of that syntax is taken by forwarding references. However with the ahead parameter directive, Circle reclaims that functionality. void f2(auto&& x) is an rvalue reference parameter, not a forwarding reference parameter. We won’t cross lvalues to it, as a result of it expects an rvalue reference.

forward3.cxx(Compiler Explorer)

// A operate with a forwarding reference parameter.
void func(auto&& x) { }    // #1

#pragma function ahead

// A operate with an rvalue reference parameter. This can be a totally different
// overload from #1.
void func(auto&& x) { }    // #2

int principal() {
  // Move an lvalue. 
  // OK: This matches #1 and never #2.
  int x = 1;

  // Move an xvalue.
  // ERROR: That is ambiguous, as a result of it matches each #1 and #2.

The compiler is aware of the distinction between a legacy forwarding reference parameter and a [feature] rvalue reference parameter. They’ll certainly overload each other. The 2 func declarations are totally different features. This overloading makes it protected to incorporate operate declarations right into a file with the [forward] function activated. A redeclaration/definition will not change the which means of that present code. As a substitute, a brand new operate declaration is created which overloads the outdated one.


  • Reserved phrases: dyn, impl, interface and make_dyn.
  • Editions: [edition_2023]

C++ has two methods for organizing features with respect to a receiver sort:

  1. Object-oriented design, the place strategies are written as a part of a category definition, and the receiver object is the implicit class object.
  2. Free features, the place strategies are overloaded, and the receiver sort is the overloaded operate parameter sort. Overload resolution is the sophisticated course of by which a selected overloaded is chosen from an overload set when a operate name is tried.

Some fashionable languages, notably Rust and Swift, embrace a 3rd technique to manage features: exterior polymorphism. Rust calls this mechanism traits. Swift calls it protocols. Carbon calls it interfaces. The tried C++0x extension known as them ideas. (No shut relation to C++20 ideas.) Circle calls them interfaces.

Most easily, an interface declares a set of features. An impl assertion implements the interface strategies for a selected sort, however exterior to that sort’s definition. This creates a free coupling between knowledge and strategies, versus the robust coupling of object-oriented programming.

polymorphism.cxx(Compiler Explorer)

// Courses: strategies are sure with knowledge.
struct A1 {
  void print() const;

struct B1 {
  void print() const;

// Overloading: free features are overloaded by their receiver sort.
struct A2 { };
void print(const A2& a);

struct B2 { };
void print(const B2& b);

// Interfaces: varieties externally implement interfaces.
// Slightly than operate overloading, interfaces are carried out by varieties.
#pragma function interface

interface IPrint {
  void print() const;

struct A3 { };
struct B3 { };

impl A3 : IPrint {
  void print() const;

impl B3 : IPrint {
  void print() const;

void name() {
  A1 a1;
  a1.print();          // A member operate name.

  A2 a2;
  print(a2);           // A free operate name.

  A3 a3;
  a3.IPrint::print();  // An interface operate name.
  1. In object-oriented design, the receiver sort is the kind of the encircling class. You possibly can entry its worth with the this or self key phrases.
  2. With overloading of free features, the receiver sort is a few operate parameter slot, which is overloaded for all collaborating varieties. Within the above instance, the print free operate is overloaded to help operations on A2 and B2.
  3. With interfaces, a receiver-type agnostic interface is outlined, after which carried out for every receiver sort. As with member features, overload decision is not wanted to name interface strategies on a sort.

Following Rust, Carbon does not help operate overloading. It requires that you simply manage features as interface strategies. The [edition_carbon_2023] version allows interfaces and disables operate overloading, to extra carefully adhere to the Carbon design objectives.

However it’s not essential to disable operate overloading. Rust does, however Swift retains it. Interface group of features can co-exist with member features and free features. My recommendation is to make use of whichever technique finest addresses your drawback.

From a C++ perspective, an enormous power of interfaces is its glorious suitability for delivering customization factors. We need a language mechanism for customization points makes a forceful case that Rust traits are the way in which to do this. [interface] is the equal for C++.

As a serious aspect profit, type erasure builds on all of this. It is low on boilerplate, and permits you to simply swap between static calls to interface strategies and dynamic calls to interface strategies.

Interface definitions

The [interface] function brings in 4 new key phrases:

  • interface – a blueprint for an impl. There are non-template interfaces, interface main templates, interface partial templates and interface specific specializations. Interfaces are just like courses: they might inherit different interfaces, they’ve member lookup, they’ve strategies.
  • impl – a set of strategies named in an interface and carried out for a sort. impl is a main template entity with two parameters: a sort and an interface. Certified member lookup of an expression (of any object sort, even non-class varieties) the place the nested-name-specifier is an interface causes implicit era of an impl for the sort on the left-hand aspect. Then member lookup continues in that impl. The impl’s strategies notionally “lengthen” the sort’s definition externally. That is non-invasive extension.
  • dyn – an summary sort template (the place specializations have sizeof and alignof 1 in order that libraries see them as full varieties) that erases the sort behind an interface. dyn<IFace> implements IFace, which means impl<dyn<IFace>, IFace>, the impl test, is true. Every of that impl’s strategies ahead calls on the dyn sort to the whole sort implementing the interface through a dyn pointer. The dyn pointer is the dimensions of two regular pointers. It comprises a pointer to the info object, and a pointer to a dyntable, which is an externalized digital desk, with slots for every impl methodology and slots for auxiliary info like a deleting dtor, std::type_info pointer, and full sort measurement and alignment.
  • make_dyn – an operator that takes a pointer to a whole object and yields a dyn pointer. This does the acutal type-erasure. When this instruction is lowered throughout codegen, the dyn desk is emitted to the module’s IR.

Let’s take a detailed take a look at interface:

interface [interface-name] [auto] : [base-interfaces] {
  [explicit] [static] [return-type] func([parameters]) [cv-ref] [noexcept-spec] [default-spec] {
    // Non-obligatory Default operate implementation.

You possibly can have interface templates along with non-templated interfaces. Interface templates are parameterized similar to class templates. Moreover, you possibly can parameterize any template entity (class templates, choice templates, operate templates, variable templates, interface templates and ideas) with interface and interface template parameters, that are a brand new language entity in Circle.

Interfaces allow a large spectrum of help between explicitness and implicitness. A lot of the C++0x ideas work was caught up in arguments as to the place to peg the design alongside this spectrum. Simplifying the use of concepts makes the argument for implicit idea map era.

I feel it is smart to let organizations do what they need, and keep away from one-size matches all prescriptions that elevate discord. There are sufficient dials on the interface to get any impact you are in search of:

  • auto – Mark an interface definition as auto to allow implicit era of impls. Do that once you count on many of the performance to be offered by the interface’s personal operate default definitions. Turning this on reduces explicitness. The auto token comes after the interface identify, the identical place the place the contextual key phrase last happens in a class-virt-specifier.
  • specific – Mark a technique as specific to stop its implementation in an impl from being picked up implicitly from a member operate within the sort. For instance, in case your methodology is named write, and your interface expects some very particular conduct, it could be applicable to mark the strategy specific, particularly if the interface is marked auto, in order that an unrelated write methodology is not introduced in inadvertently. Turning this on will increase explicitness.
  • default – The default-specifier considers a value-dependent expression earlier than allowing default operate era. Every interface has a Self declaration, which is a dependent sort that’s substituted when the interface generates an impl. We are able to constrain the supply of default implementations with checks on the receiver sort.
  • static – Interface strategies could also be static (which means they do not want an precise object on the left-hand aspect) or non-static (the default).
  • cv-ref – Non-static interface strategies could also be const-volatile certified. This results how this/self is typed contained in the implementation.

Self is a dependent sort alias that is implicitly declared inside interfaces and interface templates. It is a placeholder for the to-be-determined receiver sort. For interface templates, the interface’s identify is implicitly declared as an injected-interface-name, just like the injected-class-name at school templates. It behaves like an interface, except given a template-argument-list, wherein case it behaves like an interface template.

To leap into the deep finish, contemplate the best way to annotate a clone operate to ship worth semantics in a sort erasure container like Rust’s Field sort.

template<interface IFace>
interface IClone auto : IFace {
  // The injected-interface-name implies that IClone is IClone when used like
  // a template and IClone<IFace> when used like an interface. That is related
  // to injected-class-name at school templates.
  std::unique_ptr<dyn<IClone>> clone() const 
  default std::is_copy_constructible_v<Self> {
    // Present a default implementation for impls that do not implement clone.
    return std::unique_ptr<dyn<IClone>>(new Self(self));

This interface does the fitting factor in all conceivable circumstances.

  • It is marked auto, so customers do not have to offer their impl.
  • The clone methodology shouldn’t be marked specific, so impls will search for appropriate clone member features within the receiver sort. There isn’t any danger of binding the fallacious clone methodology, as a result of the return sort of clone is so particular: the member operate would wish to return std::unique_ptr<dyn<IClone<IFace>>>. If it is doing that, when else may it do in addition to clone?
  • Lastly, the clone operate’s default implementation is guarded in opposition to failure by a default-specifier. If the sort does not present a clone member operate, the compiler evaluates the default-specifier after substituting within the receiver sort for Self. If the expression evaluates false, or if there is a substitution failure, the sort does not fulfill the interface, and impl era fails. Importantly, it fails in a SFINAE-friendly method. This system shouldn’t be necessarilly ill-formed, like it could have been had it tried to instantiate the physique of the defaulted clone operate. default-specifier needs to be acquainted to C++ programmers, as a result of it is just like the requires-specifier, however it’s evaluated throughout part of program translation.

IClone‘s definition is completely distinct from its template parameter’s necessities. It does not know or care about IFace. IFace might be substituted with an auto interface, or not. IFace‘s strategies may have specific strategies, and it may have operate definitions guarded by default-specifiers. This design encourages native reasoning, by inserting all of the dials for the express/implicit tradeoffs contained in the interface definition.


The impl is rather like a Rust’s impl. It specifies how a sort implements an interface. However my implementation leverages C++’s partial template deduction mechanism to permit for very versatile era impls.

[optional template-head] [requires-clauses]
impl type-id : interface-name {
  // operate impls.

Each the type-id and interface-name elements of the declaration could also be templated. The compiler treats this as a partial or specific specialization of a single main template:

template<typename Kind, interface IFace>
impl __impl;

This can be a fully new language entity. It is extremely generic, but it requires minimal wording as a result of it builds on the present partial template argument deduction framework.

print_impl.cxx(Compiler Explorer)

#pragma function interface self
#embrace <iostream>

interface IScale {
  // Self relies within the interface context, and non-dependent
  // within the impl context.
  void scale(Self x);

interface IPrint {
  void print() const;

// Implement IScale on double. 
impl double : IScale {
  // Self is an alias for double, so may write this as 
  // void scale(double x)
  void scale(Self x) {
    // There is no such thing as a implicit object. You could use `this` or `self`
    // to entry objects of non-static member features.
    std::cout<< "impl<double, IScale>::scale(): " << self << " to ";
    self *= x;
    std::cout<< self << "n";

// A partial template that can bear successfull argument 
// deduction for arithmetic varieties.
template<typename T> requires(T~is_arithmetic)
impl T : IPrint {
  void print() const {
    std::cout<< "impl<" + T~string + ", IPrint>::print(): "<< self<< "n";

int principal() {
  double x = 100;
$ circle print_impl.cxx
$ ./print_impl
impl<double, IScale>::scale(): 100 to 220
impl<double, IPrint>::print(): 220

The impl syntax impl type-id : interface-name is supposed to counsel inheritance. It is as if your sort is externally inheriting the interface, similar to it could be internally inheriting a base class.

external_impl.cxx(Compiler Explorer)

#pragma function interface self
#embrace <iostream>

interface IPrint {
  void print() const;

impl double : IPrint {
  void print() const;

template<typename T> requires(T~is_arithmetic)
impl T : IPrint {
  void print() const;

// Out-of-line definition for impl<double, IPrint>::print.
// This has exterior linkage.
void impl<double, IPrint>::print() const {          // #1
  std::cout<< "specific specialization: "<< self<< "n";

// Out-of-line definition for impl<T, IPrint>::print. 
// This has inline linkage, as a result of it is a template entity.
template<typename T> requires (T~is_arithmetic)
void impl<T, IPrint>::print() const {               // #2
  std::cout<< "partial template: "<< self<< "n";

int principal() {
  (3.14).IPrint::print();  // Calls the express specialization.
  (101ul).IPrint::print(); // Calls the partial specialization.
$ circle external_impl.cxx
$ ./external_impl
specific specialization: 3.14
partial template: 101

Out-of-line impl operate definitions are permitted. These are supposed to have the identical semantics as out-of-line member operate definitions. impl<type-id, interface-name>:: serves as a nested-name-specifier for the aim of out-of-line definitions. Perform #1, which is a non-template operate, has exterior linkage, which means that it could solely be outlined in a single translation unit in this system. Perform #2, which is a templated operate (particularly a non-function template inside an impl partial template), nonetheless has inline linkage, as if it had been outlined contained in the impl-specifier.

This function was chosen to leverage programmer familiarity with object-oriented programming, to cut back the training burden. For those who usually write your class-specifier in a header file and your member operate definitions in a .cxx file, you possibly can hold doing that with interfaces and impls!

