Moments ago, the ISO C++ committee completed its third meeting of C++26, held in Tokyo, Japan. Our hosts, Woven by Toyota, arranged for high-quality facilities for our six-day meeting from Monday through Saturday. We had over 220 attendees, about two-thirds in-person and the others remote via Zoom, formally representing 21 nations. That makes it roughly tied numerically for our largest meeting ever, roughly the same attendance as Prague 2020 that shipped C++20 just a few weeks before the pandemic lockdowns. — However, note that it’s not an apples-to-apples comparison, because the pre-pandemic meetings were all in-person, and since the pandemic they have been hybrid. But it is indicative of the ongoing strong participation in C++ standardization.

At each meeting we regularly have new attendees who have never attended before, and this time we welcomed over 30 new first-time attendees, mostly in-person, who were counted above. (These numbers are for technical participants, and they don’t include that we also had observers, including a class of local high school students visiting for part of a day, similarly to how a different local high school class did at our previous meeting in Kona in November. We are regularly getting high-school delegations as observers these days, and to them once again welcome!)

The committee currently has 23 active subgroups, 16 of which met in parallel tracks throughout the week. Some groups ran all week, and others ran for a few days or a part of a day and/or evening, depending on their workloads. You can find a brief summary of ISO procedures here.

This week’s meeting: Meeting #3 of C++26

At the previous two meetings in June and November, the committee adopted the first 63 proposed changes for C++26, including many that had been ready for a couple of meetings while we were finishing C++23 and were just waiting for the C++26 train to open to be adopted. For those highlights, see the June trip report and November trip report.

This time, the committee adopted the next set of features for C++26, and made significant progress on other features that are now expected to be complete in time for C+26.

Here are some of the highlights… note that these links are to the most recent public version of each paper, and some were tweaked at the meeting before being approved; the links track and will automatically find the updated version as soon as it’s uploaded to the public site.

Adopted for C++26: Core language changes/features

The core language adopted 10 papers, including the following…

P2573R2 “=delete(“should have a reason”)” by Yihe Li does the same for =delete as we did for static_assert: It allows writing a string as the reason, which makes it easier for library developers to give high-quality compile-time error messages to users as part of the compiler’s own error message output. Thanks, Yihe!

Here is an example from the paper that will now be legal and generate an error message similar to the one shown here:

class NonCopyable
{
public:
    // ...
    NonCopyable() = default;

    // copy members
    NonCopyable(const NonCopyable&)
        = delete("Since this class manages unique resources, \
copy is not supported; use move instead.");
    NonCopyable& operator=(const NonCopyable&)
        = delete("Since this class manages unique resources, \
copy is not supported; use move instead.");
    // provide move members instead
};

<source>:16:17: error: call to deleted
constructor of 'NonCopyable': Since this class manages unique resources, copy is not supported; use move instead.
    NonCopyable nc2 = nc;
                ^     ~~

P2795R5 “Erroneous behavior for uninitialized reads” by Thomas Köppe is a major change to C++ that will help us to further improve safety by providing a tool to reduce undefined behavior, especially that it removes undefined behavior for some cases of uninitialized objects.

I can’t do better than quote from the paper:

Summary: We propose to address the safety problems of reading a default-initialized automatic variable (an “uninitialized read”) by adding a novel kind of behaviour for C++. This new behaviour, called erroneous behaviour, allows us to formally speak about “buggy” (or “incorrect”) code, that is, code that does not mean what it should mean (in a sense we will discuss). This behaviour is both “wrong” in the sense of indicating a programming bug, and also well-defined in the sense of not posing a safety risk.

…

With increased community interest in safety, and a growing track record of exploited vulnerabilities stemming from errors such as this one, there have been calls to fix C++. The recent P2723R1 proposes to make this fix by changing the undefined behaviour into well-defined behaviour, and specifically to well-define the initialization to be zero. We will argue below that such an expansion of well-defined behaviour would be a great detriment to the understandability of C++ code. In fact, if we want to both preserve the expressiveness of C++ and also fix the safety problems, we need a novel kind of behaviour.

…

Reading an uninitialized value is never intended and a definitive sign that the code is not written correctly and needs to be fixed. At the same time, we do give this code well-defined behaviour, and if the situation has not been diagnosed, we want the program to be stable and predictable. This is what we call erroneous behaviour.

In other words, it is still an “wrong” to read an uninitialized value, but if you do read it and the implementation does not otherwise stop you, you get some specific value. In general, implementations must exhibit the defined behaviour, at least up until a diagnostic is issued (if ever). There is no risk of running into the consequences associated with undefined behaviour (e.g. executing instructions not reflected in the source code, time-travel optimisations) when executing erroneous behaviour.

Adding the notion of “erroneous behavior” is a major change to C++’s specification, that can help not only with uninitialized reads but also can be applied to reduce other undefined behavior in the future. Thanks, Thomas!

Adopted for C++26: Standard library changes/features

The standard library adopted 18 papers, including the following…

In the “Moar Ranges!” department, P1068R11 “Vector API for random number generation” by Ilya Burylov, Pavel Dyakov, Ruslan Arutyunyan, Andrey Nikolaev, and Alina Elizarova addresses the situation that, when you want one random number, you likely want more of them, and random number generators usually already generate them efficiently in batches. Thanks to their paper, this will now work:

std::array<std::uint_fast32_t, arrayLength> intArray;
std::mt19937 g(777);

std::ranges::generate_random(intArray, g);

// The above line will be equivalent to this:
for(auto& e : intArray)
    e = g();

In the “if you still didn’t get enough ‘Moar Ranges!’” department, P2542 “views::concat” by Hui Xie and S. Levent Yilmaz provides an easy way to efficiently concatenate an arbitrary number of ranges, via a view factory. Thanks, Hui and Levent! Here is an example from the paper:

std::vector<int> v1{1,2,3}, v2{4,5}, v3{};
std::array  a{6,7,8};
auto s = std::views::single(9);

std::print("{}\n", std::views::concat(v1, v2, v3, a, s)); 
// output:  [1, 2, 3, 4, 5, 6, 7, 8, 9]

Speaking of concatenation, have you ever wished you could write “my_string_view + my_string” and been surprised it doesn’t work? I sure have. No longer: P2591R4 “Concatenation of strings and string views” by Giuseppe D’Angelo adds operator+ overloads for those types. Thanks, Giuseppe, for finally getting us this feature!

P2845 “Formatting of std::filesystem::path” by Victor Zverovich (aka the King of Format) provides a high-quality std::format formatter for filesystem paths that addresses concerns about quoting and localization.

A group of papers by Alisdair Meredith removed some (mostly already-deprecated) features from the standard library. Thanks for the cleanup, Alisdair!

