Draft FAQ: Why does the C++ standard ship every three years?

WG21 has a strict schedule (see P1000) by which we ship the standard every three years. We don’t delay it.

Around this time of each cycle, we regularly get questions about “but why so strict?”, especially because we have many new committee members who aren’t as familiar with our history and the reasons why we do things this way now. And so, on the pre-Cologne admin telecon last Friday, several of the chairs encouraged me to write down the reasons why we do it this way, and some of the history behind the decision to adopt this schedule.

I’ve now done that by adding a FAQ section to the next draft of P1000, and I’ve sent a copy of it to the committee members now en route to Cologne. That FAQ material will appear in the next public revision of P1000 which will be in the post-meeting mailing a few weeks from now.

In the meantime, because the draft FAQ might be of general public interest too, here is a copy of that material. I hope you find it largely useful, occasionally illuminating, and maybe even a bit entertaining.

Now off to the airport… see (many of) you in Cologne. We are expecting about 220 people… by far our largest meeting ever. More about that in the post-meeting trip report after the meeting is over…


FAQs

(As of pre-Cologne, July 2019) There are bugs in the standard, so should we delay C++20?

Of course, and no.

We are on our planned course and speed: Fixing bugs is the purpose of this final year, and it’s why this schedule set the feature freeze deadline for C++“20” in early “19” (Kona), to leave a year to fix bugs including to get a round of international comments this summer. We have until early 2020 (three meetings: Cologne, Belfast, and Prague) to apply that review feedback and any other issue resolutions and bug fixes.

If we had just another meeting or two, we could add <feature> which is almost ready, so should we delay C++20?

Of course, and no.

Just wait a couple more meetings (post-Prague) and C++23 will be open for business and <feature> can be the first thing voted into the C++23 working draft. For example, that’s what we did with concepts; it was not quite ready to be rushed from its TS straight into C++17, so the core feature was voted into draft C++20 at the first meeting of C++20 (Toronto), leaving plenty of time to refine and adopt the remaining controversial part of the TS that needed a little more bake time (the non-“template” syntax) which was adopted the following year (San Diego). Now we have the whole thing.

This feels overly strict. Why do we ship releases of the IS at fixed time intervals (3 years)?

Because it’s one of only two basic project management options to release the C++ IS, and experience has demonstrated that it’s better than the other option.

What are the two project management options to release the C++ IS?

I’m glad you asked.

There are two basic release target choices: Pick the features, or pick the release time, and whichever you pick means relinquishing control over determining the other. It is not possible to control both at once. They can be summarized as follows:

Elaborating:

(1) “What”: Pick the features, and ship when they’re ready; you don’t get to pick the release time. If you discover that a feature in the draft standard needs more bake time, you delay the world until it’s ready. You work on big long-pole features that require multiple years of development by making a release big enough to cover the necessary development time, then try to stop working on new features entirely while stabilizing the release (a big join point).

This was the model for C++98 (originally expected to ship around 1994; Bjarne originally said if it didn’t ship by about then it would be a failure) and C++11 (called 0x because x was expected to be around 7). This model “left the patient open” for indeterminate periods and led to delayed integration testing and release. It led to great uncertainty in the marketplace wondering when the committee would ship the next standard, or even if it would ever ship (yes, among the community, the implementers, and even within the committee, some had serious doubts in both 1996 and 2009 whether we would ever ship the respective release). During this time, most compilers were routinely several years behind implementing the standard, because who knew how many more incompatible changes the committee would make while tinkering with the release, or when it would even ship? This led to wide variation and fragmentation in the C++ support of compilers available to the community.

Why did we do that, were we stupid? Not exactly, just inexperienced and… let’s say “optimistic,” for (1) is the road paved with the best of intentions. In 1994/5/6, and again in 2007/8/9, we really believed that if we just slipped another meeting or three we’d be done, and each time we ended up slipping up to four years. We learned the hard way that there’s really no such thing as slipping by one year, or even two.

