C++20 STL Features: 1 Year of Development on GitHub

Stephan T. Lavavej - MSFT

My CppCon 2020 talk, “C++20 STL Features: 1 Year of Development on GitHub”, is now available on YouTube. The slides are available on GitHub as PDF and original PPTX.

The talk contains complete examples (not snippets!) of several C++20 features: integer comparison functions, constexpr algorithms, uniform container erasure, atomic_ref, and span.

Here are the important links from the end of the talk:

Finally, at the end of the talk I had time to answer a half-dozen questions, but there were many more. Here are those extra questions and my answers:


Q: Why do you squash pull requests instead of just merging them?

A: This significantly simplifies the branch’s history, since one squashed commit == one PR. You can still view the PR’s history on GitHub. Merges would create highly non-linear history (making it hard to figure out when things changed and why; the MSVC internal git repo is full of non-linear merges so we have unfortunate extensive experience with that). Most of the information from non-squashed merges would be uninteresting too – basically code review feedback, fixing bugs during development, etc. For highly unusual situations I could imagine wanting to sequence a PR as a series of commits that are then rebased-and-merged to the default branch, which we’d need to temporarily enable via policy, but generally having that history in the PR is sufficient.


Q: Regarding the atomic_ref, why not just specify relaxed access when you don’t want to pay the atomic penalty?

A: My understanding is that relaxed is still significantly more expensive than plain operations. For example, on x86/x64 for MSVC, atomic increments are implemented by _InterlockedIncrement which provides full sequential consistency, even if you asked for relaxed; I’ve heard that this costs somewhere around 10-100 cycles, whereas a plain increment is half a cycle or less. Even on ARM/ARM64, where there are _Meow_nf intrinsics (“no fence”) for relaxed, I believe they still imply additional costs compared to plain logic.


Q: Were you already expecting that open-sourcing your STL would improve the STL’s team throughput? Or were you afraid that collaborating with third-party contributors would carry too much overhead?

A: Great question – this was one of the top things we thought/worried about on the road to open-sourcing. I’d say we were prepared to take an overhead/throughput cost in the short term, while hoping for throughput improvements in the long term – and were pleasantly surprised that the short-term costs were less than expected, and that we’re already enjoying throughput gains – e.g. midpoint/lerp were lingering because we didn’t have deep numeric expertise, until statementreply contributed an amazing PR analyzing and fixing the remaining issues. I believe that major throughput gains are still to come – my plan/dream for C++23 and beyond is that proposals will be written with implementations based on our STL, such that a PR is ready to be reviewed and merged as soon as the proposal is accepted by WG21. (Bonus points for simultaneous contribution to libc++.) That will improve Standardization quality/throughput as well as the implementation.


Q: For shipped binaries is there an integration with Microsoft public facing symbols and sources servers so debugger will pull in correct version of sources during debugging?

A: The answer here is that there is no change to how the VS product is built and interacts with the symbol server, so everything will continue to work. GitHub is where we do all development, and we ensure that the repo is binary-identical to the MS-internal src/vctools/crt/github tree by replicating PRs over to MSVC. From there, the product is built, the sources are packaged into the VS Installer, and the PDBs are uploaded to the symbol server. In the far future, we may build official binaries through the GitHub CI system and then bundle them into VS through some mechanism – but we’re unsure how to do that right now, and it would involve a lot of work for unclear payoff. We should be able to achieve most of the time savings by simply finishing our build system migration and then getting the MS-internal MSVC MSBuild system (so much MS! 😹) to invoke the CMake/Ninja build system we use for GitHub; we already have such CMake invocations for the LLVM ASAN support libraries.


Q: Did you encounter cases when design in the standard isn’t as practical as it should be? Did you report this to the committee?

A: Yes, this happens fairly frequently. There’s a distinction between “this design isn’t great for implementers and/or users” and “this specification is unclear/inconsistent with other practice/internally inconsistent/violates conservation of momentum”. For the former (suboptimal design), we sometimes mention it to the Library Evolution Working Group, especially as new features are being developed, but it’s generally “too late” after a feature has been accepted into the Working Paper. (Not always, since features can be revised before the International Standard is published; one place this happened was span which received an unsigned size_type before C++20 was completed.) The latter (bogus specification) is common, and we report those to the Library Working Group (as LWG issues) which can usually be quickly resolved. In the meantime, we use our best judgement to implement what’s possible and what the Standard “should have said”.


Q: Why <charconv> does not work with wchar_t?