The impl reserved phrase has one other use: the impl-expression exams if an impl is obtainable for a sort and interface. That is just like evaluating a C++20 idea.

impl_test.cxx(Compiler Explorer)

#pragma function interface
#embrace <type_traits>
#embrace <string>

interface IFace { };

template<typename T> requires(T~is_arithmetic)
impl T : IFace { };

// impl is outlined for all arithmetic varieties.
static_assert(impl<uint8_t, IFace>);
static_assert(impl<int16_t, IFace>);
static_assert(impl<lengthy lengthy, IFace>);
static_assert(impl<lengthy double, IFace>);

// impl is undefined for all different varieties.
static_assert(!impl<void, IFace>);
static_assert(!impl<const char*, IFace>);
static_assert(!impl<std::string, IFace>);
static_assert(!impl<int[10], IFace>);

Interface identify lookup

Thus far we have been utilizing certified identify lookup, akin to (3.14).IPrint::print() to name interface strategies on objects. For those who name loads of strategies, unqualified identify lookup, like (3.14).print() is much more handy.

Because of the separate definitions of impls and kinds, we won’t simply write one thing like (3.14).print() — how does the compiler know which interface we need to name the print methodology from? Usually the compiler does not even know that you simply intend to implement an interface for some type–the generic IPrint implementation I have been displaying is specialised over a template parameter and constrained with a T~is_arithmetic test.

Now we have to place an impl in scope to make use of unqualified identify lookup to name interface strategies. There are two methods to do that, the handbook, described right here, and the automated method, with interfaces in templates.

  • utilizing impl type-id-list : interface-list; – Put all of the impls within the outer product of type-id-list and interface-list in scope.

impl_scope.cxx(Compiler Explorer)

#pragma function interface self
#embrace <iostream>

interface IPrint {
  void print() const;

template<typename T> requires(T~is_arithmetic)
impl T : IPrint {
  void print() const {
    std::cout<< T~string + ": "<< self<< "n";

int principal() {
  // Put these 5 impls in scope.
  utilizing impl quick, int, lengthy, float, double : IPrint;

  // As a result of their impls are in scope, we will use
  // unqualified member entry to name IPrint::print.

  // Error: 'print' shouldn't be a member of sort unsigned.
  // (6u).print();
$ circle impl_scope.cxx
$ ./impl_scope
quick: 1
int: 2
lengthy: 3
float: 4.4
double: 5.55

We are able to name print on quick, int, lengthy, float and double varieties with out qualifying the interface identify. using-impl-declaration tells the compiler to think about these interface scopes throughout member lookup.

Interfaces in templates

The facility of interfaces is let free when utilized in templated code. Not like Rust and Swift, which use early type-checking of their generics, C++ makes use of late type-checking. That is much more versatile with respect to supporting all kinds of template parameter sorts, variadics, and so forth. And it is extra forgiving to the person. The tradeoff is that errors could also be issued from deep inside a library, slightly than on the level of a name, in code that’s extra acquainted to its programmer. Late-binding could also be carried out with the [generic] function, if some open questions are resolved.

When used with templates, interfaces function “tremendous ideas.” As a substitute of evaluating a boolean-valued expression, the compiler will test if a sort implements one or a collection of interfaces. auto-marked interfaces could even be implicitly carried out by varieties that implement all their necessities.

There’s new syntax for becoming interfaces into template sort parameters:

  • template<typename T : IFace1 & IFace2 & IFace3> – The template parameter T should implement the &-separated record of interfaces.

This syntax has two results:

  1. There’s an implicit constraint on the template that fails if the template parameter doesn’t implement all of the listed interfaces.
  2. Throughout template instantiation, the impl over T for every of the listed interfaces is put into scope, in order that unqualified identify lookup can be utilized to name interface strategies. That is like injecting a using-impl-declaration within the subsequent template definition.

interface_template.cxx(Compiler Explorer)

#pragma function interface self
#embrace <iostream>

interface IPrint {
  void print() const;

interface IScale {
  void scale(double x);

template<typename T : IPrint & IScale>
void func(T& obj) {

impl double : IPrint {
  void print() const {
    std::cout<< self<< "n";

impl double : IScale {
  void scale(double x) {
    self *= x;

int principal() {
  double x = 100;

  int y = 101;

  // Error: int doesn't implement interface IPrint.
  // func(y);
$ circle interface_template.cxx 
$ ./interface_template

The IPrint & IScale necessities on the template parameter declaration are constraints. It is just like writing requires impl<T, IPrint> && impl<T, IScale> after the template-header. However it additionally has the helpful aspect impact of bringing each these impls into the scope of the operate, so you possibly can name print and scale with out utilizing certified names. Trying to cross an int argument raises an error, as a result of int doesn’t implement both of these interfaces.

Interface packs

interface_template2.cxx(Compiler Explorer)

#pragma function interface self

interface IPrint {
  void print() const;

interface IScale {
  void scale(double x);

interface IUnimplemented { };

// IFace is an interface pack.
// Increase IFace into the interface-list that constrains T.
template<interface... IFace, typename T : IFace...>
void func(T& obj) {

impl double : IPrint {
  void print() const { }

impl double : IScale {
  void scale(double x) { }

int principal() {
  double x = 100;
  func<IPrint, IScale>(x);

  // Error: double doesn't implement interface IUnimplemented
  func<IPrint, IScale, IUnimplemented>(x);

Interfaces and interface templates are first-class language entities. The template system has grown to accommodate them as template parameter kinds. The primary template parameter takes a pack of interfaces, and the template parameter declaration T expands that pack into an interface-list. This sort of flexibility is afforded by C++’s late type-checking.

Interface inheritance

interface_template3.cxx(Compiler Explorer)

#pragma function interface self
#embrace <iostream>

// IGroup inherits a pack of interfaces.
// It is marked 'auto', which means that if a sort implements its
// necessities, an implicit impl is generated for it.
// Because it has no interface strategies, the one necessities are that
// the sort implement all base interfaces IFaces.
template<interface... IFaces>
interface IGroup auto : IFaces... { };

interface IPrint {
  void print() const;

interface IScale {
  void scale(double x);

interface IUnimplemented { };

template<interface IFace, typename T : IFace>
void func(T& obj) {

impl double : IPrint {
  void print() const { }

impl double : IScale {
  void scale(double x) { }

int principal() {
  double x = 100;
  func<IGroup<IPrint, IScale>>(x);

  // Error: double doesn't implement interface IUnimplemented
  func<IGroup<IPrint, IScale, IUnimplemented>>(x);

Circle’s interfaces are modeled on C++ courses. Since courses can inherit courses, interfaces can inherit interfaces. Interfaces may even inherit packs of interfaces with a base-specifier-list. The IGroup interface template takes an interface pack and inherits from its growth. It is marked auto, which means an implicit impl could be generated for a sort that satisfies all of its necessities. On this case, since IGroup has no interface strategies, the one necessities are that the sort implement all its base interfaces.

Interface composition boasts related strengths as class composition. Critically, the power to gather a gaggle of interfaces and bind them right into a single interface drastically improves the flexibleness of type erasure.

Kind erasure and dyn

There have been many, many, many, many convention talks on C++ dynamic polymorphism, or “sort erasure”. I feel the clearest remedy for individuals who do not have a mastery of this concept is Klaus Iglberger’s CppCon 2021 presentation. There is a good weblog publish on the subject here.

There are various advanced libraries that attempt to cut back the boilerplate burdens of implementing sort erasure. However I feel it is time that sort erasure change into a first-class language function. Rust does it, everybody likes it, and with interfaces accessible in C++, it is a simple step to dynamic polymorphism.

Rust delivers dynamic polymorphism with trait objects. You specialize the dyn generic over a trait that solely has dispatchable features, which means non-templated features that haven’t any reference to the receiver sort of their declarations. These function pure digital features within the C++ sense.

Circle’s dyn sort is used the identical method. dyn, the reserved phrase, is a sort template. However its specializations aren’t courses. They’re dyns, a brand new language entity. Like summary base courses, which they mannequin, they’re abstract types that implicitly implement the interfaces they’re specialised on. As with all summary varieties, you possibly can’t declare objects and you’ll’t copy them. You possibly can work with dyn pointers or dyn lvalues.

  • dyn<interface-name> – a dyn sort, specialised on an interface. It implements impl<dyn<interface-name>, interface-name>.
  • make_dyn<interface-name>(expr) – given a pointer expression, generate a dyntable and populate it with operate pointers to every member operate implementation from the interface-name.

The purpose of dyn is to offer a base pointer sort that includes the interface, however not the sort being erased.

dyn.cxx(Compiler Explorer)

#pragma function interface self
#embrace <iostream>
#embrace <string>

interface IFace { 
  void print() const;

impl int : IFace { 
  void print() const { 
    std::cout<<"int.print = "<< self<< "n";

impl std::string : IFace {
  void print() const {
    std::cout<<"string.print = "<< self<< "n";

int principal() {
  // dyn<IFace> implements IFace.
  static_assert(impl<dyn<IFace>, IFace>);

  // Get a pointer to the type-erased dyn<IFace>.
  int x = 100;
  dyn<IFace>* p1 = make_dyn<IFace>(&x);

  // Name its interface methodology. Look, it does not say 'int' anyplace!

  // Kind erase string and name IFace::print.
  std::string s = "My nifty string";
  dyn<IFace>* p2 = make_dyn<IFace>(&s);
$ circle dyn.cxx
$ ./dyn
int.print = 100
string.print = My nifty string

int and std::string implement the interface IFace. This interface is dispatchable, which means it has no operate templates and no features with dependent varieties (probably from the Self sort). We are able to sort erase it.

  1. Declare an object. x is placed on the stack.
  2. Move the thing’s deal with to make_dyn, specialised on the interface we need to sort erase via. On this case, make_dyn<IFace>(&x).
  3. The end result object is a pointer to dyn: dyn<IFace>*. That is used similar to an summary base class pointer in C++’s digital operate polymorphism.
  4. Name IFace‘s strategies via the dyn pointer. The kind of the thing has been erased.

Dyn pointers are totally different from all different C++ pointers: they’re twice as massive. These fats pointers embrace two fields:

  1. A pointer to the info object. On this case, &x or &s.
  2. A pointer to the dyntable for the impl.

C++ courses with digital features (or digital base claasses) are known as dynamic courses.) They hold hidden vtable pointers at the beginning of their knowledge, forward of named non-static knowledge members. When calling a digital operate via a base class pointer, the vtable pointer is loaded from the thing knowledge. The offset of the digital operate pointer throughout the vtable is added to the vtable pointer, and that knowledge is loaded, and the ensuing operate pointer is invoked.

With sort erasure, we do not have the comfort of a vtable pointer sure up with an object’s knowledge. The entire level is exterior polymorphism, the place we outline the interface relationships exterior of the class-specifier. Consequently, because the vtable pointer is not a part of the thing knowledge, we cross across the dyntable pointer as a part of the dyn pointer, doubling its measurement.

Do not fret over the efficiency implications of utilizing fats pointers. The additional pointer measurement is offset by the truth that we do not have to load the dyntable pointer from reminiscence like we now have to load the vtable pointer. Fats pointers are handed by register, and we will simply extract from register.

Kind erasure and the heap

Kind erasure will get highly effective when you possibly can handle a assortment of type-erased objects. You are going to need to allocate them on the heap. With exterior polymorphism/digital features, you’d use a container like this:

std::vector<std::unique_ptr<BaseType>> vec;

For sort erasure, it is excellent to maintain on utilizing std::unique_ptr. This time, the unique_ptr tracks a dyn specialization:

std::vector<std::unique_ptr<dyn<IFace>>> vec;

By reusing the usual containers, I hope programmers can reuse their acquainted coding idioms. The psychological shift is nearly going from digital operate polymorphism to exterior polymorphism, not about studying an unique programming paradigm.

dyn2.cxx(Compiler Explorer)

#pragma function interface self ahead template_brackets
#embrace <iostream>
#embrace <string>
#embrace <vector>
#embrace <reminiscence>

// make_type_erased is like std::make_unique, but it surely returns a 
// *sort erased* unique_ptr. There are two specific template parameters:
// 1. The sort to allocate on the heap.
// 2. The interface of the return dyn sort.
// That is library code. Write it as soon as, file it away.
template<typename Kind, interface IFace>
std::unique_ptr!dyn!IFace make_unique_dyn(ahead auto... args) {
  return std::unique_ptr!dyn!IFace(make_dyn<IFace>(new Kind(ahead args...)));

// Outline an interface. This serves as our "summary base class".
interface IPrint {
  void print() const;

// Implement for arithmetic varieties.
template<typename T> requires(T~is_arithmetic)
impl T : IPrint {
  void print() const {
    std::cout<< T~string + ": "<< self<< "n";

// Implement for string.
impl std::string : IPrint {
  void print() const {
    std::cout<< "std::string: "<< self<< "n";

// Implement for a container of type-erased varieties.
impl std::vector!std::unique_ptr!dyn!IPrint : IPrint {
  void print() const {
    std::cout<< "std::vector!std::unique_ptr!dyn!IPrint:n";
    for(const auto& obj : self) {
      // Loop via all components. Print out 2 areas to indent.
      std::cout<< "  ";

      // Invoke the type-erased print operate.

int principal() {
  std::vector!std::unique_ptr!dyn!IPrint vec;

  // Allocate and a push an unsigned quick : IPrint;
  vec.push_back(make_unique_dyn!<unsigned quick, IPrint>(2));

  // Allocate and push an int : IPrint.
  vec.push_back(make_unique_dyn!<int, IPrint>(5));

  // Allocate and push a double : IPrint.
  vec.push_back(make_unique_dyn!<double, IPrint>(3.14));

  // Allocate and push a string : IPrint.
  vec.push_back(make_unique_dyn!<std::string, IPrint>("Good day sort erasure"));

  // Loop over all components and name the print() interface methodology.
  // This can be a homogeneous, type-erased interface for heterogeneous knowledge.