Fortunately, this has changed, with option (2)…

(2) “When”: Pick the release time, and ship what features are ready; you don’t get to pick the feature set. If you discover that a feature in the draft standard needs more bake time, you yank it and ship what’s ready. You can still work on big long-pole features that require multiple releases’ worth of development time, by simply doing that work off to the side in “branches,” and merging them to the trunk/master IS when they’re ready, and you are constantly working on features because every feature’s development is nicely decoupled from an actual ship vehicle until it’s ready (no big join point).

This has been the model since 2012, and we don’t want to go back. It “closes the patient” regularly and leads to sustaining higher quality by forcing regular integration and not merging work into the IS draft until it has reached a decent level of stability, usually in a feature branch. It also creates a predictable ship cycle for the industry to rely on and plan for. During this time, compilers have been shipping conforming implementations sooner and sooner after each standard (which had never happened before), and in 2020 we expect multiple fully conforming implementations the same year the standard is published (which has never happened before). This is nothing but goodness for the whole market – implementers, users, educators, everyone.

Also, note that since we went to (2), we’ve also been shipping more work (as measured by big/medium/small feature counts) at higher quality (as measured by a sharp reduction in defect reports and comments on review drafts of each standard), while shipping whatever is ready (and if anything isn’t, deferring just that).

How serious are we about (2)? What if a major feature by a prominent committee member was “almost ready”… we’d be tempted to wait then, wouldn’t we?

Very serious, and no.

We have historical data: In Jacksonville 2016, at the feature cutoff for C++17, Bjarne Stroustrup made a plea in plenary for including concepts in C++17. When it failed to get consensus, Stroustrup was directly asked if he would like to delay C++17 for a year to get concepts in. Stroustrup said No without any hesitation or hedging, and added that C++17 without concepts was more important than a C++18 or possibly C++19 with concepts, even though Stroustrup had worked on concepts for about 15 years. The real choice then was between: (2) shipping C++17 without concepts and then C++20 with concepts (which we did), or (1) renaming C++17 to C++20 which is isomorphic to (2) except for skipping C++17 and not shipping what was already ready for C++17.

What about something between (1) and (2), say do basically (2) but with “a little” schedule flexibility to take “a little” extra time when we feel we need to stabilize a feature?

No, because that would be (1).

The ‘mythical small slip’ was explained by Fred Brooks in The Mythical Man-Month, with the conclusion: ”Take no small slips.”

For a moment, imagine we did slip C++20. The reality is that we would be switching from (2) back to (1), no matter how much we might try to deny it, and without any actual benefit. If we decided to delay C++20 for more fit-and-finish, we will delay the standard by at least two years. There is no such thing as a one-meeting or three-meeting slip, because during this time other people will continue to (rightly) say “well my feature only needs one more meeting too, since we’re slipping a meeting let’s add that too.” And once we slip at least two years, we’re saying that C++20 becomes C++22 or more likely C++23… but we’re already going to ship C++23! — So we’d still be shipping C++23 on either plan, and the only difference is that we’re not shipping C++20 in the meantime with the large amount of fully-baked work that’s ready to go, and making the world wait three more years for it. Gratuitously, because the delay will not benefit those baked features, which is most or all of them.

So the suggestion amounts to “let’s make C++20 be C++22 or C++23,” and the simple answer is “yes, we’re going to have C++23 too, but in addition to C++20 and not instead of it.” To delay C++20 actually means to skip C++20, instead of releasing the great good work that is stable and ready, and there’s no benefit to doing that.

But feature X is broken / needs more bake time than we have bugfix time left in C++20!

No problem! We can just pull it.

In that case, someone needs to write the paper aimed at EWG or LEWG (as appropriate) that shows the problem and/or the future doors we’re closing, and proposes removing it from the IS working draft. Those groups will consider it, and if they decide the feature is broken (and plenary agrees), that’s fine, the feature will be delayed to C++next. We have actually done this before, with C++0x concepts.

But under plan (1), we would be delaying, not only that feature, but the entire feature set of C++20 to C++23! That would be… excessive.