A: That’s a question for Jens Maurer who proposed the feature. My understanding is that charconv was meant as a minimal API, and the idea was that it would be primarily used with JSON and other APIs where char is sufficient. However, converting wchar_t to char and back, even for the limited purposes of float parsing, is highly inconvenient/slow, and to_chars ended up being much faster than anyone in L[E]WG realized was possible at the time (as Ulf Adams invented Ryu and Ryu Printf after the feature was accepted!), so the overhead of wchar_t conversion became even more significant. While charconv is extremely complicated, making it handle wchar_t would be a very simple matter of templatizing the codepaths that interact with the characters; the tables and core algorithm would not need to be replicated.


Q: Did the decision of open sourcing the code come top-down or the team had to fight up the chain to convince management this is a good idea?

A: An interesting question 😸 I think I can say that it was a bottom-up decision – Mahmoud Saleh (my boss, the VC Libraries dev lead) drove the process of getting approval, with support from the rest of the MSVC chain. We did have to convince our ultrabosses that it was a good idea, but it wasn’t a fight – it was a useful exercise of thinking through the rationale, the costs/benefits, and the consequences to working in the open. The top-down change in strategy definitely made this possible – going open-source was unthinkable for the MS of 10 years ago, and now we’re continually looking for places where it makes sense, including for foundational components like the STL and .NET Core (we spoke to that team as part of going open-source to understand the challenges and opportunities we were about to face, they were extremely helpful).

The opportunities that we’re looking for are where we can advance the interests of the entire C++ community, so when programmers think about the future of C++, they’ll naturally think of Microsoft. For example, all C++ programmers benefit when the major toolchains support the latest features, in a timely manner, at a high level of quality – so Microsoft has invested a ton of developer-years of effort in catching up in conformance, to the point where MSVC is often the first to implement new features. The STL was the most compelling opportunity to open-source for several reasons: it’s a relatively small code base and test suite (large in absolute terms – it’s half of the Standard, after all! – but smaller than the compiler or other massive projects), we were already shipping its source code for viewing so it was “just” a question of changing the license, the library is evolving increasingly quickly, and (perhaps most importantly) the library tends not to be deeply interconnected, so it’s possible to add or change things without understanding and changing everything else. Now that we have an open-source Standard Library like GCC’s libstdc++ and Clang/LLVM’s libc++, we hope that it’ll be easier to propose library features for Standardization, in a form that works well on all platforms.


Q: What is the best way to learn all the latest STL features? Is there an on-line cookbook? Functional style? Is there an expert on your team writing a book?

A: I would say that the best way to is to implement them 😹😹 None of the STL maintainers have time to write a book, but we’re working with Tyler Whitney from the Microsoft Docs team as he adds documentation for the various features we’ve implemented over the last few years. cppreference is also a good source of information built up by the community. I generally think that the best way to learn a feature, other than implementing it, is to try using it in toy examples first, to become familiar with the basics in a simple clean environment, followed by using it in a basic way in a real codebase, before getting to advanced uses. Trying to immediately use a new feature in a production codebase can be a headache since you might not immediately see when an issue is caused by attempting to use the feature itself incorrectly, or whether it’s caused by an interaction with the codebase (“I know how to use this feature generally, so what’s wrong here – oh, it’s that it requires copyability, but this type is move-only, okay” or whatever). If you find a better technique, let me know! It is also possible to read the Library Standardese directly – it is very detailed. The downsides are that it’s written in a somewhat strange style, and occasionally information is “hidden” elsewhere (e.g. the container specifications are highly centralized in an unusual way), but it’s generally possible to find function signatures and basic type requirements and value preconditions that way. The Core Language Standardese is much much harder to understand for ordinary humans (versus extraordinary compiler developers) – but of course I would say that, since I’m a library developer who specifically works on the STL because it’s easy compared to compiler development 🤣


Q: Is this part of the VS 2019 16.8.0 Preview 3.0?

A: Yes, all of the features that I described are available in that release today. We consider them to be at production quality, with the usual caveats that Preview releases aren’t “go-live” supported by VS, and that /std:c++latest is technically considered experimental and subject to change. (Note that we can and have broken ABI for /std:c++latest features – ABI lockdown will happen when we complete C++20 and add /std:c++20 in celebration. So anything built with /std:c++latest does need to be continually built with the latest toolset – but that shouldn’t be a problem if you want to live on the leading edge of C++!)


Q: When is vNext going to become a concrete version?