  // When vec goes out of scope, its destructor calls unique_ptr's destructor,
  // and that calls the dyn-deleting destructor saved within the dyntable of
  // every sort. For varieties with trivial destructors, that is simply the
  // deallocation operate.
  // *All sources get cleaned up*.
$ circle dyn2.cxx
$ ./dyn2
  unsigned quick: 2
  int: 5
  double: 3.14
  std::string: Good day sort erasure

This design, which by in massive copies Rust, appears and looks like idiomatic C++. It builds on the instinct we have already got from coping with digital features and sophistication inheritance. That is recognizable polymorphism. Solely in its implementation particulars are we involved that it is exterior polymorphism.

This design is also in step with our understanding of summary base courses. A dyn sort is like an summary base. You possibly can’t instantiatiate it or assign to it. You possibly can type and cross round pointers or references to it. And you may name strategies on it.

The make_unique_dyn operate is modeled on std::make_unique. It has a tougher job, so it takes two specific template parameters. The sort parameter, which comes first, signifies the kind of the thing to allocate on the heap. Further arguments are forwarded to the sort’s constructor. The second specific parameter is the interface behind which we sort erase entry to the thing. make_unique_dyn makes use of the make_dyn operator to yield a fats pointer. Marvelously, the dyn pointer is completely appropriate with std::unique_ptr, although it is 16 bytes and has some bizarre properties. The Circle compiler implements delete-expression to name the implicitly-defined dyn-deleting destructor through the dyntable. All this additional plumbing simply works with std::unique_ptr.

We load up a vector with 4 type-erased unique_ptr-wrapped objects and name print() on the vector, which calls print() on its members. As a result of the vector sort additionally implements IPrint, we will retailer vectors of type-erased objects within the vector of type-erased objects!. Exterior polymorphism lends itself to recursive knowledge constructions, even when the unique varieties weren’t supposed to be composed in that method.

Worth semantics containers

In dyn2.cxx, we wrap a dyn object in a unique_ptr. This kind gives:

  • default constructor – initialize with a nullptr.
  • transfer constructor – detach from the rhs and connect to the lhs.
  • transfer task operator – detach from the rhs and connect to the lhs.
  • destructor – name the dyn-deleting destructor on the type-erased pointer.

Nonetheless, C++ worth semantics usually calls for 2 extra features:

  • copy constructor – clone the rhs and connect to the lhs.
  • copy task operator – clone the rhs and connect to the lhs.

We need to add copy constructor and task semantics to our type-erased containers. Rust does this with a Box type. With the [interface] function, an equal Field sort could be written in C++. Field wraps a unique_ptr<dyn> and dietary supplements the lacking copy constructor and task operators with calls to IClone::clone.

// Create a unique_ptr that wraps a dyn.
template<typename Kind, interface IFace>
std::unique_ptr!dyn!IFace make_unique_dyn(ahead auto... args) {
  return std::unique_ptr!dyn!IFace(make_dyn<IFace>(new Kind(ahead args...)));

// Implicitly generate a clone interface for copy-constructible varieties.
template<interface IFace>
interface IClone auto : IFace {
  // The default-clause causes SFINAE failure to guard this system from
  // being ill-foremd if IClone is tried to be implicitly instantiated 
  // for a non-copy-constructible sort.
  std::unique_ptr!dyn!IClone clone() const 
  default(Self~is_copy_constructible) {
    // Move the const Self lvalue to make_unique_dyn, which causes copy
    // building.
    return make_unique_dyn!<Self, IClone>(self);

IClone is probably the most subtle little bit of code on this New Circle doc. It makes use of loads of superior options for excellent expressiveness. Let’s break it down:

  • IClone is an interface template parameterized on interface IFace. It inherits IFace.
  • IClone is marked auto, which means it could be implicitly carried out by varieties that fulfill all its necessities. The necessities are its clone operate and the necessities of its base interface.
  • The clone operate has an inline operate definition. When the compiler tries to construct an impl for a sort over IClone, and there’s no impl-provided clone operate, and there’s no exactly-matching clone operate as one of many sort’s member features, it will probably instantiate the definition within the interface because the definition of final resort. This definition calls make_unique_dyn and passes the const Self lvalue, which causes Self’s copy constructor to be known as.
  • The clone operate has a default-clause, a part of the language particular to interfaces. When the interface methodology has an inline definition, the default-clause is evaluated in the midst of testing if the impl is glad. If the default-clause evaluates to false, then the defaulted interface operate can’t be used to fulfill the impl, and impl era fails. This can be a important mechanism, as a result of with out it, this system can be left ill-formed, failing to name the copy constructor invoked inside make_unique_dyn. The default-clause serves as a guard, shifting the failure to a SFINAE context. It serves the identical goal because the C++20 requires-clause, shifting the purpose of failure to a recoverable context. The distinction between the 2 is that requires-clause is evaluated earlier than a operate is named, and default-clause is evaluated earlier than a default implementation is generated.

The impact of IClone’s is that customers do not should do something to choose a sort into this interface, so long as the sort helps copy building. If a sort does not help copy building, you possibly can nonetheless manually implement IClone.

dyn3.cxx(Compiler Explorer)

#pragma function interface ahead self template_brackets
#embrace <reminiscence>
#embrace <iostream>

// That is all goes right into a library.

// Create a unique_ptr that wraps a dyn.
template<typename Kind, interface IFace>
std::unique_ptr!dyn!IFace make_unique_dyn(ahead auto... args) {
  return std::unique_ptr!dyn!IFace(make_dyn<IFace>(new Kind(ahead args...)));

// Implicitly generate a clone interface for copy-constructible varieties.
template<interface IFace>
interface IClone auto : IFace {
  // The default-clause causes SFINAE failure to guard this system from
  // being ill-foremd if IClone is tried to be implicitly instantiated 
  // for a non-copy-constructible sort.
  std::unique_ptr!dyn!IClone clone() const 
  default(Self~is_copy_constructible) {
    // Move the const Self lvalue to make_unique_dyn, which causes copy
    // building.
    return make_unique_dyn!<Self, IClone>(self);

template<interface IFace>
class Field {
  utilizing Ptr = std::unique_ptr!dyn!IClone!IFace;

  Field() = default;

  // Enable direct initialization from unique_ptr!dyn!IClone!IFace.
  specific Field(Ptr p) : p(std::transfer(p)) { }
  Field(Field&&) = default;
  Field(const Field& rhs) {
    // Copy constructor. That is how we clone.
    p = rhs.p->clone();

  Field& operator=(Field&& rhs) = default;
  Field& operator=(const Field& rhs) {
    // Clone right here too. We won't name the sort erased sort's task, 
    // as a result of the lhs and rhs could have unrelated varieties which might be solely 
    // widespread of their implementation of IFace.
    p = rhs.p->clone();
    return self;

  // Return a dyn<IFace>*. That is reached through upcast from dyn<IClone<IFace>>*.
  // It is doable as a result of IFace is a base interface of IClone<IFace>.
  // If the person needs to clone the thing, it ought to achieve this via the Field.
  dyn!IFace* operator->() noexcept {
    return p.get();

  void reset() {

  Ptr p;

template<typename Kind, interface IFace>
Field!IFace make_box(ahead auto... args) {
  return Field!IFace(make_unique_dyn!<Kind, IClone!IFace>(ahead args...));

// That is the user-written half. Little or no boilerplate.
interface IText {
  void print() const;
  void set(std::string s);
  void to_uppercase();

impl std::string : IText {
  void print() const {
    // Print the deal with of the string and its contents.
    std::cout<< "string.IText::print ("<< &self<< ") = "<< self<< "n";
  void set(std::string s) {
    std::cout<< "string.IText::set known asn";
    self = std::transfer(s);
  void to_uppercase() {
    std::cout<< "string.IText::to_uppercast known asn";
    for(char& c : self)
      c = std::toupper(c);

int principal() {
  Field x = make_box!<std::string, IText>("Good day dyn");

  // Copy assemble a clone of x into y.
  Field y = x;

  // Mutate x.

  // Print each x and y. y nonetheless refers back to the authentic textual content.

  // Copy-assign y again into x, which has the unique textual content.
  x = y;

  // Set a brand new textual content for y.
  y->set("A brand new textual content for y");

  // Print each.
$ circle dyn3.cxx
$ ./dyn3
string.IText::print (0x553eb0) = Good day dyn
string.IText::to_uppercast known as
string.IText::print (0x553eb0) = HELLO DYN
string.IText::print (0x5542f0) = Good day dyn
string.IText::set known as
string.IText::print (0x554320) = Good day dyn
string.IText::print (0x5542f0) = A brand new textual content for y

We get copy and assign mechanics with out std::string having to subscribe to the IClone interface. Since strings are copy-constructible, IClone’s default clone implementation is used to generate impl<std::string, IClone<IPrint>> with out requiring something of the programmer.

The Field class clones object managed by the argument in each its copy constructor and replica task operators. We won’t name the task operator on the type-erased object, as a result of the left and proper hand sides of the task may not be the identical sort! All we all know is that their type-erased varieties, dyn!IClone!IText, are the identical. For those who observe the pointers within the output, you will see that x = y has the impact of adjusting the deal with of x‘s object. The unique object at 0x553eb0 will get changed with a clone of y (0x5542f0), allotted at 0x554320.

Examine the textual content from this pattern that the person writes (the interface-specifier for IText and its impl by std::string) with the boilerplate required by unextended C++. For those who’re a succesful C++ programmer not adopting interfaces to flee the complexity of operate overloading, you should still love them for dynamic sort erasure.


  • Reserved phrases: fn, in and var.
  • Interactions: Modifications syntax for operate, parameter and object declarations.
  • Editions: [edition_carbon_2023]

[new_decl_syntax] is a piece in progress to interchange the declaration syntax in C++ with one thing a lot clearer. The fn token is reserved for declaring features. The var token is reserved for declaring objects and knowledge members.

This function resolves some context-sensitivities which C++ inherited from C. For instance, is a * b declaring a pointer to sort a and calling it b? Or is it multiplying to values? With [new_decl_syntax], you’d want to put in writing var b : a*; to get the previous.

Enhancing C++ to ship the form of expertise builders count on from a programming language at the moment is tough partly as a result of C++ has a long time of technical debt amassed within the design of the language. It inherited the legacy of C, together with textual preprocessing and inclusion. On the time, this was important to C++’s success by giving it prompt and prime quality entry to a big C ecosystem. Nonetheless, over time this has resulted in important technical debt starting from integer promotion guidelines to advanced syntax with “probably the most vexing parse”.

Difficulties improving C++

The Carbon mission cites the most vexing parse as a motivation to begin a brand new syntax. The [new_decl_syntax] resolves that stunning parse with out requiring improvement of a wholly new toolchain.

most_vexing_parse.cxx(Compiler Explorer)

struct a_t { };

struct b_t { 
  int x; 

int principal() {
  // Most vexing parse: This isn't an object declaration. It is a operate
  // declaration in block scope. a_t() is parsed like a operate parameter
  // slightly than an initializer.
  b_t obj(a_t());

  // Error: cannot entry obj.x, as a result of obj is not an object, it is a 
  // operate identify.
  obj.x = 1;

Essentially the most vexing parse is the thing declaration b_t obj(a_t()). Sadly, the C++ grammar does not see that as an object declaration, however slightly as a operate declaration like this: b_t obj(a_t param). The parentheses within the initializer a_t() are parsed as superfluous parentheses round a non-existing declaration-id.

most_vexing_parse2.cxx(Compiler Explorer)

#pragma function new_decl_syntax

struct a_t { }

struct b_t { 
  fn b_t(a : a_t);
  var x : int; 

fn principal() -> int {
  // Okay. Essentially the most vexing parse has been resolved. That is explicitly
  // a variable declaration, not a operate declaration.
  var obj : b_t = a_t();

  // Okay. obj actually is an object.
  obj.x = 1;

The [new_decl_syntax] requires marking operate declarations with fn and object and knowledge member declarations with var. Moreover, the kind of the thing and its identify are textually by a : token, eliminating the prospect of confusion.

Perform declarations

fn.cxx(Compiler Explorer)

#pragma function new_decl_syntax

// Clearer operate declaration syntax. At all times use trailing-return-type.
fn func1(x: int) -> double { return 3.14; }

// Works like regular with templates.
template<typename T>
fn func2(x: T) -> int {
  return (int)x;

// Parameter packs use main dots.
template<typename... Ts>
fn func3(...args: Ts);

// Or use an invented template parameter pack.
fn func4(...args: auto);

// C-style ellipsis parameters are indicated as regular.
fn func5(p: const char*, ...);

struct obj_t {
  // Particular member features are declared within the typical method, however now have
  // an unambiguous syntax.
  fn obj_t() = default;
  fn ~obj_t() = default;

  // For conversion features, the return sort is implied by the 
  // operate identify.
  fn operator int() const {
    return 1;

  // Extraordinary member features.
  fn func() const -> int {
    return 100;
fn-declaration:  fn [storage-class] function-name ([parameter-list])[cv-qualifiers] [->trailing-return-type] [definition]
parameter-list:  parameter
                 parameter-list, parameter
parameter:       [...] identify : type-id
                 identify : type-id = default-argument

The brand new declaration syntax is Rust-like. Write fn, then the operate identify, then the parameter record, then the cv-qualifiers, then an elective trailing-return-type. For those who do not specify the trailing-return-type, it is assumed to be void, besides within the case of conversion features (just like the operator int() within the instance), the place it is set to the kind of the conversion.

Relying in your stage of talent, this can be a worthwhile or a reasonably trivial enchancment to your programming expertise. Nonetheless, it tremendously simplifies the operation of tooling, as a result of many potentially-ambiguous paths within the grammar are walled off thanks to those new tokens.

Object declarations

var.cxx(Compiler Explorer)

#pragma function new_decl_syntax placeholder_keyword
#embrace <iostream>

// Normal syntax.
var x0 : int;         // Default-initalization.
var x1 : int = 100;   // Copy-initialization.