Does (2) mean “major/minor” releases?

No. We said that at first, before we understood that (2) really simply means you don’t get to pick the feature set, not even at a “major/minor” granularity.

Model (2) simply means “ship what’s ready.” That leads to releases that are:

  • similarly sized (aka regular medium-sized) for “smaller” features because those tend to take shorter lead times (say, < 3 years each) and so generally we see similar numbers completed per release; and
  • variable sized (aka lumpy) for “bigger” features that take longer lead times (say, > 3 years each) and each IS release gets whichever of those mature to the point of becoming ready to merge during that IS’s time window, so sometimes there will be more than others.

So C++14 and C++17 were relatively small, because a lot of the standardization work during that time was taking place in long-pole features that lived in proposal papers (e.g., contracts) and TS “feature branches” (e.g., concepts).

C++20 is a big release …

Yes. C++20 has a lot of major features. Three of the biggest all start with the letters “co” (concepts, contracts, coroutines) so perhaps we could call it co_cpp20. Or co_dependent. Wait, we’re digressing.

… and so aren’t we cramming a lot into a three-year cycle for C++20?

No, see “lumpy” above.

C++20 is big, not because we did more work in those three years, but because many long-pole items (including at least two that have been worked on in their current form since 2012, off to the side as P-proposals and TS “branches”) happened to mature and get consensus to merge into the IS draft in the same release cycle.

It has pretty much always been true that major features take many years. The main difference between plan (1) for C++98 and C++11 and plan (2) now is: In C++98 and C++11 we held the standard until they were all ready, now we still ship those big ones when they’re ready but we also ship other things that are ready in the meantime instead of going totally dark.

C++20 is the same 3-year cycle as C++14 and C++17; it’s not that we did more in these 3 years than in the previous two 3-year cycles, it’s just that more long-pole major features became ready to merge. And if any really are unready, fine, we can just pull them again and let them bake more for C++23. If there is, we need that to be explained in a paper that proposes pulling it, and why, for it to be actionable.

In fact, I think the right way to think about it is that C++14+17+20 taken as a whole is our third 9-year cycle (2011-2020), after C++98 (1989-1998) and C++11 (2002-2011), but because we were on plan (2) we also shipped the parts that were ready at the 3- and 6-year points.

Isn’t it better to catch bugs while the product is in development, vs. after it has been released to customers?

Of course.

But if we’re talking about that as a reason to delay the C++ standard, the question implies two false premises: (a) it assumes the features haven’t been released and used before the standard ships (many already have production usage experience); and (b) it assumes all the features can be used together before the standard ships (they can’t).

Elaborating:

Re (a): Most major C++20 features have been implemented in essentially their current draft standard form in at least one shipping compiler, and in most cases actually used in production code (i.e., has already been released to customers who are very happy with it). For example, coroutines (adopted only five months ago as of this writing) has been used in production in MSVC for two years and in Clang for at least a year with very happy customers at scale (e.g., Azure, Facebook).

Re (b): The reality is that we aren’t going to find many feature interaction problems until users are using them in production, which generally means until after the standard ships, because implementers will generally wait until the standard ships to implement most things. That’s why when we show any uncertainty about when we ship, what generally happens is that implementations wait – oh, they’ll implement a few things, but they will hit Pause on implementing the whole thing until they know we’re ready to set it in stone. Ask <favorite compiler> team what happened when they implemented <major feature before it was in a published standard>. In a number of cases, they had to implement it more than once, and break customers more than once. So it’s reasonable for implementers to wait for the committee to ship.

Finally, don’t forget the feature interaction problem. In addition to shipping when we are ready, we need time after we ship to find and fix problems with interactions among features and add support for such interactions that we on typically cannot know before widespread use of new features. No matter how long we delay the standard, there will be interactions we can’t discover until much later. The key is to manage that risk with design flexibility to adjust the features in a compatible way, not to wait until all risk is done.

The standard is never perfect… don’t we ship mistakes?