• P2867R2 “Remove Deprecated strstreams From C++26”
• P2869R4 “Remove Deprecated shared_ptr Atomic Access APIs from C++26”
• P2872R3 “Remove wstring_convert From C++26”

P3142R0 “Printing Blank Lines with println” by Alan Talbot is small but a nice quality-of-life improvement: We can now write just println() as an equivalent of println(“”). But that’s not all: See the yummy postscript in the paper. (See, Alan, we do read the whole paper. Thanks!)

Those are some of the “bigger” or “likely of wide interest” papers as just a few highlights… this week there were 28 papers adopted in all, including other great work on extensions and fixes for the C++26 language and standard library.

Aiming for C++26 timeframe: Contracts

The contracts proposal P2900 “Contracts for C++” by Joshua Berne, Timur Doumler, Andrzej Krzemieński, Gašper Ažman, Tom Honermann, Lisa Lippincott, Jens Maurer, Jason Merrill, and Ville Voutilainen progressed out of the contracts study group, SG21, and was seen for the first time in the Language (EWG) and Library (LEWG) Evolution Working Groups proper. Sessions started in LEWG right on the first day, Monday afternoon, and EWG spent the entire day Wednesday on contracts, with many of the members of the safety study group, SG23, attending the session. There was lively discussion about whether contracts should be allowed to have, or be affected by, undefined behavior; whether contracts should be used in the standard library; whether contracts should be shipped first as a Technical Specification (TS, feature branch) in the same timeframe as C++26 to gain more experience with existing libraries; and other aspects… all of these questions will be discussed again in the coming months, this was just the initial LEWG and EWG full-group design review that generated feedback to be looked at. The subgroups considered the EWG and LEWG groups’ feedback later in the week in two more sessions on Thursday and Friday, including in a joint session of SG21 and SG23.

Both SG21 and SG23 will have telecons about further improving the contracts proposal between now and our next hybrid meeting in June.

On track for targeting C++26: Reflection

The reflection proposal P2996R2 “Reflection for C++26” by Wyatt Childers, Peter Dimov, Barry Revzin, Andrew Sutton, Faisal Vali, and Daveed Vandevoorde progressed out of the reflection / compile-time programming study group, SG7, and was seen by the main evolution groups EWG and LEWG for the first time on Tuesday, which started with a joint EWG+LEWG session on Tuesday, and EWG spent the bulk of Tuesday on its initial large-group review aiming for C++26. Then SG7 continued on reflection and other topics, including a presentation by Andrei Alexandrescu about making sure reflection adds a few more small things to fully support flexible generative programming.

Other progress

All subgroups continued progress. A lot happened that other trips will no doubt cover, but I’ll call out two things.

One proposal a lot of people are watching is P2300 “std::execution” (aka “executors”) by Michał Dominiak, Georgy Evtushenko, Lewis Baker, Lucian Radu Teodorescu, Lee Howes, Kirk Shoop, Michael Garland, Eric Niebler, and Bryce Adelstein Lelbach, which was already design-approved for C++26. It’s a huge paper (45,000 words, of which 20,000 words is the standardese specification! that’s literally a book… my first Exceptional C++ book was 62,000 words), so it has been taking time for the Library Wording subgroup (LWG) to do its detailed review of the specification wording, and at this meeting LWG spent a quarter of the meeting completing a first pass through the entire paper! They will continue to work in teleconferences on a second pass and are now mildly optimistic of completing P2300 wording review at our next meeting in June.

And one more fun highlight: We all probably suspected that pattern matching is a “ground-shaking” proposed addition to future C++, but in Tokyo during the pattern matching session there was a literal earthquake that briefly interrupted the session!

Thank you to all the experts who worked all week in all the subgroups to achieve so much this week!

What’s next

Our next meeting will be in St. Louis, MO, USA in June hosted by Bill Seymour.

Wrapping up

Thank you again to the over 210 experts who attended on-site and on-line at this week’s meeting, and the many more who participate in standardization through their national bodies!

But we’re not slowing down… we’ll continue to have subgroup Zoom meetings, and then in just three months from now we’ll be meeting again in person + Zoom to continue adding features to C++26. Thank you again to everyone reading this for your interest and support for C++ and its standardization.

I generally give one or two courses a year on C++ and related technologies. This year, on April 22-25, I’ll be giving a live online public course for four half-days, on the topic of high-performance low-latency coding in C++ — and the early registration discount is available for a few more days until this Thursday:

Effective Concurrency with Herb Sutter

High performance and low latency code, via concurrency and parallelism

22-25th April 2024, from 14:00 – 18:00 CEST daily

Participants in this intensive course will acquire the knowledge and skills required to write high-performance and low-latency code on today’s modern systems using modern C++. Presented by Alfasoft.

See the course link for details and a syllabus of topics that will be covered.

The times are intended to be friendly to the home time zones of attendees anywhere in EMEA and also to early risers in the Americas. If you live in a part of the world where these times can’t work for you, and you’d like another offering of the course that is friendlier to your home time zone, please email Alfasoft to let them know!

Because “high-performance low-latency” is kind of C++’s bailiwick, and because it’s my course, you’ll be unsurprised to learn that the topics and code focus on C++ and include coverage of modern C++17/20/23 features. But we are polyglots, after all… so don’t be overly shocked that I may sometimes show a few code examples in other popular languages, if only for comparison and to show how the other half lives.

Today, the ISO C++ committee completed its second meeting of C++26, held in Kona, HI, USA.

Our hosts, Standard C++ Foundation and WorldQuant, arranged for high-quality facilities for our six-day meeting from Monday through Saturday. We had over 170 attendees, about two-thirds in-person and the others remote via Zoom, formally representing 21 nations. Also, at each meeting we regularly have new attendees who have never attended before, and this time there were over a dozen new first-time attendees, mostly in-person; to all of them, once again welcome!

The committee currently has 23 active subgroups, most of which met in parallel tracks throughout the week. Some groups ran all week, and others ran for a few days or a part of a day and/or evening, depending on their workloads. You can find a brief summary of ISO procedures here.

This week’s meeting: Meeting #2 of C++26

At the previous meeting in June, the committee adopted the first 40 proposed changes for C++26, including many that had been ready for a couple of meetings and were just waiting for the C++26 train to open to be adopted. For those highlights, see the previous trip report.

This time, the committee adopted the next set of features for C++26. It also made significant progress on other features that are now expected to be complete in time for C++26 — including contracts and reflection.

Here are some of the highlights…

Adopted for C++26: Core language changes/features

The core language adopted four papers, including P2662R3 “Pack indexing” by Corentin Jabot and Pablo Halpern officially adds support for using [idx] subscripting into variadic parameter packs. Here is an example from the paper that will now be legal:

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

int main() {
    static_assert( first_plus_last(1, 2, 10) == 11 );
}

For those interested in writing standards proposals, I would suggest looking at this and its two predecessors P1858 and P2632 as well written papers: The earlier papers delve into the motivating use cases, and this paper has a detailed treatment of other design alternatives considered and why this is the one chosen. Seeing only the end result of T...[0] would be easy to call “obvious” in hindsight, but it’s far from the only option and this paper’s analysis shows a thorough consideration of alternatives, including their effects on existing and future code and future language evolution.