// Good syntax for sort inference.
var y := 10;          // Copy-initialization.

// Put the decl-specifier-seq within the regular place.
var static z := 5;

// You need to use a braced-initializer for varieties that help it.
var array : int[] { 1, 2, 3 };

// We do not want 'var' for operate parameter declarations. That is assumed.
fn foo(x : int, y : double) -> int { return 1; }

// Get a operate pointer. You must give the operate parameter a reputation, 
// however you should use the placeholder _.
// The declarator syntax hasn't been redesigned, so you continue to want a base sort
// in operate varieties.
var fp1 : int(*)(_ : int, _ : double) = &foo;
var fp2 : auto(*)(_ : int, _ : double)->int = &foo;
var fp3 := &foo;   // Use sort inference

struct foo_t {
  var x : int;

  // Put the storage-class-specifier proper after 'var'.
  var static y : double;

// Use var-declaration for non-type template parameters.
template<var A : int, var B : int>
fn func();

template<typename... Ts>
struct tuple {
  // A member pack declaration. Use main ...
  var ...m : Ts;

fn principal()->int {
  // Use var for declaring init-statement variables for loops.
  for(var i := 0; i < 5; ++i)
    std::cout<< "for: "<< i<< "n";

  // Use var with 'in' for ranged-for statements. This replaces the ':'
  // in Normal C++.
  for(var i in 5)
    std::cout<< "ranged for: "<< i<< "n";

  // Use var for situation objects in if-statements.
  if(var i := y * y)
    std::cout<< "if: "<< i<< "n";
var-declaration: var [storage-class] [...] identify : type-id;
                 var [storage-class] identify : [type-id] = init;
                 var [storage-class] identify : [type-id] { init };

For object and member declarations, write var, then the declaration identify, then a colon. Right here, you possibly can write a type-id for the thing, or elide that and use sort inference when an initializer is offered. The sort inference type is agreeable, because the = is collapsed into the :, like var x := 5.

To declare a member pack declaration with [new_decl_syntax], write a pack-expansion token ... earlier than the member identify.

Whereas [new_decl_syntax] remains to be beneath improvement, many types of declaration have been up to date:

  • Variables
  • Knowledge members
  • Situation objects (eg if-statement)
  • Init-statement objects (eg for-statement)
  • Ranged-for declarations – use in to separate the declaration from its initializer.
  • Non-type template parameters

We may go additional, and replace all types of template parameters, eg,

  var           NonType   : int,              // a non-type parameter
                Kind      : typename,         // a sort parameter
  template<...> Temp      : typename,         // a sort template parameter 
  template<...> var Var   : auto,             // a variable template paremeter
  template<...> Idea   : idea,          // an idea parameter
                Interface : interface,        // an interface parameter
  template<...> ITemp     : interface,        // an interface template parameter
                Namespace : namespace,        // a namespace paremeter
                Common : template auto     // a common parameter

Does this achieve us something? The existing forms are already unambiguous and straightforward to learn.

One aspect good thing about [new_decl_syntax] is that union, struct/class and choice definitions now not share syntax with different declarations. This implies we will drop the semicolon after the closing brace in a class-specifier. Eg,

struct A { }   // OK!
struct B { }   // OK!

This can be a modest enchancment, but it surely does assist builders coming from different languages which do not require semicolons there.







A prvalue of arithmetic, unscoped enumeration, pointer, or pointer-to-member sort could be transformed to a prvalue of sort bool. A zero worth, null pointer worth, or null member pointer worth is transformed to false; every other worth is transformed to true.


In Normal C++, pointer varieties could also be implicitly transformed to bool. There are conditions wherein this may be very complicated.

pointer_to_bool1.cxx(Compiler Explorer)

#embrace <iostream>
#embrace <string>

void func(const std::string& s) {
  std::cout<< s<< "n";

void func(bool b) {
  std::cout<< (b ? "true" : "false") << "n";

int principal() {
  // Prints "true"!!! We wished the std::string overload!
  func("Good day world!");
$ circle pointer_to_bool1.cxx
$ ./pointer_to_bool1 

Now we have two overloads of func: one taking a const std::string& and one taking a bool. We cross the string fixed “Good day world!”. And the compiler calls the bool model of func. This is not what we would like in any respect. Anyone can fall into this entice.

The [no_implicit_pointer_to_bool] function disables implicit conversions of tips that could bools, besides within the context of a contextual conversion to bool, such because the situation of an if-statement.

pointer_to_bool2.cxx(Compiler Explorer)

#pragma function as
#embrace <iostream>
#embrace <string>

void func(const std::string& s) {
  std::cout<< s<< "n";

void func(bool b) {
  std::cout<< (b ? "true" : "false") << "n";

int principal() {
  // Prints "true"!!! We wished the std::string overload!
  func("Good day world!");

  // Decide into security.
  #pragma function no_implicit_pointer_to_bool
  // Error: no implicit conversion from const char* to bool.
  func("Good day world!");

  // Explicitly forged to a string. This works.
  func("Good day world!" as std::string);

  // We are able to opt-back into implicit conversions and match the bool overload.
  func("Good day world!" as _);
$ circle pointer_to_bool2.cxx
error: pointer_to_bool2.cxx:21:8
[no_implicit_pointer_to_bool]: no implicit conversion from const char* to bool
  func("Good day world!"); 

The implicit conversion that selected the undesirable overload is prohibited beneath this function, making this system ill-formed. You possibly can invoke the specified overload of func with specific as-expression forged to std::string. Or you possibly can revert to the traditional conduct with the as-expression as _, which permits prohibit the prohibit conversions. (However they don’t seem to be implicit anymore, because you’re explicitly permitting them.)





  • Interactions: Disables interal promotions throughout regular arithmetic conversions.
  • Editions: [edition_2023]

A prvalue of an integer sort apart from bool, char8_t, char16_t, char32_t, or wchar_t whose integer conversion rank ([conv.rank]) is lower than the rank of int could be transformed to a prvalue of sort int if int can signify all of the values of the supply sort; in any other case, the supply prvalue could be transformed to a prvalue of sort unsigned int.

A prvalue of sort char8_t, char16_t, char32_t, or wchar_t ([basic.fundamental]) could be transformed to a prvalue of the primary of the next varieties that may signify all of the values of its underlying sort: int, unsigned int, lengthy int, unsigned lengthy int, lengthy lengthy int, or unsigned lengthy lengthy int. If not one of the varieties in that record can signify all of the values of its underlying sort, a prvalue of sort char8_t, char16_t, char32_t, or wchar_t could be transformed to a prvalue of its underlying sort.


C++ mechanically promotes values of integral varieties smaller than int to int throughout usual arithmetic conversions. This can be a stunning operation, because the end result sort of your expression is not going to match the kinds of your operands. Most different languages do not do that. I feel it is a stunning operation, particularly since unsigned varieties are promoted to signed int (however not at all times). The [no_integral_promotions] function disables this.

promote.cxx(Compiler Explorer)

int principal() {
  char           x = 1;
  unsigned char  y = 2;
  quick          z = 3;
  unsigned quick w = 4;

  // Integral varieties smaller than int are mechanically promoted to int
  // earlier than arithmetic.
  static_assert(int == decltype(x * x), "promote to int");
  static_assert(int == decltype(y * y), "promote to int");
  static_assert(int == decltype(z * z), "promote to int");
  static_assert(int == decltype(w * w), "promote to int");

  char8_t  a = 'a';
  char16_t b = 'b';
  char32_t c = 'c';
  wchar_t  d = 'd';
  static_assert(int      == decltype(a * a), "promote to int");
  static_assert(int      == decltype(b * b), "promote to int");
  static_assert(unsigned == decltype(c * c), "promote to unsigned");
  static_assert(int      == decltype(d * d), "promote to int");

  // Flip this very stunning conduct off.
  #pragma function no_integral_promotions
  static_assert(char           == decltype(x * x), "doesn't promote to int");
  static_assert(unsigned char  == decltype(y * y), "doesn't promote to int");
  static_assert(quick          == decltype(z * z), "doesn't promote to int");
  static_assert(unsigned quick == decltype(w * w), "doesn't promote to int");

  static_assert(char8_t  == decltype(a * a), "doesn't promote to int");
  static_assert(char16_t == decltype(b * b), "doesn't promote to int");
  static_assert(char32_t == decltype(c * c), "doesn't promote to unsigned");
  static_assert(wchar_t  == decltype(d * d), "doesn't promote to int");



Unsigned arithmetic doesn’t overflow. Overflow for signed arithmetic yields undefined conduct ([expr.pre]).


In Normal C++, addition, subtraction and multiplication on signed integer varieties that causes overflow is undefined conduct. Throughout code era, signed integer arithmetic could create a poison value, which opens up sure optimizations which might change the execution of your program. If the optimizations are literally utilized is unspecified. A program that executes accurately when compiled with -O0, could fail mysteriously when compiled with -O2. This can be attributable to signed integer overflow.

See this post for an instance of stunning conduct resulting from UB.

On the {hardware}, signed and unsigned addition, subtraction and multiplication all use the identical opcode. They’re actually the identical factor. However compiler frontends know the distinction between signed and unsigned varieties, and emit signed integer operations with these poison values.

The [no_signed_overflow_ub] function turns off undefined conduct throughout signed integer overflow.

signed_overflow.cxx(Compiler Explorer)

int add_test1(int x, int y) {
  return x + y;
int sub_test1(int x, int y) {
  return x - y;
int mul_test1(int x, int y) {
  return x * y;

#pragma function no_signed_overflow_ub

int add_test2(int x, int y) {
  return x + y;
int sub_test2(int x, int y) {
  return x - y;
int mul_test2(int x, int y) {
  return x * y;
$ circle signed_overflow.cxx -S -emit-llvm -O2
$ extra signed_overflow.ll
; ModuleID = 'signed_overflow.cxx'
source_filename = "signed_overflow.cxx"
goal datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128-i128:128"
goal triple = "x86_64-pc-linux-gnu"

; Perform Attrs: norecurse nounwind readnone willreturn
outline i32 @_Z9add_test1ii(i32 %0, i32 %1) local_unnamed_addr #0 {
  %3 = add nsw i32 %1, %0
  ret i32 %3

; Perform Attrs: norecurse nounwind readnone willreturn
outline i32 @_Z9sub_test1ii(i32 %0, i32 %1) local_unnamed_addr #0 {
  %3 = sub nsw i32 %0, %1
  ret i32 %3

; Perform Attrs: norecurse nounwind readnone willreturn
outline i32 @_Z9mul_test1ii(i32 %0, i32 %1) local_unnamed_addr #0 {
  %3 = mul nsw i32 %1, %0
  ret i32 %3

; Perform Attrs: norecurse nounwind readnone willreturn
outline i32 @_Z9add_test2ii(i32 %0, i32 %1) local_unnamed_addr #0 {
  %3 = add i32 %1, %0
  ret i32 %3

; Perform Attrs: norecurse nounwind readnone willreturn
outline i32 @_Z9sub_test2ii(i32 %0, i32 %1) local_unnamed_addr #0 {
  %3 = sub i32 %0, %1
  ret i32 %3

; Perform Attrs: norecurse nounwind readnone willreturn
outline i32 @_Z9mul_test2ii(i32 %0, i32 %1) local_unnamed_addr #0 {
  %3 = mul i32 %1, %0
  ret i32 %3

attributes #0 = { norecurse nounwind readnone willreturn }

This instance file generates add, subtract and multiply ops with undefined conduct, after which with out it. The nsw token within the LLVM disassembly stands for “No Signed Wrap,” which means if signed overflow happens, the results of the instruction is a poison worth. Enabling the function turns off the nsw flag.




  • Interactions: 0-valued literals aren’t null pointer constants.
  • Editions: [edition_2023]

A null pointer fixed is an integral literal with worth zero or a prvalue with sort std::nullptr_t.


The [no_zero_nullptr] function makes it so 0 literals aren’t null pointer constants. The nullptr key phrase (and the std::nullptr_t first-class language entity) was solely launched in C++11. Previous to that, everybody used a macro NULL, which was outlined to 0L. As a result of we now have nullptr, we do not want this functionality, and this function disables it.

nullptr.cxx(Compiler Explorer)

void func(const int*);

int principal() {
  func(nullptr);  // OK
  func(0);        // OK

  #pragma function no_zero_nullptr
  func(nullptr);  // OK
  func(0);        // Error
$ circle nullptr.cxx
error: nullptr.cxx:9:8
can not convert prvalue int to const int*
  func(0);        // Error 


A standard request of C++ customers is the addition of a particular placeholder identify key phrase which might be used to instantiate variables with scope lifetime which have a singular unutterable identify – helpful for varieties like std::scoped_lock. There have been makes an attempt to do that previously, however most had been shut down resulting from the potential of identify collisions between the placeholder syntax and present symbols.

P1881 Epochs

The [placeholder_keyword] function implements this concept from the Epochs proposal, turning _ (underscore) right into a reserved phrase that creates nameless declarations. This function works with each the usual declaration syntax and [new_decl_syntax]

placeholder.cxx(Compiler Explorer)

#pragma function placeholder_keyword

// You need to use a placeholder in parameter-declaration. It is clearer than
// leaving the declarator unnamed.
void f1(int _, double _) {
  // Allow any variety of placeholder identify objects.
  auto _ = 1;
  auto _ = 2.2;

  // Error: object should be computerized period. We'd like a non-placeholder identify
  // for identify mangling static period objects.
  static auto _ = 3;

  // Error: '_' shouldn't be an expression.