Yes.

If we see a feature that’s not ready, yes we should pull it.

If we see a feature that could be better, but we know that the change can be done in a backward-compatible way, that’s not a reason to not ship it now; it can be done as an extension in C++next.

We do intentionally ship features we plan to further improve, as long as we have good confidence we can do so in a backward-compatible way.

But shouldn’t we aim to minimize shipping mistakes?

Yes. We do aim for that.

However, we don’t aim to eliminate all risk. There is also a risk and (opportunity) cost to not shipping something we think is ready. So far, we’ve been right most of the time.

Are we sure that our quality now is better than when were on plan (1)?

Yes.

By objective metrics, notably national body comment volume and defect reports, C++14 and C++17 have been our most stable releases ever, each about 3-4 times better on those metrics than C++98 or C++11. And the reason is because we ship regularly, and put big items into TS branches first (including full wording on how they integrate with the trunk standard) and merge them later when we know they’re more baked.

In fact, since 2012 the core standard has always been maintained in a near-ship-ready state (so that even our working drafts are at least as high quality as the shipped C++98 and C++11 standards). That never happened before 2012, where we would often keep the patient open with long issues lists and organs lying around nearby that we meant to put back soon; now we know we can meet the schedule at high quality because we always stay close to a ship-ready state. If we wanted to, we could ship the CD right now without the Cologne meeting and still be way higher quality than C++98’s or C++11’s CDs (or, frankly, their published standards) ever were. Given that C++98 and C++11 were successful, recognizing that we’re now at strictly higher quality than that all the time means we’re in a pretty good place.

C++98 and C++11 each took about 9 years and were pretty good products …

Yes: 1989-1998, and 2002-2011.

… and C++14 and C++17 were minor releases, and C++20 is major?

Again, I think the right comparable is C++14+17+20 taken as a whole: That is our third 9-year cycle, but because we were on plan (2) we also shipped the parts that were ready at the 3- and 6-year points.

Does (2) allow making feature-based targets like P0592 for C++next?

Sure! As long as it doesn’t contain words like “must include these features,” because that would be (1).

Aiming for a specific set of features, and giving those ones priority over others, is fine – then it’s a prioritization question. We’ll still take only what’s ready, but we can definitely be more intentional about prioritizing what to work on first so it has the best chance of being ready as soon as possible.

Your “top five” ISO C++ feature proposals

The ISO C++ committee now regularly receives many more proposals than we can/should accept. For the meeting that begins this coming Monday, we have about 300 active technical papers, most targeting post-C++20. I now regularly get asked, including again a few hours ago, “how do we know which of these customers actually want and will use? what is our data that we’ve prioritized them correctly?”

So a colleague has challenged me: Can the ISO C++ committee get useful information by crowdsourcing public feedback on ISO C++ proposals? I don’t know, so let me try by asking one targeted question…

Survey: If you could pick at most five of [list of active papers] to be eventually adopted in the C++ standard in some form, which would you pick and why (ideally, how you would use them in production)?

I don’t expect you to know all of the proposals (and I know it’s a big list!), but I suspect many of you know about some of them that matter to you. I will share your feedback with the ISO C++ committee and post the results publicly. Thanks again for all of you who’ve already provided feedback in our spring C++ developer survey, that feedback is being used by the committee and the experiment here is try for finer-grained feedback at the proposal level. If this works perhaps we can explore things like allowing community comment/upvote on these issues.

Thank you in advance for your time, it’s appreciated and helpful.

Herb

Note: PLEASE use the survey (not the comments section on this post) to respond to this question, because the survey form makes it easier to track and use the data.

Please do use the comments section below for other discussion, including suggestions about other ways the committee might usefully engage the community.

Guy Steele on designing a programming language for library building (OOPSLA ’98 keynote)

Classic, and timeless. (HT: Patricia Aas, Tony Van Eerd and Peter Sommerlad)

Note that when he says “growing a language” he doesn’t mean literally the language itself — it’s not a talk about language evolution. Rather, he’s talking about enabling users to write rich and powerful abstractions in that language without having to go beg their language designer and compiler vendor to build them into the compiler every time.