Adopted for C++26: Standard library changes/features

The standard library adopted 19 papers, including the following…

The biggest, and probably this meeting’s award for “proposal being worked 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 help of Bob Steagall, Guy Davidson, Andrew Lumsdaine, and Davis Herring. If you want to do efficient linear algebra, you don’t want to write your own code by hand; that would be slow. Instead, you want a library that is tuned for your target hardware architecture and ready for par_unseq vectorized algorithms, for blazing speed. This is that library. For detailed rationale, see in particular sections 5 “Why include dense linear algebra in the C++ Standard 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 will have direct support for format strings not known at compile time and be able to opt out of compile-time format string checks.

P2546R5 “Debugging support” by René Ferdinand Rivera Morell, building on prior work by Isabella Muerte in P1279, adds std::breakpoint(), std::breakpoint_if_debugging(), and std::is_debugger_present(). This standardizes prior art already available in environments from Amazon Web Services to Unreal Engine and more, under a common standard API that gives the programmer full runtime control over breakpoints, including (quoting from the paper):

• “allowing printing out extra output to help diagnose problems,
• executing extra test code,
• displaying an extra user interface to help in debugging, …
• … breaking when an infrequent non-critical condition is detected,
• allowing programmatic control with complex runtime sensitive conditions,
• breaking on user input to inspect context in interactive programs without needing to switch to the debugger application,
• and more.”

I can immediately think of times I would have used this in the past month, and probably you can too.

Those are some of the “bigger” papers as highlights… there were 16 papers other adopted too, including more extensions and fixes for the C++26 language and standard library.

On track for targeting C++26: Contracts

The contracts subgroup, SG21, decided several long-open questions that needed to be answered to land contracts in C++26. Perhaps not the most important one, but the one that’s the most visible, is the contracts syntax: This week, SG21 approved pursuing P2961R2 “A natural syntax for contracts” by Jens Maurer and Timur Doumler as the syntax for C++26 contracts. The major visible change is that instead of writing contracts like this:

// previous 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, changing “assert” to “contract_assert”… pretty much everyone would prefer “assert,” if only it were backward-compatible, but in this new syntax it would hit an incompatibility with the C assert macro:

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

I already had a contracts implementation in my cppfront compiler, which used the previous [[ ]] syntax (because, when I have nothing clearly better, I try to follow syntax in existing/proposed C++). So, once P2961 was approved in the subgroup on Tuesday morning, I decided to take Tuesday afternoon to implement the change to this syntax, except that I kept the nice word “assert” because I can do that without a breaking change in my experimental alternate syntax. The work ended up taking not quite an hour, including to update the repo’s own code where I’m using contracts myself in the compiler and its unit tests. You can check out the diff in these | commits. My initial personal reaction, as an early contracts user, is that I like the result.

There are a handful of design questions still to decide, notably the semantics of implicit lambda capture, consteval, and multiple declarations. Six contracts telecons have been scheduled between now and the next meeting in March in Tokyo. The group is aiming to have a feature-complete proposal for Tokyo to forward to other groups for review.

Today when this progress was reported to the full committee, there was applause. As there should be, because this week’s progress increases the confidence that the feature is on track for C++26!

Note that “for C++26” doesn’t mean “that’s still three years away, maybe my kids can use it someday.” It means the feature has to be finished in just the next 18 months or so, and once it’s finished that unleashes implementations to be able to confidently go implement it. It’s quite possible we may see implementations available sooner, as we do with other popular in-demand draft standard features.

Speaking of major features that made great progress this meeting to be confidently on track for C++26…

On track for targeting C++26: Reflection

The reflection subgroup, SG7, saw two experience reports from people actively using the prototype implementation of P2996 by Lock3 Software: 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 can see from the titles, these were serious attempts to try out reflection for major use cases. Both experience reports supported P2996R1, so…

The group then voted unanimously to adopt P2996R1 “Reflection for C++26” by Wyatt Childers, Peter Dimov, Barry Revzin, Andrew Sutton, Faisal Vali, and Daveed Vandevoorde and forward it on to the main Evolution and Library Evolution subgroups targeting C++26. This is a “core” of static reflection that is useful enough to solve many important problems, while letting us also plan to continue building on it further post-C++26.

This is particularly exciting for me personally, because we desperately need reflection in C++, and based on this week’s progress now is the first time I’ve felt confident enough to mention a target ship vehicle for this super important feature.

Perhaps the most common example of reflection is “enum to string”, so here’s that example:

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

Note that the above uses some of the new reflection syntax, but this is just the implementation… the new syntax stays encapsulated there. The code that uses enum_to_string gets to not know anything about reflection, and just use the function:

enum Color { red, green, blue };
static_assert(enum_to_string(Color::red) == "red");
static_assert(enum_to_string(Color(42)) == "<unnamed>");

See the paper for much more detail, including more about enum-to-string in section 2.6.

Adding to the excitement, Edison Design Group noted that they expect to have an experimental implementation available on Godbolt Compiler Explorer by Christmas.

P2996 builds on the core of the original Reflection TS, and mainly changes the “top” and “bottom” layers that we knew we would likely change from the TS:

• At the “top” or programming model layer, P2996 avoids having to do temp<late,meta<pro,gram>>::ming to use the API and lets us write something more like ordinary C++ code instead.
• And at the “bottom” implementation layer, it uses a value-based implementation which is more efficient to implement.

This doesn’t mean the Reflection TS wasn’t useful; it was instrumental. Progress to this point would have been slower if we hadn’t been able to do the TS first, and we deeply appreciate all the work that went into that, as well as the new progress to move forward with P2996 as the reflection feature targeting C++26.

After the unanimous approval vote to forward this paper for C++26, there was a round of applause in the subgroup.

Then today, when this progress toward targeting C++26 was reported to the whole committee in the closing plenary session, the whole room was filled with sustained applause.

Other progress

Many other subgroups continued to make progress during the week. Here are a few highlights…

SG1 (Concurrency) will be working on out-of-thin-air issues for relaxed atomics at a face-to-face meeting or telecon between meetings. They are still on track to move forward with std::execution and SIMD parallelism for C++26, and SIMD was reviewed in the Library Evolution (LEWG) main subgroup; these features, in the words of the subgroup chair, will make C++26 a huge release for the concurrency and parallelism group.

SG4 (Networking) continued working on updating the networking proposal for std::execution senders and receivers. There is a lot of work still to be done and it is not clear on whether networking will be on track for C++26.