// Works with [new_decl_syntax] too.
#pragma function new_decl_syntax

// [new_decl_syntax] requires parameter names, so we should use placeholders
// if we would like them unnamed.
fn f2(_ : int, _ : double) {
  // Allow any variety of placeholder identify objects.
  var _ := 1;
  var _ := 2.2;

  // Error: object should be computerized period. We'd like a non-placeholder identify
  // for identify mangling static period objects.
  var static _ := 3;
$ circle placeholder.cxx
error: placeholder.cxx:12:15
[placeholder_keyword]: solely objects with computerized storage period could have placeholder names
  static auto _ = 3; 

error: placeholder.cxx:15:8
[placeholder_keyword]: '_' is the placeholder key phrase and never an expression

error: placeholder.cxx:30:3
[placeholder_keyword]: solely objects with computerized storage period could have placeholder names
  var static _ := 3; 

Observe from epochs:


  • Interactions: Requires { } after control-flow assertion constructs if, else (except instantly adopted by if), for, whereas, do and swap.
  • Editions: [edition_carbon_2023]

Code that’s simple to learn, perceive, and write: Now we have a choice to offer just one technique to do issues, and elective braces are inconsistent with that. It is also simpler to know and parse code that makes use of braces, and defends in opposition to the potential of goto fail-style bugs, whether or not unintended or malicious.

P0632 Require braces

The Carbon mission retains its personal assortment of proposals, additionally with four-digit numbers, which might be separate from C++ proposals. P0632 Require braces stipulates that braces should be used within the scope that follows management move statements.

The Circle function [require_control_flow_braces] implements this variation.

require_control_flow_braces.cxx(Compiler Explorer)

See Also

#pragma function require_control_flow_braces

int principal() {
  int x = 0;

  if(1) { 
    ++x;          // OK.

  if(1)           // Error.

  for(int i = 0; i < 10; ++i) {
    x *= 2;       // OK

  for(int i = 0; i < 10; ++i)
    x *= 2;       // Error.

  whereas(false) {
    x *= 3;       // OK

    x *= 3;       // Error.
$ circle require_control_flow_braces.cxx
error: require_control_flow_braces.cxx:11:5
[require_control_flow_braces]: anticipated braces { } after control-flow assertion

error: require_control_flow_braces.cxx:18:5
[require_control_flow_braces]: anticipated braces { } after control-flow assertion
    x *= 2;       // Error. 

error: require_control_flow_braces.cxx:25:5
[require_control_flow_braces]: anticipated braces { } after control-flow assertion
    x *= 3;       // Error. 


  • Interactions: Modifications std::initializer_list overload decision guidelines.
  • Editions: [edition_2023]

Presently, variable initialization can subtly and massively change which means relying on what syntax is used. For example, std::vector{4, 4} is wildly totally different from std::vector(4, 4). Many agree that this conduct is problematic (particularly in template definitions), and that it prevents builders from uniformly utilizing curly braces in every single place, thus defeating the aim of uniform initialization.

P1881 Epochs

The [safer_initializer_list] function adjustments the conduct of list-initialization [over.match.list], in order that even when an std::initializer_list constructor is discovered ([over.match.list]/1.1), overload decision considers all constructors ([over.match.list]/1.2). If an std::initializer_list constructor is present in 1.1, and one other constructor is present in 1.2, then the match is ambiguous and object initialization fails. The person ought to then disambiguate the initializer with an additional set of braces.

safer_initializer_list.cxx(Compiler Explorer)

#embrace <vector>

int principal() {
  std::vector<int> v1(4, 4);   // OK, [4, 4, 4, 4]
  std::vector<int> v2{4, 4};   // OK, [4, 4] - Stunning!

  #pragma function safer_initializer_list
  std::vector<int> v3(4, 4);   // OK, [4, 4, 4, 4]
  std::vector<int> v4{4, 4};   // ERROR, Ambiguous initialization
  std::vector<int> v5{{4, 4}}; // OK, [4, 4]

This instance is taken from [P1881 Epochs]. The stunning initializer for v2 is flagged as ambiguous when [safer_initializer_list] is enabled. The person resolves the paradox with an additional brace set.


  • Reserved phrases: self.
  • Interactions: Disables this key phrase.
  • Editions: [edition_2023]

The [self] function replaces the this pointer with the self lvalue expression. Whereas the token stays reserved, it’s unlawful to seek advice from this.



In C++, expressions akin to a & b << c * 3 are legitimate, however the which means of such an expression is unlikely to be readily obvious to many builders. Worse, for circumstances akin to a & 3 == 3, there’s a clear supposed which means, particularly (a & 3) == 3, however the precise which means is one thing else — on this case, a & (3 == 3).

Shouldn’t have a complete ordering of priority ranges. As a substitute, outline a partial ordering of priority ranges. Expressions utilizing operators that lack relative orderings should be disambiguated by the developer, for instance by including parentheses; when a program’s which means depends upon an undefined relative ordering of two operators, will probably be rejected resulting from ambiguity.

P0555 Operator precedence

I like to consider this different operator syntax when it comes to operator silos.

Carbon operator precedence

When you’re in a silo, you might parse further operator tokens in the identical silo. To maneuver to a special silo, write parentheses.

The [simpler_precedence] function implements operator silos in a type that is related, however not similar to, that thought of within the Carbon design. For one factor, in Circle’s model, >> and << could also be parsed in sequence, which permits us to proceed utilizing iostreams insertion and extraction operators.

precedence.cxx(Compiler Explorer)

int principal() {
  const int masks  = 0x07;
  const int worth = 0x03;

  static_assert(3 != masks & worth);

  #pragma function simpler_precedence

  static_assert(3 == masks & worth);

This function fixes the priority of bitwise AND, OR and XOR operations, by making them larger priority than comparability and relational operators, however nonetheless placing them in their very own silos.



switch_break.cxx(Compiler Explorer)

#embrace <iostream>
#pragma function switch_break

int principal() {
  for(int arg = 1; arg < 8; ++arg) {
    int x = 0;
    swap(arg) {
      case 1:
      case 2:
        x = 100;
        // implicit break right here.

      case 3:
      case 4:
        x = 200;
        [[fallthrough]];  // Use the fallthrough attribute from C++17.
                          // Successfully "goto case 5"

      case 5:
      case 6:
        x = 300;

        // Help conditional fallthroughs, so long as the following assertion
        // is a label.
        if(6 == arg)
          [[fallthrough]];  // Successfully "goto default".

      // implicit break right here. It stops arg==5, however not arg==6.
        x = 400;

    std::cout<< arg<< " -> "<< x<< "n";
$ circle switch_break.cxx
$ ./switch_break
1 -> 100
2 -> 100
3 -> 300
4 -> 300
5 -> 300
6 -> 400
7 -> 400


The [template_brackets] function drops the < token because the sigil for template-argument-list. !< > is used as an alternative. This kinds a brace pair and by no means requires a person’s disambiguation (the template key phrase after a dependent identify and earlier than <). It’s miles simpler for instruments to parse. It is simpler for people to put in writing. It gives extra constant use of template-argument-list over all features of the language. The < that opens template-parameter-list stays unchanged, as a result of it was by no means an issue.

lambda.cxx(Compiler Explorer)

#embrace <cstdio>

int principal() {
  // Outline a lambda that takes two template arguments.
  auto f = []<typename T, typename U>() {
    places("T = {}, U = {}".format(T~string, U~string));

  // Name it the outdated method.
  f.template operator()<int, double>();

  // Name it the brand new method.
  #pragma function template_brackets
  f!<int, double>();
$ circle lambda.cxx
$ ./lambda
T = int, U = double
T = int, U = double

Importantly, the !< > template-argument-list syntax makes passing template arguments to callables like lambda objects, a lot simpler. That is an unambiguous syntax to be used with [].

This syntax is the solely technique to cross template arguments to lifting lambdas.

Abbreviated template arguments

When [template_brackets] is enabled, you can too specify a single-argument template-argument-list proper after the ! with out utilizing brackets. This abbreviated-template-argument type is impressed by D’s template-single-argument.

There are three sorts of entities you possibly can specify after the !:

  1. an id-expression
  2. a simple-type-name
  3. every other single token

In my expertise, this helps code readibility, as a result of the person does not should mentally stability brackets.

single_argument.cxx(Compiler Explorer)

#pragma function template_brackets
#embrace <vector>
#embrace <string>

// Use the total template-parameter-list type. There is no such thing as a abbreviated type.
template<int N>
struct obj_t { };

template<typename X>
void func(int i) { }

int principal() {
  // Use an abbreviated type of template-argument-list.

  // 1. ! id-expression
  std::vector!std::string v1;

  // 2. ! simple-type-name
  std::vector!int v2;

  // 3. ! a single token
  obj_t!100 v3;

  // This works effectively for calling operate templates.


  • Reserved phrases: None.
  • Interactions: comma-expression.
  • Editions: [edition_2023]

The [tuple] function gives language syntax to type tuple varieties and tuple expressions.

Tuple varieties:

  • (-) – An empty tuple sort.
  • (_type-id_,) – A 1-tuple sort.
  • (_type-id_, _type-id_ [, _type-id_ ...]) – A comma-separated n-tuple sort.

Tuple expressions:

  • (,) – An empty tuple expression.
  • (_expression_,) – A 1-tuple expression.
  • (_expression, _expression_ [, _expression_ ...])– A comma-separate n-tuple expression.

Inside parenthesis, the comma-operator/discard-operator is successfully hidden. If you wish to discard an expression within parenthesis, which could be helpful within mem-initializer-list constructs, carry out an specific forged of the discarded operand to void.

auto x = (a, b);         // This can be a 2-tuple
auto y = ((void)a, b);   // This discards a and returns b.

To entry a user-defined operator,, both write it exterior of parenthesis, write the decision syntax operator,(a, b), or flip off the [tuple] function.

tuple1.cxx(Compiler Explorer)

#pragma function tuple
#embrace <tuple>
#embrace <iostream>

// An empty sort tuple is spelled (-).
utilizing T0 = (-);
static_assert(T0 == std::tuple<>);

// A 1-tuple is spelled (type-id,)
utilizing T1 = (int*,);
static_assert(T1 == std::tuple<int*>);

// Increased-level tuples are comma separated.
utilizing T2 = (int*, double[5]);
static_assert(T2 == std::tuple<int*, double[5]>);

int principal() {

  // An empty tuple expression is (,).
  std::tuple<> tup0 = (,);

  // A 1-tuple expression is (expr, ).
  std::tuple<int> tup1 = (5,);

  // Increased-level tuple expressions are comma separated.
  std::tuple<int, double> tup2 = (5, 3.14);

  // Revert to the discard operator by casting the primary component to void.
  int x = ((void)printf("Good day not a tuplen"), 5);
  std::cout<< "x = "<< x<< "n";
$ circle tuple1.cxx
$ ./tuple1
Good day not a tuple
x = 5


  • Reserved phrases: ref, refmut, protected and unsafe.

Rust references signify a borrow of an owned worth. You possibly can borrow via any variety of non-mutable references, or precisely one mutable reference. This technique proves to the compiler’s borrow checker that you simply aren’t mutating an object from two totally different locations, and that you simply aren’t mutating or studying from a useless object.

This performance needs to be a high-priority for C++ extension. A hypothetical [borrow_checker] can introduce ref and refmut varieties, which signify these borrows. protected and unsafe modifiers can apply to things and to lexical scopes, to point which objects could be accessed via the checked references.

C++11 modified the basic expression mannequin by codifying worth classes. Presently there are three:

Naming an object, parameter or operate produces an lvalue expression. Copying an object produces a prvalue. Materializing an object produces an xvalue.

I am not educated on this subject, however I feel it could be fruitful so as to add two extra worth classes for borrow checking:

If you identify an object you get an lvalue expression. If you identify an object with ref you get a ref expression. If you identify an object with refmut you get a refmut expression. All worth classes (besides prvalue) are viral to their subobjects.

I feel this suggests both new reference varieties (known as ref and refmut) or parameter directives, in order that the borrowed state of an object or subobject is communicated between operate calls.

Objects could also be declared protected, which denies the person the power to entry it by lvalue. Moreover, a scope could be declared protected (or by protected by default relying on a function), so that each one objects declared within the scope are protected.

I feel we’d like a two-pronged method for introducing borrow checking:

  1. Make it opt-in, in order that customers can dip their toe in and write their new code with checking.
  2. Present a safe-by-default function, to create a migration path for present code. Set the safe-by-default function in your mission’s pragma.feature file, and resolve the “cannot entry lvalue in a protected scope” errors till your information are totally borrow-checked.


Most fashionable languages have syntaxes which might be context free (or extra exactly, LALR), successfully which means {that a} parse tree could be produced with out the compiler needing to carry out semantic evaluation. C++ may be very context delicate. That makes writing compilers exhausting, and writing non-compiler tooling very very tough.

Does the context sensitivity C++’s grammar make issues tougher for people? In some circumstances, for certain. The [new_decl_syntax] function clarifies operate and object declarations, and resolves some context sensitivities within the language, for instance, x * y is at all times an expression with [new_decl_syntax], it is by no means the declaration of a pointer object. The [template_brackets] syntax replaces < > template-argument-list, which in dependent contexts requires user-disambiguation the template key phrase, with !< >, which is unambiguous.

For the long-term well being of the C++ ecosystem, it is most likely good to think about including a context-free grammar. The syntax needs to be orthogonal to the semantics of the language, which means you possibly can swap out the present syntax and swap within the new one, with out altering the which means of your program.

The SPECS mission, A Modest Proposal: C++ Resyntaxed from 1996 aimed to reskin C++ with a context free grammar. Essentially the most important change is the brand new declarator syntax, which reads from left-to-right, slightly than in a complicated clockwise spiral.