Steele’s argument in the back half of the talk is on point, on the importance of imbuing a programming language with a few well-chosen “patterns” that allow, and guide, specific kinds of extensibility. His argument is a great description of why I’m pursuing metaclasses for C++, because we should have a convenient way for programmers to write their own Words of Power like “value_type” or “interface” as libraries instead of forcing them to go bother their local compiler writer (or local standards committee) for a language extension each time for that sort of thing. And his “as a pattern” description is a great summary of why I’m proposing them as “just” a sugar to apply a compile-time function in a very specific place and in a very constrained way, and not going anywhere near making a mutable language which would be crazy not worth pursuing.

A theme: Simplifying C++ (& CppCast podcast)

This week I was happy to join Rob Irving and Jason Turner on their great CppCast podcast. I chose “Simplifying C++” as the theme, because all of the active work I’ve chosen to do on C++ these days is on the common theme of simplifying how we teach, learn, and use C++… the “C++ UX” you might say.

Here’s the podcast: CppCast: Simplifying C++ with Herb Sutter (length: 1:07)

And here’s a table that summarizes my active projects that we mentioned in the podcast, and what specific parts of C++ that I hope each might help simplify.

You’ll notice that three common “simplification” themes get their own summary columns: Can this project reduce what we have to teach/know to use C++, so that C++ can be easier to learn and use? Can it reduce or even eliminate classes of errors, so that we can program with greater confidence? And can it reduce the need for future language features, to help reduce future complexity as C++ continues to grow and evolve?

Finally, here is the list with links:

Elements of Modern C++ Style and C++ Core Guidelines (CppCon 2014, 2015)
Lifetime profile and gcpp opt-in tracing GC allocator (CppCon 2015, 2016, 2018)
Spaceship <=> comparisons (CppCon 2017, adopted C++20)
Reflection+metaclasses (ACCU 2017, CppCon 2017 & 2018)
Lightweight exception handling (ACCU 2019, CppCon 2019)
Lightweight RTTI (CppCon 2019)

Incidentally, this pattern is why I chose the title “Thoughts on a more powerful and simpler C++ (5 of N)” for my CppCon talk last year — it was the 5th CppCon plenary session I’d given on this theme, so I thought I might as well make the theme explicit.

I hope you enjoy the podcast.

EuroLLVM Lifetime talk by Gábor Horváth and Matthias Gehre

At CppCon 2018, I gave an update of my Lifetime analysis work that makes common cases of pointer/iterator/range/etc. dangling detectable at compile time (the spec is here in the C++ Core Guidelines GitHub repo). During that talk, we mentioned and demo’d two implementations: as a Visual C++ extension by Kyle Reed and Neil MacIntosh, and in a fork of Clang by Matthias Gehre and Gábor Horváth who both also joined me on-stage.

Last month at EuroLLVM, Matthias and Gábor gave a 30-minute progress report talk about their work to upstream the Lifetime rules implementation into Clang, “Implementing the C++ Core Guidelines’ Lifetime Safety Profile in Clang.” They give a summary of the approach and how it maps to Clang internals, show initial results from testing against Clang’s and LLVM’s own sources, and end with a nice roadmap for the next steps of incrementally upstreaming the work to Clang trunk.

If you care about making C++ safer, I recommend taking a look… it’s a great summary and update. Thanks, Matthias and Gábor and Kyle and Neil for their collaboration on this project!

Trip report: Winter ISO C++ standards meeting (Kona)

A few minutes ago, the ISO C++ committee completed its winter meeting in Kona, HI, USA, hosted with thanks by Plum Hall, NVIDIA, and the Standard C++ Foundation. As usual, we met for six days Monday through Saturday, including most evenings. This and the previous meeting were the biggest ISO C++ meetings in our 29-year history, and this time we had a new record of 13 voting national bodies represented in person: Bulgaria, Canada, Czech Republic, Finland, France, Germany, Netherlands, Poland, Russia, Spain, Switzerland, United Kingdom, and United States. For more details about our size increase, including how we adapted organizationally to handle the load, see my San Diego “pre-trip” report and my San Diego trip report.