SG9 (Ranges) set a list of features and priorities for ranges for C++26. There are papers that need authors, including ones that would be good “first papers” for new authors, so please reach out to the Ranges chair, Daisy Hollman, if you are interested in contributing toward a Ranges paper.

SG15 (Tooling) considered papers on improving modules to enable better tooling, and work toward the first C++ Ecosystem standard.

SG23 (safety) subgroup made further progress towards safety profiles for C++ as proposed by Bjarne Stroustrup, and adopted it as the near-term direction for safety in C++. The updated paper will be available in the next mailing in mid-December.

Library Evolution (LEWG) started setting a framework for policies for new C++ libraries. The group also made progress on a number of proposals targeting C++26, including std::hive, SIMD (vector unit parallelism), ranges extensions, and std::execution, and possibly some support for physical units, all of which made good progress.

Language Evolution (EWG) worked on improving/forwarding/rejecting many proposals, including a set of discussions about improving undefined behavior in conjunction with the C committee, including eight papers about undefined behavior in the preprocessor. The group also decided to pursue doing a full audit of “ill-formed, no diagnostic required” undefined behavior that compilers currently are not required to detect and diagnose. The plan for our next meeting in Tokyo is to spend a lot of time on reflection, and prepare for focusing on contracts.

Thank you to all the experts who worked all week in all the subgroups to achieve so much this week!

What’s next

Our next meeting will be in Tokyo, Japan in March hosted by Woven by Toyota.

Wrapping up

Thank you again to the over 170 experts who attended on-site and on-line at this week’s meeting, and the many more who participate in standardization through their national bodies!

But we’re not slowing down… we’ll continue to have subgroup Zoom meetings, and then in just four months from now we’ll be meeting again in person + Zoom to continue adding features to C++26. Thank you again to everyone reading this for your interest and support for C++ and its standardization.

My Thursday CppCon talk is now online.

Note: There’s already a Reddit thread for it, so if you want to comment on the video I suggest you use that thread instead of creating a new one.

At CppCon 2022, I argued for why we should try to make C++ 10x simpler and 50x safer, and this update is an evolution of the update talk I gave at C++ Now in May, with additional news and demos.

The “Dart plan” and the “TypeScript plan”

The back half of this talk clearly distinguishes between what I call the “Dart plan” and the “TypeScript plan” for aiming at a 10x improvement for an incumbent popular language. Both plans have value, but they have different priorities and therefore choose different constraints… most of all, they either embrace up-front the design constraint of perfect C++ interop and ecosystem compatibility, or they forgo it (forever; as I argue in the talk, it can never be achieved retroactively, except by starting over, because it’s a fundamental up-front constraint). No one else has tried the TypeScript plan for C++ yet, and I see value in trying it, and so that’s the plan I’m following for cppfront.

When people ask me “how is cppfront different from all the other projects trying to improve/replace C++?” my answer is “cppfront is on the TypeScript plan.” All the other past and present projects have been on the Dart plan, which again is a fine plan too, it just has different priorities and tradeoffs particularly around

• full seamless interop compatibility with ISO Standard C++ code and libraries without any wrapping/thunking/marshaling,
• full ecosystem compatibility with all of today’s C++ compilers, IDEs, build systems, and tooling, and
• full standards evolution support with ISO C++, including not creating incompatible features (e.g., a different concepts feature than C++20’s, a different modules system than C++20’s) and bringing all major new pieces to today’s ISO C++ evolution as also incremental proposals for today’s C++.

See just the final 10 minutes of the talk to see what I mean — I demo’d syntax 2 “just working” with four different IDEs’ debuggers and visualizers, but I could also have demo’d that profilers just work, build systems just work, and so on.

I call my experimental syntax 2 (aka Cpp2) “still 100% pure C++” not only because cppfront translates it to 100% today’s C++ syntax (aka Cpp1), but because:

• every syntax-2 type is an ordinary C++ type that ordinary existing C++ code can use, recognized by tools that know C++ types including IDE visualizers;
• every syntax-2 function is an ordinary C++ function that ordinary existing C++ code can use, recognized by tools that know C++ functions including debuggers to step into them;
• every syntax-2 object is an ordinary C++ object that ordinary existing C++ code can use;
• every syntax-2 feature can be (and has been) brought as a normal ISO C++ standards proposal to evolve today’s syntax, because Cpp2 embraces and follows today’s C++ standard and guidance and evolution instead of competing with them;

and that’s because I want a way to keep writing 100% pure C++, just nicer.

“Nicer” means: 10x simpler by having more generality and consistency, better defaults, and less ceremony; and 50x safer by having 98% fewer vulnerabilities in the four areas of initialization safety (guaranteed in Cpp2), type safety (guaranteed in Cpp2), bounds safety (on by default in Cpp2), and lifetime safety (still to be implemented in cppfront is the C++ Core Guidelines Lifetime static analysis which I designed for Cpp2).

Cpp2 and cppfront don’t replace your C++ compilers. Cpp2 and cppfront work with all your existing C++ compilers (and build systems, profilers, debuggers, visualizers, custom in-house tools, test harnesses, and everything else in the established C++ ecosystem, from the big commercial public C++ products to your team’s internal bespoke C++ tools). If you’re already using GCC, Clang, and/or MSVC, keep using them, they all work fine. If you’re already using CMake or build2, or lldb or the Qt Creator debugger, or your favorite profiler or test framework, keep using them, it’s all still C++ that all C++ tools can understand. There’s no new ecosystem.

There are only two plans for 10x major improvement. (1-min clip) This is the fundamental difference with all the other attempts at a major improvement of today’s C++ I know of, which are all on the Dart plan — and those are great projects by really smart people and I hope we all learn from each other. But for my work I want to pursue the TypeScript plan, which I think is the only major evolution plan that can legitimately call itself “still 100% C++.” That’s important to me, because like I said at the very beginning of my talk last year (1-min clip), I want to encourage us to pursue major evolution that brings C++ itself forward and to double down on C++, not switch to something else — to aim for major C++ evolution directed to things that will make us better C++ programmers, not programmers of something else.

I’m spending time on this experiment first of all for myself, because C++ is the language that best lets me express the programs I need to write, so I want to keep writing real C++ types and real C++ functions and real C++ everything else… just nicer.

Thanks again to the over 120 people who have contributed issues and PRs to cppfront, and the many more who have provided thoughtful comments and feedback! I appreciate your help.

Since the 2022-12-31 year-end mini-update and the 2023-04-30 spring update, progress has continued on cppfront. (If you don’t know what this personal project is, please see the CppCon 2022 talk on YouTube for an overview, and the CppNow 2023 talk on YouTube for an interim update.)