From the SPECS paper:

The next are easy C++ summary declarators:
    int               // integer
    int *             // pointer to integer
    int *[3]          // array of three tips that could integer
    int (*)[3]        // pointer to array of three integers
    int *()           // operate having no parameters, returning pointer to integer
    int (*)(double)   // pointer to operate of double, returning an integer

The equal SPECS sort IDs are:
    int                // integer
    ^ int              // pointer to integer
    [3] ^ int          // array of three tips that could integer
    ^ [3] int          // pointer to array of three integers
    (void -> ^int)     // operate having no parameters, returning pointer to integer
    ^ (double -> int)  // pointer to operate taking a double, returning an integer

I feel it is a particular enchancment. Will we need to simply exchange C++’s terrible declarator syntax with a [new_declarator_syntax], or is a totally new grammar definitely worth the studying curve it places on customers? A giant benefit can be to simplify tooling. In the long run, this may most likely show to be definitely worth the retraining prices, but it surely’s not essentially excessive precedence.

SPECS is twenty-five years old-fashioned. It will take extra effort to amend and complement this design to resyntax the present model of the language. However resyntaxing the language shouldn’t be very tough from a compiler engineering standpoint, and have pragmas would help this with out breaking any dependencies. Solely new code can be written within the new syntax, and it could proceed to function with present code within the outdated syntax.


Cyclone was a analysis compiler for a memory-safe dialect of C. It had many safety-oriented options, chief amongst them was fats pointers for performing run-time bounds checking.

From Cyclone: A safe dialect of C:

int strlen(const char ?s) {
  int i, n;
  if (!s) return 0;
  n = s.measurement;
  for (i = 0; i < n; i++,s++)
    if (!*s) return i;
  return n;

? is Cyclone’s declarator for a fats pointer. This features a pointer, and a rely of components left within the vary. This strlen implementation searches for inner null characters. Within the case of a string that is not null terminated, the loop will not learn previous the top of the info, as a result of it has accessed the rely of components, s.measurement. In an bizarre C/C++ compiler, this system would run off the top of the info and into undefined conduct.

Rust adopted this fats pointer concept and put it into manufacturing. Out-of-bounds accesses in Rust will elevate a panic.

C++ could be prolonged with run-time bounds checking beneath a [cyclone_pointers] function.


In C++, templates are late-checked, which means the semantic necessities of statements could also be deferred till instantiation when the constituent varieties are depending on template parameters. In contrast, Rust generics are early-checked, which means the person should outline an implementation, together with relating dependent kinds of generic parameters (known as related varieties). The first purpose of early-checking is to maneuver error checking nearer to definition, in order that error messages present what the person did fallacious, slightly than pointing deep inside library code. The draw back of early-checking is that it does not deal with variadics, non-type template parameters, or interface template parameters.

Within the 2000s, early-checked generics N2773 Proposed Wording for Concepts had been added to the C++ working draft earlier than being eliminated previous to C++11. A analysis function, [generic], would add a generic key phrase and help early-checked variations of all of the language entities that now help templates:

  • operate generics
  • class generics
  • alternative generics
  • variable generics
  • interface generics

I made an enormous effort on this path, however there have been too many open questions to shut the loop.


  • Reserved phrases: meta and emit.

Circle basic centered on compile-time execution. It was an train rotating the language from the runtime to the compile-time area: the compiler built-in a whole ABI-aware interpreter, and will execute any code throughout translation, together with making international operate calls to compiled libaries. This allowed configuration-driven program era, the place a translation unit may open a file (like a .json or .csv file), learn the contents at compile time, and programmatically generate varieties and AST by branching on the loaded knowledge.

The key phrase for activating this conduct was @meta. I used the @ character to prefix reserved tokens with out clashing with identifiers in present code. With the pragma function mechanism, that is now not crucial. A [meta] function can reserved bizarre identifiers.

template<typename... Ts>
struct tuple {
  @meta for(int i : sizeof...(Ts))
    Ts...[i] @("m", i);

That is the Circle Traditional tuple definition, circa 2017. The @meta token applies to the for-statement, and causes compile-time execution of the loop throughout template instantiation. The loop steps over every component of the parameter pack, and at every step emits a knowledge member declaration: we all know it is a knowledge member declaration, as a result of it is a non-meta assertion, and the inner-most enclosing non-meta scope is a class-specifier.

Ts...[i] is a pack subscript, which yields one component of the sort parameter pack at every step. @("m", i) is a dynamic identify, which evaluates all operands, converts them to strings, concatenates them, and adjustments the end result to an identifier. For a three-element tuple, the info member names are m0, m1 and m2.

template<typename... Ts>
struct tuple {
  Ts ...m;

With New Circle’s member pack declarations, tuple is much more concise, compiles far more rapidly, and seems much less unique. Moreover, as a result of the format of the sort is thought throughout definition, it may be used with mixture CTAD to mechanically deduce the kinds of the parameter pack components from an initializer.

As Circle’s metaprogramming turned extra subtle, a lot of Circle Traditional’s @meta use circumstances turned much less compelling. I am refactoring and modernizing the compile-time execution “meta” system. The brand new design focuses on “heavy lifting” in metaprogramming, high-level and elaborate duties for which the extra succinct operators in New Circle aren’t well-suited.


  • Reserved phrases: in, out, inout, copy, and transfer.

There’s lengthy been a want to make use of parameter directives to point the move of knowledge via a operate. This might exchange the function of reference varieties in doing this. References reminiscence, they do not convey intent. A set of parameter-passing directives would assist the person convey intent to the compiler, and the compiler would select an environment friendly implementation.

D0708 Parameter passing describes the issue and makes some first makes an attempt at architecting a vocabulary of parameter directives. Nonetheless, the semantics should be nailed down, and wording needs to be generated, earlier than this performance could be carried out because the [parameter_directive] function.

Template parameter sorts

Normal C++ helps three sorts of template parameters. Circle helps 9 sorts:

  1. Non-type parameter
  2. Kind parameter
  3. Kind template parameter
  4. Variable template parameter – template-header ‘auto’ identify
  5. Idea parameter – template-header ‘idea’ identify
  6. Interface parameter – ‘interface’ identify
  7. Interface template parametertemplate-header ‘interface’ identify
  8. Namespace parameter – ‘namespace’ identify
  9. Common parameter – ‘template auto’ identify (P1985R1)

For templated parameter sorts (sort template, variable template, idea and interface template), it is typically handy to not should specify the sorts of templates anticipated from the argument. We are able to merely wildcard the template-header with a pack of common parameters, which can match any form of template:

    template<template auto...> - match templates of any parameterization.

As a result of this assemble happens so ceaselessly, Circle gives an abbreviated type:

    template<...> - match templates of any parameterization.

Namespace parameters are a brand new function. This enables parameterization of certified identify lookup. It is helpful for specializing a template with a model of code, when code is versioned by namespace. To cross a namespace argument, simply cross the identify of the namespace. To specify the worldwide namespace, cross :: because the template argument.

template_params.cxx(Compiler Explorer)

#pragma function interface
#embrace <iostream>
#embrace <ideas>
#embrace <vector>

                auto      nontype,
                typename  sort,
  template<...> typename  type_template,
  template<...> auto      var_template,
  template<...> idea   concept_,
                interface interface_,
  template<...> interface interface_template, 
                namespace namespace_,
  template      auto      common
> void f() {
  std::cout<< "nontype            = {}n".format(nontype~string);
  std::cout<< "sort               = {}n".format(sort~string);
  std::cout<< "type_template      = {}n".format(type_template~string);
  std::cout<< "var_template       = {}n".format(var_template~string);
  std::cout<< "idea            = {}n".format(concept_~string);
  std::cout<< "interface          = {}n".format(interface_~string);
  std::cout<< "interface_template = {}n".format(interface_template~string);
  std::cout<< "namespace          = {}n".format(namespace_~string);
  std::cout<< "common          = {}n".format(common~string);

interface IPrint { };

template<interface IBase>
interface IClone : IBase { };

int principal() {
    5,                 // non-type
    char[3],           // sort
    std::basic_string, // sort template
    std::is_signed_v,  // variable template
    std::integral,     // idea
    IPrint,            // interface
    IClone,            // interface template
    std,               // namespace
    void               // common
$ circle template_params.cxx -std=c++20
$ ./template_params
nontype            = 5
sort               = char[3]
type_template      = std::basic_string
var_template       = std::is_signed_v
idea            = std::integral
interface          = IPrint
interface_template = IClone
namespace          = std
common          = void

This expanded help for template parameters will increase the composability of language entities, with out having to wrap non-type entities in courses.

namespace.cxx(Compiler Explorer)

#embrace <iostream>

// Model your code by namespace.
namespace v1 { 
  void func() {
    std::cout<< "Referred to as v1::func()n";

namespace v2 { 
  void func() {
    std::cout<< "Referred to as v2::func()n";

// Parameterize a operate template over a namespace.
template<namespace ns>
void f() {
  // Certified dependent identify lookup.

int principal() {
  // Specialize the template primarily based on model.
$ circle namespace.cxx
$ ./namespace
Referred to as v1::func()
Referred to as v2::func()

You possibly can manage code into model namespaces, and specialize utility code on these namespaces. That is fairly frictionless in comparison with the present observe of getting to prepare all of your declarations as class members.

tuple_like_of.cxx(Compiler Explorer)

#embrace <iostream>
#embrace <tuple>

// Use Circle member traits to show the std::tuple_elements of a tuple-like
// right into a pack. Consider the idea C on every tuple_element of T.
// They need to all consider true.
template<typename T, template<typename> idea C>
idea tuple_like_of = (... && C<T~tuple_elements>);

// Constrain func to tuple-like integral varieties. Observe we're passing an idea
// as a template parameter.
void func(tuple_like_of<std::integral> auto tup) { 
  std::cout<< "func known as with sort {}n".format(decltype(tup)~string);

int principal() {
  func(std::tuple<int, unsigned quick>());    // OK
  func(std::array<char, 5>());                // OK
  func(std::pair<quick, char>());             // OK
  // func(std::tuple<int, const char*>());    // Error
$ circle tuple_like_of.cxx -std=c++20
$ ./tuple_like_of
func known as with sort std::tuple<int, unsigned quick>
func known as with sort std::array<char, 5ul>
func known as with sort std::pair<quick, char>

You possibly can compose ideas which might be parameterized on different ideas, or interface templates which might be parameterized on different interfaces. The tuple_like_of idea takes an idea C as a template parameter, and if the receiver sort T is tuple-like (which means there exists a partial or specific specialization of std::tuple_size for it), all these component varieties are extracted and examined via C. If C<T~tuple_elements> is true for every component, then the fold-expression evaluates true, and the idea is glad.

This type of composition is made a lot simpler when the compiler helps parameterization for added sorts language entities.

Overload units as arguments

Normal C++ does not allow the passing of operate templates or overload units as operate parameters. Typical utilization is to create a forwarding wrapper across the overload set, with std::bind or a lambda-expression. N3617 “Lifting overload sets into function objects” (2013) by Philipp Juschka makes the case for “lifting lambdas,” that are callable, trivially-copyable, empty class varieties that present entry to the overload set via an implicitly-generated operator().

Circle implements the lifting lambda utilizing N3617’s []id-expression syntax. If the id-expression names a non-member operate, overload set or using-declaration, a brand new sort is minted which could be handed by worth and invoked.

lifting1.cxx(Compiler Explorer)

#embrace <vector>
#embrace <iostream>
#embrace <cassert>

template<typename It, typename F>
auto find_extreme(It start, It finish, F f) {
  assert(start != finish);

  auto x = *start++;
  whereas(start != finish)
    x = f(x, *start++);

  return x;

int principal() {
  std::vector<int> vec { 10, 4, 7, 19, 14, 3, 2, 11, 14, 15 };

  // Move a lifting lambda for the max and min operate templates.
  auto max = find_extreme(vec.start(), vec.finish(), []std::max);
  auto min = find_extreme(vec.start(), vec.finish(), []std::min);

  std::cout<< "min is "<< min<< "n";
  std::cout<< "max is "<< max<< "n";
$ circle lifting1.cxx
$ ./lifting1
min is 2
max is nineteen

We won’t cross std::max on to a operate, as a result of operate parameters want varieties, and an overload set (and performance templates) do not have varieties. The lifting-expression []std::max creates a brand new sort which could be handed by worth. Calling it performs argument deduction, overload decision, and calls the fitting specialization of std::max.

Unqualified and certified identify lookup

lifting2.cxx(Compiler Explorer)

#embrace <iostream>

namespace ns1 {
  struct item_t { };

  // That is discovered with ADL.
  void f(item_t) {
    std::cout<< "known as ns1::f(item_t)n";

namespace ns2 {
  void f(double) {
    std::cout<< "known as ns2::f(double)n";

template<typename T>
void f(T) {
  std::cout<< "known as ::f({})n".format(T~string);

void doit(auto callable, auto arg) {
  // Invoke the lifting lambda. 
  // * If the lambda was fashioned with unqualified lookup, ADL is used to
  //   discover the candidate. Utilizing-declarations encountered throughout unqualified
  //   lookup could inject further candidates for overload decision.
  // * If the lamdba was fashioned with certified lookup, ADL shouldn't be used.

int principal() {
  // Make an ADL name to f. The argument sort int has no related 
  // namespaces, so solely ::f is a candidate.
  doit([]f, 1);

  // Make an ADL name to f. The argument sort has ns1 as an related
  // namespace. Each ::f and ns1::f are candidates, however ns1::f is the
  // higher match.
  doit([]f, ns1::item_t{});

  // Make a certified name to f. The related namespaces of item_t aren't
  // thought of, as a result of ADL solely occurs with unqualified lookup.
  doit([]::f, ns1::item_t{});

  // Unqualified identify lookup finds the alias-declaration for ns2::f.
  // This turns into one of many candidates, although it isn't a member of 
  // an related of the argument sort double. That is precisely the 
  // std::swap trick.
  utilizing ns2::f;
  doit([]f, 3.14);
$ circle lifting2.cxx
$ ./lifting2
known as ::f(int)
known as ns1::f(item_t)
known as ::f(ns1::item_t)
known as ns2::f(double)

There are loads of concerns made when calling features. When the identify of a operate in a name expression in as unqualified-id, and unqualified lookup doesn’t discover

  1. a declaration of a category member, or
  2. a operate declaration inhabiting block scope, or
  3. a declaration not of a operate or operate template,

then argument-dependent lookup (ADL) searches namespaces related to the kinds of the decision’s arguments for added candidates. Circle’s lifting lambdas help this functionality. If an unqualified-id is specified after the [], then ADL could produce further candidates for overload decision on the level of the decision. Features and overload units named in using-declarations are added to the candidate set on the level of the lifting-expression. This enables customers to breed the std::swap trick.