Thank you to all of the hundreds of people who participate in ISO C++ in person and electronically. Below, I want to at least try to recognize by name many of the authors of the proposals we adopted, but nobody succeeds with a proposal on their own. C++ is a team effort – this wouldn’t be possible without all of your help. So, thank you, and apologies for not being able to acknowledge everyone by name.

Notes:

  • Some of the links below are to papers that will not be published until the post-meeting mailing a few weeks from now, and so the links will start working at that time.
  • You can find a brief summary of ISO procedures here.

C++20 feature freeze

Per our official C++20 schedule, this was the last meeting to approve features for C++20, so we gave priority to proposals that might make C++20 or otherwise could make solid progress, and we deferred other proposals to be considered at a future meeting — not at all as a comment on the proposals, but just for lack of time at this meeting. (I’ve been holding back from publishing updates to my own P0707 and P0709 proposals, generation+metaclasses and lightweight exception handling, for this very reason.)

So we now know most of the final feature set of C++20! At our next meeting in July, we expect to formally adopt a few additional features that were design-approved at this meeting but didn’t complete full wording specification review this week, and then at the end of the July meeting we will launch the primary international comment ballot (aka CD ballot) for C++20.

Two more major features adopted for C++20: Modules and coroutines

Again, I want to acknowledge the primary proposal authors by name, who did a lot of the heavy lifting. But none of this would be possible without the hard work of scores of people at this meeting and over the past few years including in alternative proposals.

Modules (Gabriel Dos Reis, Richard Smith) was adopted for C++20. Modules are a new alternative to header files that deliver a number of key improvements, notably isolating the effects of macros and enabling scalable builds. As I’ve said in talks (another example), I personally find this feature very significant for several reasons, but the most fundamental is that this is the first time in about 35 years that C++ has added a new feature where users can define a named encapsulation boundary. Until now, we have had three such language features that let programmers create their own Words of Power by (a) giving a user-defined name to (b) something whose implementation is hidden. They are: the variable (which encapsulates the current value), the function (which encapsulates code and behavior), and the class (which encapsulates both to deliver a bunch of state and functions together). Even other major features such as templates are ways to adorn or parameterize those three fundamental features. To these three we now add a fourth: the module (which encapsulates all three to deliver a bunch of them together). That’s a fundamental reason underlying why modules enable further improvements that can now follow on in future C++ evolution.

Coroutines (Gor Nishanov) was adopted for C++20. A number of other authors that have their own coroutine proposals have also done great work to give feedback to this design and inform the committee in the process of showing their own designs, and the adopted coroutines design is expected to adopt some of their additional programming capabilities in the future as well as we continue to build on it after C++20. At this meeting, we had many detailed design discussions, including notably these papers that will be available in a few weeks when the post-meeting mailing is posted: a unified consensus report from all coroutines proposers describing the strengths and weaknesses of all their proposals, and a consensus report from implementers from all major C++ compilers about the tradeoffs and feasibility of the basic facilities required by the various coroutines proposals. Both of these papers are very educational and highly recommended.

Other changes approved for C++20

A number of other smaller changes were adopted as well.

