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Autumn ISO C++ requirements assembly (Kona, HI, USA) – Sutter’s Mill

Autumn ISO C++ requirements assembly (Kona, HI, USA) – Sutter’s Mill

2023-11-12 23:41:01

At present, the ISO C++ committee accomplished its second assembly of C++26, held in Kona, HI, USA.

Our hosts, Commonplace C++ Basis and WorldQuant, organized for high-quality services for our six-day assembly from Monday by means of Saturday. We had over 170 attendees, about two-thirds in-person and the others distant by way of Zoom, formally representing 21 nations. Additionally, at every assembly we often have new attendees who’ve by no means attended earlier than, and this time there have been over a dozen new first-time attendees, principally in-person; to all of them, as soon as once more welcome!

The committee at present has 23 energetic subgroups, most of which met in parallel tracks all through the week. Some teams ran all week, and others ran for just a few days or part of a day and/or night, relying on their workloads. You will discover a short abstract of ISO procedures here.

This week’s assembly: Assembly #2 of C++26

On the earlier assembly in June, the committee adopted the primary 40 proposed adjustments for C++26, together with many who had been prepared for a few conferences and had been simply ready for the C++26 practice to open to be adopted. For these highlights, see the previous trip report.

This time, the committee adopted the following set of options for C++26. It additionally made important progress on different options that at the moment are anticipated to be full in time for C++26 — together with contracts and reflection.

Listed below are a number of the highlights…

Adopted for C++26: Core language adjustments/options

The core language adopted 4 papers, together with P2662R3 “Pack indexing” by Corentin Jabot and Pablo Halpern formally provides help for utilizing [idx] subscripting into variadic parameter packs. Right here is an instance from the paper that can now be authorized:

template <typename... T>
constexpr auto first_plus_last(T... values) -> T...[0] {
    return T...[0](values...[0] + values...[sizeof...(values)-1]);

int most important() {
    static_assert( first_plus_last(1, 2, 10) == 11 );

For these serious about writing requirements proposals, I might counsel this and its two predecessors P1858 and P2632 as effectively written papers: The sooner papers delve into the motivating use circumstances, and this paper has an in depth remedy of different design alternate options thought-about and why that is the one chosen. Seeing solely the top results of T...[0] could be straightforward to name “apparent” in hindsight, but it surely’s removed from the one choice and this paper’s evaluation exhibits a radical consideration of alternate options, together with their results on present and future code and future language evolution.

Adopted for C++26: Commonplace library adjustments/options

The usual library adopted 19 papers, together with the next…

The largest, and possibly this assembly’s award for “proposal being labored on the longest,” is P1673R13, “A free function linear algebra interface based on the BLAS” by Mark Hoemmen, Daisy Hollman, Christian Trott, Daniel Sunderland, Nevin Liber, Alicia Klinvex, Li-Ta Lo, Damien Lebrun-Grandie, Graham Lopez, Peter Caday, Sarah Knepper, Piotr Luszczek, and Timothy Costa, with the assistance of Bob Steagall, Man Davidson, Andrew Lumsdaine, and Davis Herring. If you wish to do environment friendly linear algebra, you don’t wish to write your individual code by hand; that might be sluggish. As an alternative, you need a library that’s tuned in your goal {hardware} structure and prepared for par_unseq vectorized algorithms, for blazing pace. That is that library. For detailed rationale, see specifically sections 5 “Why embody dense linear algebra within the C++ Commonplace Library?” and 6 “Why base a C++ linear algebra library on the BLAS?”

P2905R2 “Runtime format strings”  and P2918R2 “Runtime format strings II” by Victor Zverovich builds on the C++20 format library, which already supported compile-time format strings. Now with this pair of papers, we can have direct help for format strings not recognized at compile time and be capable to choose out of compile-time format string checks.

P2546R5 “Debugging support” by René Ferdinand Rivera Morell, constructing on prior work by Isabella Muerte in P1279, provides std::breakpoint(), std::breakpoint_if_debugging(), and std::is_debugger_present(). This standardizes prior artwork already out there in environments from Amazon Net Providers to Unreal Engine and extra, underneath a standard customary API that provides the programmer full runtime management over breakpoints, together with (quoting from the paper):

  • “permitting printing out additional output to assist diagnose issues,
  • executing additional take a look at code,
  • displaying an additional person interface to assist in debugging, …
  • … breaking when an rare non-critical situation is detected,
  • permitting programmatic management with complicated runtime delicate situations,
  • breaking on person enter to examine context in interactive packages without having to change to the debugger utility,
  • and extra.”