Remember the fact that there isn’t a precise forwarding happening. This differs from the implementation ideas in N3617 “Lifting overload sets into function objects” (2013). Circle’s lifting lambda is pure compiler magic. It permits the person to place a ways between naming an overload set and calling it.

Template arguments for overload units

lifting3.cxx(Compiler Explorer)

#pragma function template_brackets
#embrace <iostream>

// auto operate parameters generate "invented" template parameters.
void f1(auto... x) {
  std::cout<< decltype(x)~string<< " " ...;
  std::cout<< "n";

// Watch out as a result of invented parameters are tacked to the top.
// Which may be stunning because the order of template params does not
// match the order of operate params.
template<typename Z, typename W>
void f2(auto x, auto y, Z z, W w) {
  std::cout<< decltype(x)~string<< " "
           << decltype(y)~string<< " "
           << Z~string<< " "
           << W~string<< "n";

void dispatch(auto f) {
  // Use !< > to cross a template-argument-list on an object expression. 
  // This enables specific template arguments for [].
  // For lambdas, it's shorthand for f.template operator()<char16_t, quick>.
  f!<char16_t, quick>(1, 2, 3, 4);

int principal() {
  dispatch([]f2!wchar_t);  // f2 has invented parameters, that are on the finish.
$ circle lifting3.cxx
$ ./lifting3
char16_t quick int int 
wchar_t char16_t quick int 
int int char16_t quick
quick int wchar_t char16_t

You are free to specify lifting lambda template arguments on the web site of the lambda-expression and on the web site of the decision. Because you create the lambda first, these template parameters are loaded first. On the level of the decision, the elective template-argument-list is appended to any arguments that will have been pushed when the lambda was created.

Customers ought to remember that invented template parameters in abbreviated operate templates (i.e., features taking an ‘auto’ parameter) are at all times saved user-declared template parameters within the operate. The flexibility to specify template arguments at two locations, paired with the re-sequencing of invented template parameters, ought to point out a level of warning for builders.

Lifting lambdas over customization factors

A proposal from the latest C++ mailing, P2769R0 “get_element customization point object” needs to cross a template-argument-qualified overload set (particularly std::get) as a operate parameter. However the Normal language does not help this. So the authors suggest a customization level object that memoizes the template argument as a template parameter, after which forwards to std::get.

1.3. The specified method

The nicest technique to get what we would like can be:

// The code that doesn't work as a result of std::get shouldn't be totally instantiated
std::ranges::type(v, std::much less{}, std::get<0>);

However it doesn’t work as a result of std::get is a operate template, and one can not cross operate templates as arguments with out instantiating them.

1.3. The desired approach

After all, this does work with lifting lambdas!

lifting4.cxx(Compiler Explorer)

#embrace <ranges>
#embrace <algorithm>
#embrace <vector>
#embrace <iostream>

int principal() {
  // {3, 0} ought to come AFTER {3, 1}, as a result of we're solely evaluating the
  // get<0> component, not a full tuple comparability.
  std::vector<std::tuple<int, int> > v{{3,1},{2,4},{1,7},{3,0}};

  // Use the lifting lambda []std::get<0>. That is invoked internally by
  // ranges::type to extract the 0th component from every tuple, for
  // comparability functions.
  std::ranges::type(v, std::much less{}, []std::get<0>);

  for(auto obj : v) {
    std::cout<< get<0>(obj)<< ", "<< get<1>(obj)<< "n";
$ circle lifting4.cxx -std=c++20
$ ./lifting4
1, 7
2, 4
3, 1
3, 0

Within the name to ranges::type, we cross a operate template with template arguments as a operate argument. Highly effective language options like lifting lambdas drastically cut back the necessity for the explosion of library varieties that serve solely to work round deficiencies in the usual language.

String fixed operators

Circle enriches the language with concatenation and comparability operators for string fixed operands. These are tremendously handy, as a result of Circle gives so many ways to programmatically produce string constants.

  • + – string fixed concatenation
  • ==, !=, <, <=, >, >= – string fixed comparability

The + operator concatenates two string constants and yields a brand new string fixed. The null terminator of the left-hand operand is popped off earlier than concatenated. That is basically totally different from utilizing the preprocessor’s string concatenation facility, as a result of it really works throughout program translation, slightly than throughout tokenization. If a string fixed is produced inside template instantiation with reflection, you possibly can concatenate that with one other string fixed, throughout instantiation.

The six comparability and relation operators name into std::char_traits::compare.

string_sort.cxx(Compiler Explorer)

#embrace <iostream>

enum class shapes_t {

int principal() {
  std::cout<< shapes_t~enum_names~type(_1 < _2) + "n" ...;
$ circle string_sort.cxx
$ ./string_sort

Circle has highly effective compile-time mechanisms. The ~enum_names reflection trait yields a non-type pack of string constants for every enumerator within the left-hand enumeration sort. The ~type pack algorithm kinds the weather of a pack and returns a brand new pack. We use the string comparability operator < to lexicographically type the enumerators of shapes_t, and print them to the terminal in a single go along with a pack-expansion assertion ....

String fixed formatting

Circle integrates a subset of std::format into the compiler frontend. That is uncovered via a name to the string-literal suffix .format. The format sample is specified within the string literal, and the arguments, which fill in braces within the sample, are the operands of the format name. That is differs from calling into std::format or fmtlib, as there isn’t a textual dependence on these libraries. The string formatting routines are a part of the compiler binary and execute at native speeds.

Tha fantastic thing about this method is that the results of the format is one other string fixed, and is appropriate with the string fixed comparability operators, string fixed concatenation, static_assert messages, and so forth.

format.cxx(Compiler Explorer)

// Helps arithmetic arguments.
static_assert("The reply is {}.".format(42) == "The reply is 42.");

// Helps formatted arithmetic arguments.
static_assert("That is {:04x}.".format(0xc001) == "That is c001.");

// Helps named arguments.
static_assert("My identify is {identify}.".format(identify: "Sean") == "My identify is Sean.");

// Routinely converts enums to enum names.
enum class command {

static_assert("Command is {}.".format(command::WRITE) == "Command is WRITE.");

We are able to mix Circle’s metaprogramming, static_assert strings and pack expansions to put in writing subtle exams on a operate’s preconditions.

format2.cxx(Compiler Explorer)

#embrace <type_traits>

template<typename... Ts>
void func() {
    "parameter {0}, sort {1}, shouldn't be arithmetic".format(int..., Ts~string)
  ) ...;

int principal() {
  func<int, double, char*, float>();
$ circle format2.cxx
ODR utilized by: int principal()
  func<int, double, char*, float>(); 

  instantiation: format2.cxx:4:13
  throughout instantiation of operate template void func()
  template arguments: [
    'Ts#0' = int
    'Ts#1' = double
    'Ts#2' = char*
    'Ts#3' = float
  void func() { 
    error: format2.cxx:5:3
    static_assert failed; message: "parameter 2, sort char*, shouldn't be arithmetic"

func takes a template parameter pack of varieties. We need to check that every component of the pack is an arithmetic sort. If it isn’t, we need to format a static_assert message that signifies which parameter fails the check and the kind of that parameter.

static_assert takes two operands:

  1. The situation – Ts~is_arithmetic. This can be a pack expression utilizing the type trait is_arithmetic. When static_assert will get expanded with the trailing ..., the primary component of Ts~is_arithmetic that evaluates false raises the compiler error.
  2. The message – "parameter {0}, sort {1}, shouldn't be arithmetic".format(int..., Ts~string). That is one other pack expression, with the identical pack measurement because the situation. When one of many situation component that evaluates, the corresponding pack component within the message expression undergoes template substitution. int... is the integer pack operator, and yields the present index of the pack growth, which on this case is the index of the unlawful parameter sort. The Ts~string expression is the string trait, which produces a string fixed that spells out the parameter sort.

Put all this collectively, and you’ve got the very informative, very legible compiler error:

static_assert failed; message: "parameter 2, sort char*, shouldn't be arithmetic"

Backtick identifiers

Most of the Circle options introduce new key phrases. This raises the query: how will we entry outdated identifiers as soon as they have been shadowed by new key phrases?

Backtick identifiers are the reply. Write a string inside backticks, like `this`, and it will get emitted to the token stream as an identifier with that spelling, and by no means a reserved phrase. In truth, any string could be become an identifier, together with strings with whitespace, or C escape sequences, or Unicode characters.

backtick.cxx(Compiler Explorer)

#embrace <string>
#embrace <iostream>

struct Knowledge {
  // Strings in tickmarks 
  std::string `First Title`;
  std::string `Final Title`;
  int single, `double`, triple;

int principal() {
  Knowledge knowledge { };
  knowledge.`First Title` = "Abe";
  knowledge.`Final Title` = "Lincoln";
  knowledge.single = 1;
  knowledge.`double` = 2;
  knowledge.triple = 3;

  // Use reflection to print the identify of every member and its worth.
  std::cout<< Knowledge~member_names + ": "<< knowledge~member_values<< "n" ...;
$ circle backtick.cxx
$ ./backtick
First Title: Abe
Final Title: Lincoln
single: 1
double: 2
triple: 3

Circle helps reflection on knowledge members and enumerators. That is a gap to make use of backtick identifiers to retailer strings for presentation. Title your fields `First Title` and `Final Title`, and reflection will produce a pack of string constants for these identifiers, which you’ll be able to develop out and print to the terminal, or to a logging library, or no matter you need.

Backtick identifiers:

  1. clear up the key phrase shadowing drawback, so you possibly can activate options which outline new reserved phrases with out reducing your self off from those self same identifiers, and
  2. permits this system to make use of user-facing strings to declare knowledge members and enumerators internally, and programmatically generate serialization code with reflection.

Pack subscripts and slices

Subscript or slice any form of parameter pack with ...[]. There are three kinds:

  • ...[index] – subscript the index‘th component of a parameter pack.
  • ...[begin:end] – slice a parameter pack, returning a brand new pack within the half-open interval [begin, end).
  • ...[begin:end:step] – slice a parameter pack, returning a brand new pack with components beginning at start, ending at finish, and incrementing by step at every step. If step is unfavorable, the pack order is reversed.

subscript.cxx(Compiler Explorer)

#embrace <iostream>

template<typename... Ts>
void func(Ts... x) {
  // Print the primary parameter sort and worth.
  std::cout<< Ts...[0]~string<< " "<< x...[0]<< "n";
  // Print the final parameter sort and worth.
  std::cout<< Ts...[-1]~string<< " "<< x...[-1]<< "n";

int principal() {
  func(10i16, 20i32, 30i64, 40.f, 50.0);
$ circle subscript.cxx
$ ./subscript
quick 10
double 50

This instance subscripts the primary (0) and final components of a operate parameter pack. Legitimate index operands vary from -size (a synonym for the primary component) via measurement – 1 (the final component). This covers the vary of components twice: it is handy as a result of you possibly can identify components from the top with out querying the pack measurement. -1 is the final component, -2 is the second-to-list component, and so forth.


Tuple subscripts and slices


Integer packs

std::make_index_sequence is among the worst designs in C++. You desire a pack of integers. However as an alternative of simply providing you with a pack of integers, you need to name std::make_index_sequence, which returns a trivial sort, however will match a specialization of std::index_sequence, which then has the specified integers as template arguments. That’s, as an alternative of imperatively creating the integers, you need to deduce the integers.

index_sequence.cxx(Compiler Explorer)

#embrace <utility>
#embrace <iostream>

template<typename F, size_t... Is>
void func_inner(F f, std::index_sequence<Is...>) {
  // Name f on every index Is. This can be a Circleism.
  f(Is) ...;

template<size_t N, typename F>
void func(F f) {
  // we won't do something with N right here. Now we have to infer the integers
  // from one other operate.
  func_inner(f, std::make_index_sequence<N>());

int principal() {
  func<5>([](size_t i) {
    std::cout<< "Obtained index "<< i<< "n";
$ circle index_sequence.cxx
$ ./index_sequence
Obtained index 0
Obtained index 1
Obtained index 2
Obtained index 3
Obtained index 4

With this insupportable design, we now have to create a new operate simply to obtain the generated integers. C++20 gives a slight affordance, permitting us to call template parameters of generic lambdas, in order that we will nested the internal operate within the outer operate:

index_sequence2.cxx(Compiler Explorer)

#embrace <utility>
#embrace <iostream>

template<size_t N, typename F>
void func(F f) {
  // We won't do something straight. Deduce the integer sequence right into a
  // generate lambda. C++20 solely.
  auto internal = []<size_t... Is>(F f, std::index_sequence<Is...>) {
    // Name f on every index Is. This can be a Circleism.
  internal(f, std::make_index_sequence<N>());

int principal() {
  func<5>([](size_t i) {
    std::cout<< "Obtained index "<< i<< "n";

It is faint reward to name this progress. C++20 now permits some tighter scoping, however we nonetheless are compelled to put in writing spaghetti code.