  • Extending structured bindings and reference capture of structured bindings (Nicolas Lesser) allow a name introduced as a structured binding to be used in additional ways that programmers would naturally expect to work, such as allowing them to be captured by reference in lambdas.
  • Allow initializing aggregates from a parenthesized list of values (Ville Voutilainen and Thomas Köppe) allows using () to initialize aggregates in addition to {}. See N4462 for an example of the kind of problems this fixes.
  • <=> != == (Barry Revzin) adds better language support for composability when writing <=> for types that can write a more efficient == than using the <=> operator alone. For example, vector<T> can short-circuit == comparison by first checking whether the two vectors have the same size; if they don’t, we know in constant time that they cannot be equal without comparing any of the elements. The previous guidance for such types was that in addition to <=> they should also overload == and !=, and that any types that compose such types have to know to do the same so as not to lose the optimization. With this change, we can more simply teach that for such types in addition to <=> they should also overload ==, full stop; there is no need to also overload !=, and no need for other types that use such a type to be especially careful so avoid losing the optimization .
  • Change span to use unsigned size, and add ssize() functions to get signed sizes (Jorg Brown and many others) makes std::span more convenient to use with existing STL types, while also allowing use of signed sizes and indexes via ssize() to get the benefits of signedness.
  • polymorphic_allocator (Pablo Halpern, Dietmar Kühl) allows pmr::memory_resource to be used wherever allocators can be used in the standard library.
  • Adopt vectorization policies from Parallelism TS2 (Alisdair Meredith, Pablo Halpern) further extends the parallel STL algorithms with vector execution policies that allow programs to exploit the parallelism of vector units available on modern CPUs.
  • A slew of broad papers restating the standard library in terms of our new C++20 language features of concepts and contracts that were already adopted at recent meetings. See P0788 and P1369 for a great overview of what this involves. This is part of the standard library’s intentional evolution toward replacing exceptions with contracts for preconditions (such as domain and range errors) in future C++.
  • Lots of smaller features and individual issues resolutions.

Other features targeting C++20 for the July meeting

Several other features were design-approved for C++20 this week but did not yet have their specification wording reviewed to merge into the working draft at this meeting, and they are hoped to be formally adopted at our July meeting in Cologne. These include things like text formatting, flat_map, more ranges and algorithms such as move-only views, an automatically joining thread with stop token support, and more. I’ll cover which of these are actually adopted in my July trip report, when we know what landed at that meeting.

The shape of C++20 (modulo any additional features in July)

Now that we are close to knowing the final feature set of C++20, we can see will be C++’s largest release since C++11. “Major” features include at least the following:

  • modules
  • coroutines
  • concepts
  • contracts
  • <=> spaceship
  • “lots more constexpr”: broad use of normal C++ for direct compile-time programming, without resorting to template metaprogramming (see last trip report)
  • ranges
  • calendars and time zones
  • span
  • and lots more

Combined with what came in C++14 and C++17, the C++14/17/20 nine-year cycle is arguably our biggest nine-year cycle yet alongside the previous two (C++98 and C++11) in terms of new features added. We understand that’s exciting, but we also understand that’s a lot for the community to absorb, and so I’m also pleased that along the way we’ve done things like create the Direction Group and, most recently, SG20 on Education, to help guide and absorb continued C++ evolution in our vibrant living language.

Other progress and decisions

Executors and Networking: Both of these continue to progress together, as Networking depends on Executors. We had hoped that part of Executors might be ready for C++20 but they didn’t make the cut, and both of these are now on track for soon post-C++20 (i.e., early in the C++23 cycle).

Reflection TS v1 (David Sankel, Axel Naumann) completed. The Reflection TS international comments have now been processed and the TS is approved for publication. As I mentioned in other trip reports, note again that the TS’s current template metaprogramming-based syntax is just a placeholder; the feedback being requested is on the core “guts” of the design, and the committee already knows it intends to replace the surface syntax with a simpler programming model that uses ordinary compile-time code and not <>-style metaprogramming.

Generic Scope Guard and RAII Wrapper for the Standard Library (Peter Sommerlad, Andrew Sandoval, Eric Niebler, Daniel Krügler) was adopted into the Library Fundamentals 3 TS.

Thank you again to the approximately 180 experts who attended this meeting, and the many more who participate in standardization through their national bodies! Have a good spring… our next regular WG21 meeting in July (Cologne, Germany) where we plan to send out C++20 for its major international review ballot, then spend two more meetings responding to those review comments and make other bugfixes before sending final C++20 out for its approval ballot one year from now.