I can instantly consider occasions I might have used this up to now month, and possibly you possibly can too.

These are a number of the “greater” papers as highlights… there have been 16 papers different adopted too, together with extra extensions and fixes for the C++26 language and customary library.

On observe for concentrating on C++26: Contracts

The contracts subgroup, SG21, determined a number of long-open questions that wanted to be answered to land contracts in C++26. Maybe not crucial one, however the one which’s essentially the most seen, is the contracts syntax: This week, SG21 accredited pursuing P2961R0 “A natural syntax for contracts” by Jens Maurer and Timur Doumler because the syntax for C++26 contracts. The key seen change is that as an alternative of writing contracts like this:

// earlier draft syntax
int f(int i)
    [[pre: i >= 0]]
    [[post r: r > 0]]
    [[assert: i >= 0]]
    return i+1;

we’ll write them like this, altering “assert” to “contract_assert”… just about everybody would favor “assert,” if solely it had been backward-compatible, however on this new syntax it might hit an incompatibility with the C assert macro:

// newly adopted syntax
int f(int i)
    pre (i >= 0)
    submit (r: r > 0)
    contract_assert (i >= 0);
    return i+1;

I already had a contracts implementation in my cppfront compiler, which used the earlier [[ ]] syntax (as a result of, when I’ve nothing clearly higher, I attempt to observe syntax in present/proposed C++). So, as soon as P2961 was accredited within the subgroup on Tuesday morning, I made a decision to take Tuesday afternoon to implement the change to this syntax, besides that I stored the good phrase “assert” as a result of I can try this with no breaking change in my experimental alternate syntax. The work ended up taking not fairly an hour, together with to replace the repo’s personal code the place I’m utilizing contracts myself within the compiler and its unit exams. You may try the diff in these | commits. My preliminary private response, as an early contracts person, is that I just like the outcome.

There are a handful of design questions nonetheless to determine, notably the semantics of implicit lambda seize, consteval, and a number of declarations. Six contracts telecons have been scheduled between now and the following assembly in March in Tokyo. The group is aiming to have a feature-complete proposal for Tokyo to ahead to different teams for evaluation.

At present when this progress was reported to the total committee, there was applause. As there must be, as a result of this week’s progress will increase the arrogance that the function is on observe for C++26!

Notice that “for C++26” doesn’t imply “that’s nonetheless three years away, perhaps my youngsters can use it sometime.” It means the function must be completed in simply the following 18 months or so, and as soon as it’s completed that unleashes implementations to have the ability to confidently go implement it. It’s fairly attainable we may even see implementations out there sooner, as we do with different in style in-demand draft customary options.

Talking of main options that made nice progress this assembly to be confidently on observe for C++26…

On observe for concentrating on C++26: Reflection

The reflection subgroup, SG7, noticed two expertise experiences from folks actively utilizing the prototype implementation of P2996 by Lock3 Software program: P3010R0 “Using reflection to replace a metalanguage for generating JS bindings” by Dan Katz, and P2911R1 “Python bindings with value-based reflection” by Adam Lach and Jagrut Dave. As you possibly can see from the titles, these had been severe makes an attempt to check out reflection for main use circumstances. Each expertise experiences supported P2996R1, so…

The group then voted unanimously to undertake P2996R1 “Reflection for C++26” by Wyatt Childers, Peter Dimov, Barry Revzin, Andrew Sutton, Faisal Vali, and Daveed Vandevoorde and ahead it on to the principle Evolution and Library Evolution subgroups concentrating on C++26. This can be a “core” of static reflection that’s helpful sufficient to resolve many vital issues, whereas letting us additionally plan to proceed constructing on it additional post-C++26.

That is notably thrilling for me personally, as a result of we desperately want reflection in C++, and primarily based on this week’s progress now’s the primary time I’ve felt assured sufficient to say a goal ship car for this tremendous vital function.