I’ll be giving a major update next week at CppCon. I hope to see many of you there! In the meantime, here are some notes about what’s been happening since the spring update post, including:

• Acknowledgments and thanks
• Started self-hosting
• No data left behind: Mandatory explicit discard
• requires clauses
• Generalized aliases+constexpr with ==
• Safe enum and flag_enum metafunctions
• Safe union metafunction
• What’s next

Acknowledgments: Thank you!

Thank you to all these folks who have participated in the cppfront repo by opening issues and PRs, and to many more who participated on PR reviews and comment threads! These contributors represent people from high school and undergrad students to full professors, from commercial developers to conference speakers, and from every continent except Antarctica.

Started self-hosting

I haven’t spent a lot of time yet converting cppfront’s own code from today’s syntax 1 to my alternate syntax 2 (which I’m calling “Cpp1” and “Cpp2” for short), but I started with all of cppfront’s reflection API and metafunctions which are now mostly written in Cpp2. Here’s what that reflect.h2 file compilation looks like when compiled on the command line on my laptop:

But note you can still build cppfront as all-today’s-C++ using any fairly recent C++20 compiler because I distribute the sources also as C++ (just as Bjarne distributed the cfront sources also as C).

No data left behind: Mandatory explicit discard

Initialization and data flow are fundamental to safe code, so from the outset I ensured that syntax 2 guaranteed initialization-before use, I made all converting constructors explicit by default… and I made [[nodiscard]] the default for return values (1-min talk clip).

The more I thought about [[nodiscard]], the more determined I was that data must never be silently lost, and data-lossy operations should be explicit. So I’ve decided to try an aggressive experiment:

• make “nodiscard” the law of the land, implicitly required all the time, with no opt-out…
• including when calling existing C++ libraries (including std::) that were never designed for their return values to be treated as [[nodiscard]]!

Now, I wasn’t totally crazy: See the Design note: Explicit discard for details on how I first surveyed other languages’ designers about experience in their languages — notably C#, F#, and Python. In particular, F# does the same thing with .NET APIs — F# requires explicit |> ignore to discard unused return values, including for .NET APIs that were never designed for that and were largely written in other languages. Don Syme told me it has not been a significant pain point, and that was encouraging, so I’m following suit.

My experience so far is that it’s pretty painless, and I write about one explicit discard for every 200 lines of code, even when using the C++ standard library (which cppfront does pervasively, because the C++ standard library is the only library cppfront uses). And, so far, every time cppfront told me I had to write an explicit discard, I learned something useful (e.g., before this I never realized that emplace_back started to return something since C++17! push_back still doesn’t) and I found I liked that my code explicitly self-documented it was not looking at output values… my code looked better.

The way to do an explicit discard is to assign the result to the “don’t care” wildcard. It’s unobtrusive, but explicit and clear:

 _ = vec.emplace_back(1,2,3);

Now all Cpp2-authored C++ functions are emitted as [[nodiscard]], except only for assignment and streaming operators because those are designed for chaining and every chain always ends with a discarded return.

And the whole language hangs together well: Explicit discard works very naturally with inout and out parameters too, not just return values. If you have a local variable x and pass it to an inout parameter, what if that’s the last use of the variable?

{
    x := my_vector.begin();
    std::advance(x, 2);
        // ERROR, if param is Cpp2 'inout' or Cpp1 non-const '&'
}

In this example, that call to std::advance(x, 2); is a definite last use of x, and so Cpp2 will automatically pass x as an rvalue and make it a move candidate… and presto! the call won’t compile because you can’t pass an rvalue to a Cpp2 inout parameter (the same as a Cpp1 non-const-& parameter, so this correctly detects the output side effects also when calling existing C++ functions that take references to non-const). That’s a feature, not a bug, because if that’s the last use of x that means the function is not looking at x again, so it’s ignoring the “out” value of the std::advance(x, 2) function call, which is exactly like ignoring a return value. And the guidance is the same: If you really meant to do that, just explicitly discard x‘s final value:

{
    x := my_vector.begin();
    std::advance(x, 2);
    _ = x; // all right, you said you meant it, carry on then...
}

Adding _ = x; afterward naturally makes that the last use of x instead. Problem solved, and it self-documents that the code really meant to ignore a function’s output value.

I really, really like how my C++ code’s data flow is explicit, and fully protected and safe, in syntax 2. And I’m very pleased to see how it just works naturally throughout the language — from universal guaranteed initialization, to explicit constructors by default, to banning implicitly discarding any values, to uniform treatment of returned values whether returned by return value or the “out” part of inout and out parameters, and all of it working also with existing C++ libraries so they’re safer and nicer to use from syntax 2. Data is always initialized, data is never silently lost, data flow is always visible. Data is precious, and it’s always safe. This feels right and proper to me.

requires clauses

I also added support for requires clauses, so now you can write those on all templates. The cppfront implementation was already generating some requires clauses already (see this 1-min video clip). Now programmers can write their own too.

This required a bit of fighting with a GCC 10 bug about requires-clauses on declarations, that was fixed in GCC 11+ but was never backported. But because this was the only problem I’ve encountered with GCC 10 that I couldn’t paper over, and because I could give a clear diagnostic that a few features in Cpp2 that rely on requires clauses aren’t supported on GCC 10, so far I’ve been able to retain GCC 10 as a supported compiler and emit diagnostics if you try to use those few features it doesn’t support. GCC 11 and higher are all fine and support all Cpp2 semantics.

Generalized aliases+constexpr with ==

In the April blog post, I mentioned I needed a way to write type and namespace aliases, and that because all Cpp2 declarations are of the form thing : type = value, I decided to try using the same syntax but with == to denote “always equal to.”

// namespace alias
lit: namespace == ::std::literals;

// type alias
pmr_vec: <T> type
    == std::vector<T, std::pmr::polymorphic_allocator<T>>;

I think this clearly denotes that lit is always the same as ::std::literals, and pmr_vec<int> is always the same as std::vector<int, std::pmr::polymorphic_allocator<int>>.

Since then, I’ve thought about how this should be best extended to functions and objects, and I realized the requirements seem to overlap with something else I needed to support: constexpr functions and objects. Which, after all, are functions/objects that return/have “always the same values” known at compile time…

// function with "always the same value" (constexpr function)
increment: (value: int) -> int == value+1;
    // Cpp2 lets you omit { return } around 1-line bodies

// object with "always the same value" (constexpr object)
forty_two: i64 == 42;

I particularly needed these in order to write the enum metafunctions…

Safe enum and flag_enum metafunctions

In the spring update blog post, I described the first 10 working compile-time metafunctions I implemented in cppfront, from the set of metafunctions I described in my ISO C++ paper P0707. Since then, I’ve also implemented enum and union.