A: Our plans are still tentative and subject to change, but we’re planning to work on vNext after completing C++20, in a clean switchover – that is, VS 2019 (the “v19” release series that began with VS 2015) will receive all C++20 features, then we’ll do vNext, then C++23 features will be added to vNext only – we will continue to service v19 for critical bugs and security fixes, but not new feature work. We hope to finish C++20 in 2020, then work on vNext in H1 2021 – we’re unsure how long we’ll have to work on the vNext overhaul although we expect it to be at least 6 months. (I personally hope for a year, but I also want a pony and a unicorn). At this time, we don’t yet know exactly how this will ship to users (i.e. what release).


Posted in C++


Discussion is closed. Login to edit/delete existing comments.

  • steve heller 0

    Ok, so when is the compiler going open-source? 🙂

    • Stephan T. Lavavej - MSFTMicrosoft employee 0

      No comment 😸

      However, vcruntime/vcstartup will be open-sourced soon (unless a meteor strikes). This is the low-level support code between the CRT and the STL.

  • Hristo Hristov 0

    You mention libc++. Is there any chance that you’ll contribute some of your code to libc++ in the foreseeable future? It looks like libc++ is missing concepts, ranges, etc.

    • Stephan T. Lavavej - MSFTMicrosoft employee 0

      We’re definitely interested in contributing code to libc++ (which is one of the reasons why we chose the same license). I created a PR (well, Phabricator review) for charconv several months ago, but the libc++ maintainers haven’t had time to work on it yet. We’re also happy to contribute any parts of concepts, ranges, filesystem-on-Windows, etc. that libc++ needs. At the moment, we’re extremely busy with C++20, so we haven’t had time to “drive” any contributions, but we’re happy to answer questions about the code.

  • Sekkappa Nagappan 0

    This request is not related to this blog item. I ma posting this here as I do not know where else to report it.

    The c++ team blog site has problems with embededed c++ code. Please follow this link for eg, all embedded code is garbled:


    • Stephan T. Lavavej - MSFTMicrosoft employee 0

      Thanks, I’ve forwarded your report along to people who may be able to fix it.

      • Sekkappa Nagappan 0

        Hi, still not resolved.

  • Sekkappa Nagappan 0

    Current preview versions seems to have removed coroutine support for std::future under c++ 20/c++latest . Was this intentional ? what is the alternative ?

    Essentially coroutine_traits is not defined for std::future.

    Line 1505 in future

    // Experimental coroutine support for std::future. Subject to change/removal!
    namespace experimental {
        struct coroutine_traits<future, _ArgTypes...> {
    • Stephan T. Lavavej - MSFTMicrosoft employee 0

      That’s intentional – C++20 std::future doesn’t contain support for C++20 coroutines. This is likely to change in the C++23 timeframe.

    • Gor Nishanov 0

      Yes. This is intentional. We were optimistically hoping that future.then which has been accepted as a part of Concurrency TS in 2014 will get in by 2020 and put this placeholder for users to experiment with. It was not efficient and not good for production use. (Notice that it launces a dedicated thread every time you await, since std::future does not provide a subscribe for notification interface).

      Current standardization effort goes along different lines. For example see this CppCon talk from the last year: https://www.youtube.com/watch?v=tF-Nz4aRWAM “CppCon 2019: Eric Niebler, David Hollman “A Unifying Abstraction for Async in C++””

  • Irakli Lomidze 0

    Modern C++ x20 Became better enough to think of porting existing .net code to modern C++.
    Now C++ has safe pointers, reflection, metaclasses
    Reasons for that: 1) Performance 2) Code stability (As .net developers w have to update our code every 3 years as .net core support lifecycle ) In C++ we could have decades-old code that still usable.

    Modern C++ needs to have some good libraries for developing business applications.
    1) Good ORM Like EF
    2) Good WebFramework (Like ASP.NET with or without razor)

    Is any discussions or plan in Microsoft to port some of the .net libraries to the Modern C++ ?

    • Stephan T. Lavavej - MSFTMicrosoft employee 0

      C++20 doesn’t support reflection or metaclasses – such features might appear in C++23 or beyond. The Visual C++ Libraries team has no plans to port .NET libraries to C++. Note that other teams within Microsoft develop C++ libraries (e.g. there’s a recently-announced Azure SDK for C++).

      • Irakli Lomidze 0

        Stephan Correct me if I’m wrong.
        both reflections and metaclasses released as TS. it is not part of the STD yet but, they supported by compilers. and will became STD in the next iteration.

        C++ relay needs rich libraries like .net and java has.
        It might not be a direct port of libraries, but C++ needs such things. I think. : )

        • Stephan T. Lavavej - MSFTMicrosoft employee 0

          There is a Reflection TS, but not metaclasses (there is no concrete proposal for that yet, AFAIK). There is also no guarantee that something in a TS will become Standard, although it does seem quite likely for reflection.

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