  • int...(N) – generate N integers.
  • int...(start:finish:step) – generate integers as a slice.
  • int... – yield the present pack growth index. Shorthand for int...(N) the place N is inferred from different packs within the growth.

Circle’s integer pack operator int...(N) imperatively gives a pack of N integers. You ask for it, and there it’s. You do not have to be inside a template in any respect, as packs exterior of templates are totally supported. The shorthand type int... is most helpful, particularly along with the multi-conditional operator ...?:. This returns a pack of ascending integers which is mechanically sized to the opposite packs within the growth. That’s, it gives the index of the present component of the growth, which is what the person most frequently needs.

pack_index.cxx(Compiler Explorer)

#embrace <iostream>

template<int I>
void func() {
  std::cout<< "Obtained index "<< I<< "n";

int principal() {
  // Name func as soon as for every index {0, 1, 2, 3, 4}
  func<int...(5)>() ...;

  // Or simply do it straight, with no operate name.
  std::cout<< "Even higher "<< int...(5)<< "n" ...;
$ circle pack_index.cxx
$ ./pack_index
Obtained index 0
Obtained index 1
Obtained index 2
Obtained index 3
Obtained index 4
Even higher 0
Even higher 1
Even higher 2
Even higher 3
Even higher 4

Circle makes use of packs to handle every kind of collections. They’re now not sure to template paremeters. The reflection traits return details about varieties in packs. The pack traits embrace transformations on packs. The integer pack int... is probably the most basic Circle metaprogramming operator. You will see that producing numbers is important to fixing all method of compile-time programming issues.

Member pack declarations

Circle gives member pack declarations, as proposed in P1858 “Generalized pack declarations”. This makes implementing generic courses like tuples and variants actually simple.

Use ... earlier than the id-expression in a member declarator. The names of the instantiated members have the pack index appended. For instance, a pack declaration m instantiates non-pack members named m0, m1, m2 and so forth.

member_pack.cxx(Compiler Explorer)

#embrace <iostream>

template<typename... Sorts>
struct tuple {
  [[no_unique_address]] Sorts ...m;

int principal() {
  // Declare and use the combination initializer.
  tuple<int, double, char> A {
    5, 3.14, 'X'
  std::cout<< "A:n";
  std::cout<< "  "<< decltype(A)~member_decl_strings<< ": "<< A.m<< "n" ...;

  // It even works with CTAD! Deduced via the parameter pack.
  tuple B {
    6ll, 1.618f, true
  std::cout<< "B:n";
  std::cout<< "  "<< decltype(B)~member_decl_strings<< ": "<< B.m<< "n" ...;
$ circle member_pack.cxx
$ ./member_pack
  int m0: 5
  double m1: 3.14
  char m2: X
  lengthy lengthy m0: 6
  float m1: 1.618
  bool m2: 1

Idiomatic utilization names pack knowledge members and expands them in pack-expansion statements to use an operation to each instantiated member.

member_pack2.cxx(Compiler Explorer)

#embrace <iostream>

template<typename T, int N>
struct soa {
  [[member_names(T~member_names...)]] T~member_types ...m[N];

  T get_object(int index) const {
    // Use the pack identify in member features: that is generic.
    return { m[index]... };

  void set_object(int index, T obj) {
    // Decompose obj and write into element arrays.
    m[index] = obj~member_values ...;

struct vec3_t { float x, y, z; };
utilizing my_soa = soa<vec3_t, 8>;

int principal() {
  std::cout<< my_soa~member_decl_strings + "n" ...; 

  my_soa obj;
  obj.x[0] = 1;   // Entry member names cloned from vec3_t
  obj.y[1] = 2;
  obj.z[2] = 3;
$ circle member_pack2.cxx
$ ./member_pack2
float x[8]
float y[8]
float z[8]

This instance raises the bar in sophistication. It performs struct-of-array transformations to extend efficiency from improved knowledge format. The soa template is specialised on vec3_t. The caller signifies a SIMD width of 8. The member pack declaration makes use of reflection to interrupt aside the elements of the vector sort and spam them out 8 instances in an array. Now all 8 .x elements are adjoining in reminiscence, the 8 .y elements are adjoining in reminiscence and the 8 .z elements are adjoining in reminiscence. All of that is completed generically utilizing a member pack declaration.

Multi-conditional operator

  • a ...? b : c – The multi-conditional operator. a should be a pack expression. b could also be a pack expression. c should be a non-pack expression.

The multi-conditional operator is the logical soul of Circle’s metaprogramming. It is the key operator, and I take advantage of it in every single place. It is the variadic model of the conditional operator. The packs a and b are concurrently expanded right into a cascade of conditionals. The use circumstances are innumerable.

enum_to_string.cxx(Compiler Explorer)

#embrace <iostream>

template<typename T> requires(T~is_enum)
const char* enum_to_string(T e) {
  //   return
  //     circle   == e ? "circle"           :
  //     line     == e ? "line"             :
  //     triangle == e ? "triangle"         :
  //     sq.   == e ? "sq."           :
  //     pentagon == e ? "pentagon"         :
  //                     "unknown<shapes_t>";
  return T~enum_values == e ...?
    T~enum_names : 
    "unknown <{}>".format(T~string);

enum shapes_t {
  circle, line, triangle, sq., pentagon,

int principal() {
  shapes_t shapes[] { line, sq., circle, (shapes_t)10 };
  std::cout<< enum_to_string(shapes[:])<< "n" ...;
$ circle enum_to_string.cxx 
$ ./enum_to_string 
unknown <shapes_t>

The enum_to_string operate is converts an enum worth into its string fixed illustration. It makes use of the pack expression T~enum_values == e to feed the multi-conditional operator ...?: and return every of corresponding enumerator names.

      circle   == e ? "circle"           :
      line     == e ? "line"             :
      triangle == e ? "triangle"         :
      sq.   == e ? "sq."           :
      pentagon == e ? "pentagon"         :

The return-statement expands to this equal conditional cascade. The primary matching predicate a causes the corresponding pack component b to be returned. If there is not any match, the c expression is returned.

variant_dtor.cxx(Compiler Explorer)

#embrace <type_traits>
#embrace <limits>
#embrace <vector>
#embrace <string>
#embrace <reminiscence>

static constexpr size_t variant_npos = size_t~max;

template<class... Sorts>
class variant {
  static constexpr bool trivially_destructible = 
    (... && Sorts~is_trivially_destructible);

  union { Sorts ...m; };
  uint8_t _index = variant_npos;

  // Conditionally outline the default constructor.
  constexpr variant() 
  requires(Sorts...[0]~is_default_constructible) : 
    m...[0](), _index(0) { }

  // Conditionally-trivial destructor.
  constexpr ~variant() requires(trivially_destructible) = default;
  constexpr ~variant() requires(!trivially_destructible) { reset(); }

  constexpr void reset() noexcept {
    if(_index != variant_npos) {
      int... == _index ...? m.~Sorts() : __builtin_unreachable();
      _index = variant_npos;        // set to worthless by exception.

int principal() {
  // Instantiate the variant in order that the destructor is generated.
  variant<std::string, std::vector<int>, double, std::unique_ptr<double>> vec;

Circle metaprogramming simply implements the variant destructor, a very complicated function when approached with Normal C++. It is only one line right here:

      int... == _index ...? m.~Sorts() : __builtin_unreachable();

There are three Circle options working collectively:

  1. Member pack declarations declare the variant alternate options inside a union,
  2. the integer pack operator int... yields up the index of the pack growth, and
  3. the multi-conditional operator ...?: compares the index of the pack growth with the lively variant different, and after they match, calls the destructor on the corresponding union member.

These three capabilites, used collectively, make variadic programming really feel quite a bit like bizarre programming.

Circle Crucial Arguments

Circle Crucial Arguments are a set of compile-time management move mechanisms, deployed inside template and performance argument lists. They’ll filter, loop, and amplify arguments. They are a recreation changer for superior metaprogramming.

  • if pred => generic-argument
  • if pred => generic-argument else generic-argument


Extending the language with traits and metafunctions

Reflection traits

  • Class template destructuring – the left-hand aspect is a category/alternative template specialization.

    • is_specialization – returns true if the sort is a category/alternative template specialization.
    • template – yields the category template of the specialization.
    • type_args – yields the template arguments of the specialization as sort pack.
    • nontype_args – yields the template arguments of the specialization as a non-type pack.
    • template_args – yields the template arguments of the specialization as a sort template pack.
    • var_template_args – yields the template arguments of the specialization as a variable template pack.
    • concept_args – yields the template arguments of the specialization as an idea pack.
    • interface_args – yields the template arguments of the specialization as an interface pack.
    • interface_template_args – yields the template arguments of the specialization as an interface template pack.
    • namespace_args – yields the template arguments of the specialization as a namespace pack.
    • universal_args – yields the template arguments of the specialization as a common pack.
  • Class sort traits – the left-hand aspect is a category sort.

    • base_count – the variety of public direct base courses.
    • base_offsets – byte offsets of public direct base class subobjects as a non-type pack.
    • base_types – the kinds of public direct base courses as a sort pack.
    • member_count – the variety of public non-static knowledge members.
    • member_decl_strings – declaration string constants for public non-static knowledge members as a non-type pack.
    • member_names – string constants for public non-static knowledge members as a non-type pack.
    • member_offsets – byte offsets of public non-static knowledge members as a non-type pack.
    • member_ptrs – pointers-to-data-members to public non-static knowledge members as a non-type pack.
    • member_types – kinds of public non-static knowledge members as a sort pack.
  • Class object traits – the left-hand aspect is an expression of sophistication sort.object.

    • base_values – lvalues of public direct base class subobjects as a non-type pack.
    • member_values – lvalue of public non-static knowledge members as a non-type pack.
  • Enum sort traits – the left-hand aspect is an enum sort.

    • enum_count – the variety of unique-valued enumerators.
    • enum_names – string constants for enumerators as a non-type pack.
    • enum_values – enumerator constants as a non-type pack.
  • Enum object traits – the left-hand aspect is an enum worth.

    • to_underlying – convert the enum object to an expression of its underlying integral sort. That is like std::to_underlying however as a compiler builtin and with out the C++23 requirement.
  • Perform sort traits – the left-hand aspect is a operate sort.

    • return_type – the return sort of the operate.
    • param_count – the variety of operate parameters.
    • param_types – kinds of operate parameters as a sort pack.
  • String trait – the left-hand aspect is any language entity.

    • string – a string fixed naming the entity. If an expression, the left-hand aspect should be constexpr. Not all expression varieties are supported.
  • Tuple sort traits – the left-hand aspect should be a sort that implements the std::tuple_size customization level.

    • tuple_size – the variety of tuple components.
    • tuple_elements – kinds of tuple components as a sort pack.
  • Variant sort traits – the left-hand aspect should be a sort that implements the std::variant_size customization level.

    • variant_size – the variety of variant alternate options.
    • variant_alternatives – kinds of variant alternate options as a sort pack.

Pack traits

Kind traits

  • Numeric limits – the left-hand aspect should be a sort that implements std::numeric_limits.

    • has_numeric_limits
    • is_signed
    • is_integer
    • is_exact
    • has_infinity
    • has_quiet_NaN
    • has_signaling_NaN
    • has_denorm
    • has_denorm_loss
    • round_style
    • is_iec559
    • is_bounded
    • is_modulo
    • digits
    • digits10
    • max_digits10
    • radix
    • min_exponent
    • min_exponent10
    • max_exponent
    • max_exponent10
    • traps
    • tinyness_before
    • min
    • lowest
    • max
    • epsilon
    • round_error
    • infinity
    • quiet_NaN
    • signaling_NaN
    • denorm_min
  • Kind classes – the left-hand aspect should be a sort.

    • is_void
    • is_null_pointer
    • is_integral
    • is_floating_point
    • is_array
    • is_enum
    • is_union
    • is_class
    • is_function
    • is_pointer
    • is_lvalue_reference
    • is_rvalue_reference
    • is_member_object_pointer
    • is_member_function_pointer
    • is_fundamental
    • is_arithmetic
    • is_scalar
    • is_object
    • is_compound
    • is_reference
    • is_member_pointer
  • Kind modifications – the left-hand aspect should be a sort.

    • remove_cv
    • remove_const
    • remove_volatile
    • add_cv
    • add_const
    • add_volatile
    • remove_reference
    • add_lvalue_reference
    • add_rvalue_reference
    • remove_pointer
    • add_pointer
    • make_signed
    • make_unsigned
    • remove_extent
    • remove_all_extents
    • decay
    • remove_cvref
    • underlying_type
    • unwrap_reference
    • unwrap_ref_decay
    • return_type
  • Kind properties – the left-hand aspect should be a sort.

    • is_const
    • is_volatile
    • is_trivial
    • is_trivially_copyable
    • is_standard_layout
    • has_unique_object_representations
    • is_empty
    • is_polymorphic
    • is_abstract
    • is_final
    • is_aggregate
    • is_unsigned
    • is_bounded_array
    • is_unbounded_array
    • is_scoped_enum
    • rank
  • Supported operations – the left-hand aspect should be a sort.

    • is_default_constructible
    • is_trivially_default_constructible
    • is_nothrow_default_constructible
    • is_copy_constructible
    • is_trivially_copy_constructible
    • is_nothrow_copy_constructible
    • is_move_constructible
    • is_trivially_move_constructible
    • is_nothrow_move_constructible
    • is_copy_assignable
    • is_trivially_copy_assignable
    • is_nothrow_copy_assignable
    • is_move_assignable
    • is_trivially_move_assignable
    • is_nothrow_move_assignable
    • is_destructible
    • is_trivially_destructible
    • is_nothrow_destructible
    • has_virtual_destructor
    • is_swappable
    • is_nothrow_swappable

Common traits

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