The most important thing about metafunctions is that they are compile-time library code that uses the reflection and code generation API, that lets the author of an ordinary C++ class type easily opt into a named set of defaults, requirements, and generated contents. This approach is essential to making the language simpler, because it lets us avoid hardwiring special “extra” types into the language and compiler.

In Cpp2, there’s no enum feature hardwired into the language. Instead you write an ordinary class type and just apply the enum metafunction:

// skat_game is declaratively a safe enumeration type: it has
// default/copy/move construction/assignment and <=> with 
// std::strong_ordering, a minimal-size signed underlying type
// by default if the user didn't specify a type, no implicit
// conversion to/from the underlying type, in fact no public
// construction except copy construction so that it can never
// have a value different from its listed enumerators, inline
// constexpr enumerators with values that automatically start
// at 1 and increment by 1 if the user didn't write their own
// value, and conveniences like to_string()... the word "enum"
// carries all that meaning as a convenient and readable
// opt-in, without hardwiring "enum" specially into the language
//
skat_game: @enum<i16> type = {
    diamonds := 9;
    hearts;  // 10
    spades;  // 11
    clubs;   // 12
    grand    := 20;
    null     := 23;
}

Consider hearts: It’s a member object declaration, but it doesn’t have a type (or a default value) which is normally illegal, but it’s okay because the @enum<i16> metafunction fills them in: It iterates over all the data members and gives each one the underlying type (here explicitly specified as i16, otherwise it would be computed as the smallest signed type that’s big enough), and an initializer (by default one higher than the previous enumerator).

Why have this metafunction on an ordinary C++ class, when C++ already has both C’s enum and C++11’s enum class? Because:

• it keeps the language smaller and simpler, because it doesn’t hardwire special-purpose divergent splinter types into the language and compiler
  • (cue Beatles, and: “all you need is class (wa-wa, wa-wa-wa), all you need is class (wa-wa, wa-wa-wa)”);
• it’s a better enum than C enum, because C enum is unscoped and not as strongly typed (it implicitly converts to the underlying type); and
• it’s a better enum class than C++11 enum class, because it’s more flexible…

… consider: Because an enumeration type is now “just a type,” it just naturally can also have member functions and other things that are not possible for Cpp1 enums and enum classes (see this StackOverflow question):

janus: @enum type = {
    past;
    future;

    flip: (inout this) == {
        if this == past { this = future; }
        else { this = past; }
    }
}

There’s also a flag_enum variation with power-of-two semantics and an unsigned underlying type:

// file_attributes is declaratively a safe flag enum type:
// same as enum, but with a minimal-size unsigned underlying
// type by default, and values that automatically start at 1
// and rise by powers of two if the user didn't write their 
// own value, and bitwise operations plus .has(flags), 
// .set(flags), and .clear(flags)... the word "flag_enum"
// carries all that meaning as a convenient and readable
// opt-in without hardwiring "[Flags]" specially into the
// language
//
file_attributes: @flag_enum<u8> type = {
    cached;     // 1
    current;    // 2
    obsolete;   // 4
    cached_and_current := cached | current;
}

Safe union metafunction

And you can declaratively opt into writing a safe discriminated union/variant type:

// name_or_number is declaratively a safe union/variant type: 
// it has a discriminant that enforces only one alternative 
// can be active at a time, members always have a name, and
// each member has .is_member() and .member() accessors...
// the word "union" carries all that meaning as a convenient 
// and readable opt-in without hardwiring "union" specially 
// into the language
//
name_or_number: @union type = {
    name: std::string;
    num : i32;
}

Why have this metafunction on an ordinary C++ class, when C++ already has both C’s union and C++11’s std::variant? Because:

• it keeps the language smaller and simpler, because it doesn’t hardwire special-purpose divergent splinter types into the language and compiler
  • (cue the Beatles earworm again: “class is all you need, class is all you need…”);
• it’s a better union than C union, because C union is unsafe; and
• it’s a better variant than C++11 std::variant, because std::variant is hard to use because its alternatives are anonymous (as is the type itself; there’s no way to distinguish in the type system between a variant<int,string> that stores either an employee id or employee name, and a variant<int,string> that stores either a lucky number or a pet unicorn’s dominant color).

Each @union type has its own type-safe name, has clear and unambiguous named members, and safely encapsulates a discriminator to rule them all. Sure, it uses unsafe casts in the implementation, but they are fully encapsulated, where they can be tested once and be safe in all uses. That makes @union:

• as easy to use as a C union,
• as safe to use as a std::variant… and
• as a bonus, because it’s an ordinary type, it can naturally have other things normal types can have, such as template parameter lists and member functions:

// a templated custom safe union
name_or_other: @union <T:type> type
= {
    name  : std::string;
    other : T;

    // a custom member function
    to_string: (this) -> std::string = {
        if is_name()       { return name(); }
        else if is_other() { return other() as std::string; }
        else               { return "invalid value"; }
    }
}

main: () = {
    x: name_or_other<int> = ();
    x.set_other(42);
    std::cout << x.other() * 3.14 << "\n";
    std::cout << x.to_string(); // prints "42", but is legal whichever alternative is active
}

What’s next

For the rest of the year, I plan to:

• continue self-hosting cppfront, i.e., migrate more of cppfront’s own code to be written in Cpp2 syntax, particularly now that I have enum and union (cppfront uses enum class and std::variant pervasively);
• continue working my list of pending Cpp2 features and implementing them in cppfront; and
• work with a few private alpha testers to start writing a bit of code in Cpp2, to alpha-test cppfront and also to alpha-test my (so far unpublished) draft documentation.

But first, one week from today, I’ll be at CppCon to give a talk about this progress and why full-fidelity compatibility with ISO C++ is essential (and what that means): “Cooperative C++ Evolution: Toward a TypeScript for C++.” I look forward to seeing many of you there!

Thanks again to C++ Now for inviting me to speak this year in glorious Aspen, Colorado, USA! It was nice to see many old friends again there and make a few new ones too.

The talk I gave there was just posted on YouTube, you can find it here:

At CppCon 2022, I argued for why we should try to make C++ 10x simpler and safer, and I presented my own incomplete experimental compiler, cppfront. Since then, cppfront has continued progressing: My spring update post covered the addition of types, a reflection API, and metafunctions, and this talk was given a week after that post and shows off those features with discussion and live demos.

This talk also clearly distinguishes between what I call the “Dart plan” and the “TypeScript plan” for aiming at a 10x improvement for an incumbent popular language. Both plans have value, but they have different priorities and therefore choose different constraints… most of all, they either embrace up-front the design constraint of perfect C++ interop compatibility, or they forgo it (forever; as I argue in the talk, it can never be achieved retroactively, except by starting over, because it’s a fundamental up-front constraint). No one else has tried the TypeScript plan for C++ yet, and I see value in trying it, and so that’s the plan I’m following for cppfront.