Maybe the commonest instance of reflection is “enum to string”, so right here’s that instance:

template <typename E>
    requires std::is_enum_v<E>
constexpr std::string enum_to_string(E worth) {
    template for (constexpr auto e : std::meta::members_of(^E)) {
        if (worth == [:e:]) {
            return std::string(std::meta::name_of(e));
    return "<unnamed>";

Notice that the above makes use of a number of the new reflection syntax, however that is simply the implementation… the brand new syntax stays encapsulated there. The code that makes use of enum_to_string will get to not know something about reflection, and simply use the operate:

enum Shade { crimson, inexperienced, blue };
static_assert(enum_to_string(Shade::crimson) == "crimson");
static_assert(enum_to_string(Shade(42)) == "<unnamed>");

See the paper for far more element, together with extra about enum-to-string in part 2.6.

Including to the joy, Edison Design Group famous that they count on to have an experimental implementation out there on Godbolt Compiler Explorer by Christmas.

P2996 builds on the core of the unique Reflection TS, and primarily adjustments the “prime” and “backside” layers that we knew we’d doubtless change from the TS:

  • On the “prime” or programming mannequin layer, P2996 avoids having to do temp<late,meta<professional,gram>>::ming to make use of the API and lets us write one thing extra like odd C++ code as an alternative.
  • And on the “backside” implementation layer, it makes use of a value-based implementation which is extra environment friendly to implement.

This doesn’t imply the Reflection TS wasn’t helpful; it was instrumental. Progress so far would have been slower if we hadn’t been in a position to do the TS first, and we deeply respect all of the work that went into that, in addition to the brand new progress to maneuver ahead with P2996 because the reflection function concentrating on C++26.

After the unanimous approval vote to ahead this paper for C++26, there was a spherical of applause within the subgroup.

See Also

Then right this moment, when this progress towards concentrating on C++26 was reported to the entire committee within the closing plenary session, the entire room was crammed with sustained applause.

Different progress

Many different subgroups continued to make progress in the course of the week. Listed below are just a few highlights…

SG1 (Concurrency) will likely be engaged on out-of-thin-air points for relaxed atomics at a face-to-face assembly or telecon between conferences. They’re nonetheless on observe to maneuver ahead with std::execution and SIMD parallelism for C++26, and SIMD was reviewed within the Library Evolution (LEWG) most important subgroup; these options, within the phrases of the subgroup chair, will make C++26 an enormous launch for the concurrency and parallelism group.

SG4 (Networking) continued engaged on updating the networking proposal for std::execution senders and receivers. There’s a variety of work nonetheless to be executed and it isn’t clear on whether or not networking will likely be on observe for C++26.

SG9 (Ranges) set a listing of options and priorities for ranges for C++26. There are papers that want authors, together with ones that might be good “first papers” for brand new authors, so please attain out to the Ranges chair, Daisy Hollman, in case you are serious about contributing towards a Ranges paper.

SG15 (Tooling) thought-about papers on enhancing modules to allow higher tooling, and work towards the primary C++ Ecosystem customary.

SG23 (security) subgroup made additional progress in the direction of security profiles for C++ as proposed by Bjarne Stroustrup, and adopted it because the near-term path for security in C++. The up to date paper will likely be out there within the subsequent mailing in mid-December.

Library Evolution (LEWG) began setting a framework for insurance policies for brand new C++ libraries. The group additionally made progress on a variety of proposals concentrating on C++26, together with std::hive, SIMD (vector unit parallelism), ranges extensions, and std::execution, and probably some help for bodily items, all of which made good progress.

Language Evolution (EWG) labored on enhancing/forwarding/rejecting many proposals, together with a set of discussions about enhancing undefined habits along with the C committee, together with eight papers about undefined habits within the preprocessor. The group additionally determined to pursue doing a full audit of “ill-formed, no diagnostic required” undefined habits that compilers at present will not be required to detect and diagnose. The plan for our subsequent assembly in Tokyo is to spend so much of time on reflection, and put together for specializing in contracts.

Thanks to all of the consultants who labored all week in all of the subgroups to attain a lot this week!

What’s subsequent

Our subsequent assembly will likely be in Tokyo, Japan in March hosted by Woven by Toyota.

Wrapping up

Thanks once more to the over 170 consultants who attended on-site and on-line at this week’s assembly, and the numerous extra who take part in standardization by means of their nationwide our bodies!

However we’re not slowing down… we’ll proceed to have subgroup Zoom conferences, after which in simply 4 months from now we’ll be assembly once more in particular person + Zoom to proceed including options to C++26. Thanks once more to everybody studying this in your curiosity and help for C++ and its standardization.

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