When people ask me “how is cppfront different from all the other projects trying to improve/replace C++?” my answer is “cppfront is on the TypeScript plan.” All the other past and present projects have been on the Dart plan, which again is a fine plan too, it just has different priorities and tradeoffs particularly around compatibility.

The video description has a topical guide linking to major points in the talk. Here below is a more detailed version of that topical guide… I hope you enjoy the talk!

1:00 Intro and roadmap for the talk

2:28 1. cppfront recap

2:35 – green-field experiments are great; but cppfront is about refreshing C++ itself

3:28 – “when I say compatibility .. I mean I can call any C++ code that exists today … with no shims, no thunks, no overheads, no indirections, no wrapping”

4:05 – can’t take a breaking change to existing code without breaking the world

5:22 – to me, the most impactful release of C++ was C++11, it most changed the way we wrote our code

6:20 – what if we could do C++11 again, but a coordinated set of features to internally evolve C++

6:52 – cppfront is an experiment in progress, still incomplete

7:41 – thanks to 100+ cppfront contributors!

8:00 – summary slide of features demonstrated at CppCon 2022

– safety for C++; goal of 50x fewer CVEs due to type/bounds/lifetime/init safety

– simplicity for C++; goal of 10x less to know

10:00 – 2. cppfront: what’s new

10:05 – (a) 3 smaller new features showing simplicity+safety+efficiency

10:15 – <=> from this work has already been standardized

11:05 – simplicity, safety and efficiency rarely in tension, with the right abstractions

12:55 – chained comparisons: simple, safe (mathematically), efficient (single eval)

15:08 – named loops/break/continue: simple, safe (structured), efficient

16:51 – main’s arguments: simple (std:: always available), safe (bounds/null check by default), efficient (pay only if you ask for main’s parameters)

18:30 – (b) user-defined types

19:20 – explicit `this`

20:20 – defaults: rarely write access-specifiers

21:30 – (recall from CppCon 2022: composable initialization safety with `out` parameters)

23:50 – unified `operator=`: {construct,assign}x{copy,move} is a single function (by default)

25:48 – visual for unified `operator=`

27:28 – walk through example code generation for unified `operator=`

31:35 – virtual/override/final are qualifiers on `this`

35:05 – DEMO: inheritance (GCC this time)

40:43 – easter egg

41:55 – can interleave bases and members, more control over layout and lifetime

43:10 – (c) reflection and type metafunctions

43:10 – recap overview from CppCon 2017

54:23 – DEMO: applying type metafunctions

56:10 – 3. compatibility for C++

56:35 – John Carmack on compatibility in the real world

59:40 – recall: summary of “Bridge to NewThingia” talk

1:02:05 – avoiding an adoption step function requires high fidelity compatibility

1:04:25 – C++ from C, TypeScript from JavaScript, Swift from Objective-C, Roslyn from prior compiler

1:05:45 – emphasizing and dissecting TypeScript’s compatibility story

1:07:55 – Dart: similar goal, but not designed to be compatible, and you’ll never be able to back into compatibility without starting over

1:08:55 – examples of why incompatibility costs a decade:

1:08:57 – – VC++ 6.0 to 10.0 … 12 years

1:10:28 – – Python 2 to 3 … 12 years (Python is C++’s #1 sister language)

1:18:30 – – C99 to C11 … 12 years

1:18:50 – – C++11 basic_string (approved in 2008) to 2019 support on all major platforms … 11 years

1:19:25 – the “lost decade” pattern: lack of seamless compatibility will cost you a decade in adoption

1:20:45 – three “plans”: the “10% plan”, the “Dart plan”, and the “TypeScript plan”

1:21:00 – “10% plan”: incremental evolution-as-usual

1:21:40 – so how do we get a 10x improvement?

1:21:50 – “Dart plan”: designing something new, not worry about compatible interop, competitive

1:23:20 – “TypeScript plan”: designing for something compatible, cooperative

1:25:40 – what it takes to evolve C++ compatibly, which no other effort has tried before

1:28:50 – filling in the blank: ______ for C++

Minutes ago, the ISO C++ committee finished its meeting in-person in Varna, Bulgaria and online via Zoom, where we formally began adopting features into C++26.

Our hosts, VMware and Chaos, arranged for high-quality facilities for our six-day meeting from Monday through Saturday. We had nearly 180 attendees, about two-thirds in-person and the others remote via Zoom, formally representing 20 nations. Also, at each meeting we regularly have new attendees who have never attended before, and this time there were 17 new first-time attendees, mostly in-person; to all of them, once again welcome!

The committee currently has 23 active subgroups, most of which met in parallel tracks throughout the week. Some groups ran all week, and others ran for a few days or a part of a day and/or evening, depending on their wortpkloads. You can find a brief summary of ISO procedures here.

This week’s meeting: Starting C++26

ISO C++ is on a three-year development cycle, which includes a “feature freeze” about one year before we ship and publish that edition of the standard. For example, the feature freeze for C++23 was in early 2022.

But this doesn’t mean we only have two years’ worth of development time in the cycle and the third year is bug fixes and red tape. Instead, the subgroups are a three-stage pipeline and continue concurrently working on new feature development all the time, and the feature freezes are just the checkpoints where we pause loading new features into this particular train. So for the past year, as the subgroups finished work on fit-and-finish for the C++23 features, they also increasingly worked on C++26 features.

That showed this week, as we adopted the first 40 proposed change papers for C++26, many of which had been ready for a couple of meetings and were just waiting for the C++26 train to open for loading to be adopted. Of those 40 change papers, two were “apply the resolutions for all Ready issues” papers that applied a bunch of generally-minor changes. The other 38 were individual changes, everything from bug fixes to new features like hazard pointers and RCU.

Here are some of the highlights…

Adopted for C++26: Core language changes/features

The core language adopted 11 papers, including the following. Taking them in paper number order, which is roughly the order in which work started on the paper…

P2169 “A nice placeholder with no name” by Corentin Jabot and Michael Park officially adds support for the _ wildcard in C++26. Thanks to the authors for all their research and evidence for how it could be done in a backward-compatible way! Here are some examples that will now be legal as compilers start to support draft-C++26 syntax:

std::lock_guard _(mutex);

auto [x, y, _] = f();

inspect(foo) { _ => bar; };

Some compiler needs to implement -Wunderbar.

The palindromic P2552 “On the ignorability of standard attributes” by Timur Doumler sets forth the Three Laws of Robotics… er, I mean, the Three Rules of Ignorability for standard attributes. The Three Rules are a language design guideline for all current and future standard attributes going forward… see the paper for the full rules, but my informal summary is:

• [Already in C++23] Rule 1. Standard attributes must be parseable (i.e., can’t just contain random nonsense).

• [Already in C++23] Rule 2. Removing a standard attribute can’t change the program’s meaning: It can reduce the program’s possible legal behaviors, but it can’t invent new behaviors.

• [New] Rule 3. Feature test macros shouldn’t pretend to support an attribute unless the implementation actually implements the attribute’s optional semantics (i.e., doesn’t just parse it but then ignore it).

P2558 “Add @, $, and ` to the basic character set” by Steve Downey is not a paper whose name was redacted for cussing; it’s a language extension paper that follows in C’s footsteps, and allows these characters to be used in valid C++ programs, and possibly in future C++ language evolution.

P2621 “UB? In my lexer?” by Corentin Jabot removes the possibility that just tokenizing C++ code can be a source of undefined behavior in a C++ compiler itself. (Did you know it could be UB? Now it can’t.) Note however that this does not remove all possible UB during compilation; future papers may address more of those compile-time UB sources.

P2738 “constexpr cast from void*: towards constexpr type-erasure” by Corentin Jabot and David Ledger takes another step toward powerful compile-time libraries, including enabling std::format to potentially support constexpr compile-time string formatting. Speaking of which…

P2741 “User-generated static_assert messages” by Corentin Jabot lets compile-time static_assert accept stringlike messages that are not string literals. For example, the popular {fmt} library (but not yet std::format, but see above!) supports constexpr string formatting, and so this code would work in C++26:

static_assert(sizeof(S) == 1, fmt::format("Unexpected sizeof: expected 1, got {}", sizeof(S)));

Together with P2738, an implementation of std::format that uses both of the above features would now be able to used in a static_assert.

Adopted for C++26: Standard library changes/features

The standard library adopted 28 papers, including the following. Starting again with the lowest paper number…

This first one gets the award for “being worked on the longest” (just look at the paper number, and the R revision number): P0792R14, “function_ref: A type-erased callable reference” by Vittorio Romeo, Zhihao Yuan, and Jarrad Waterloo adds function_ref<R(Args...)> as a vocabulary type with reference semantics for passing callable entities to the standard library.

P1383 “More constexpr for <cmath> and <complex>” by Oliver J. Rosten adds constexpr to over 100 more standard library functions. The march toward making increasing swathes of the standard library usable at compile time continues… Jason Turner is out there somewhere saying “Moar Constexpr!” and “constexpr all the things!”

Then, still in paper number order, we get to the “Freestanding group”:

• P2013 “Freestanding language: Optional ::operator new” by Ben Craig makes ::operator new optional in freestanding implementations.
• P2198 “Freestanding feature-test macros and implementation-defined extensions” by Ben Craig lets programs targeting freestanding implementations detect which standard library facilities are available.
• P2338 “Freestanding library: Character primitives and the C library” by Ben Craig adds to the freestanding C and C++ standard library all features that can be implemented without OS calls and space overhead, including <charconv> and char_traits.

P2510 “Formatting pointers” by Mark de Wever allows nice formatting of pointer values without incanting reinterpret_cast to an integer type first. For example, this will now work: format("{:P}", ptr);

P2530 “Hazard pointers for C++26” by Maged M. Michael, Michael Wong, Paul McKenney, Andrew Hunter, Daisy S. Hollman, JF Bastien, Hans Boehm, David Goldblatt, Frank Birbacher, and Mathias Stearn adds a subset of the Concurrency TS2 hazard pointer feature to add hazard pointer-based deferred cleanup to C++26.

P2545 “Read-Copy-Update (RCU)” by Paul McKenney, Michael Wong, Maged M. Michael, Andrew Hunter, Daisy Hollman, JF Bastien, Hans Boehm, David Goldblatt, Frank Birbacher, Erik Rigtorp, Tomasz Kamiński, Olivier Giroux, David Vernet, and Timur Doumler as another complementary way to do deferred cleanup in C++26.

P2548 “copyable_function” by Michael Hava adds a copyable replacement for std::function, modeled on move_only_function.

P2562 “constexpr stable sorting” by Oliver J. Rosten enables compile-time use of the standard library’s stable sorts (stable_sort, stable_partition, inplace_merge, and the ranges:: versions). Jason Turner is probably saying “Moar!”…

P2641 “Checking if a union alternative is active” by Barry Revzin and Daveed Vandevoorde introduces the consteval bool is_within_lifetime(const T* p) noexcept function, which works in certain compile-time contexts to find out whether p is a pointer to an object that is within its lifetime — such as checking the active member of a union, but during development the feature was made even more generally useful than just that one use case. (This is technically a core language feature, but it’s in one of the “magic std:: features that look like library functions but are actually implemented by the compiler” section of the standard, in this case the metaprogramming clause.)

P2757 “Type-checking format args” by Barry Revzin enables even more compile-time checking for std::format format strings.

Those are just 12 of the adopted papers as highlights… there were 16 more papers adopted that also apply more extensions and fixes for the C++26 standard library.

Other progress

We also adopted the C++26 schedule for our next three-year cycle. It’s the same as the schedule for C++23 but just with three years added everywhere, just as the C++23 schedule was in turn the same as the schedule for C++20 plus three years.

The language evolution subgroup (EWG) saw 30 presentations for papers during the week, mostly proposals targeting C++26, including fine-tuning for some of the above that made it into C++26 at this meeting.

The standard library evolution subgroup (LEWG) focused on advancing “big” papers in the queue that really benefit from face-to-face meetings. Notably, there is now design consensus on P1928 SIMD, P0876 Fibers, and P0843 inplace_vector, and those papers have been forwarded to the library wording specification subgroup (LWG) and may come up for adoption into C++26 at our next meeting in November. Additional progress was made on P0447 hive, P0260 Concurrent Queues, P1030 path_view, and P2781 constexpr_v.

The library wording specification subgroup (LWG) is now caught up with their backlog, and spent a lot of time iterating on the std::execution and sub_mdspan proposals (the latter was adopted this week).

The contracts subgroup made further progress on refining contract semantics targeting C++26, including to get consensus on removing build modes and having a contract violation handling API.

The concurrency and parallelism subgroup are still on track to move forward with std::execution and SIMD parallelism for C++26, which in the words of the subgroup chair will make C++26 a huge release for the concurrency and parallelism group.

Thank you to all the experts who worked all week in all the subgroups to achieve so much this week!

What’s next

Our next two meetings will be in Kona, HI, USA in November hosted by WorldQuant and the Standard C++ Foundation, and Tokyo, Japan in March hosted by Woven by Toyota.

Wrapping up

Thank you again to the nearly 180 experts who attended on-site and on-line at this week’s meeting, and the many more who participate in standardization through their national bodies!

But we’re not slowing down… we’ll continue to have subgroup Zoom meetings, and then in less than five months from now we’ll be meeting again in person + Zoom to continue adding features to C++26. Thank you again to everyone reading this for your interest and support for C++ and its standardization.