Using C++20 in 2019

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I'm starting a new C++ project that I probably will be working on and gradually extending for quite a while (at least a year). I'm trying to keep up with C++20 and I would love to start using some of the new features. I don't really care about supporting multiple compilers (GCC or Clang is enough). So far, I've been only experimenting with some of these features, but never considered using C++20 features in a real project.

Edit: My original question was about the current state of the C++20 standard and its support from compilers. I've been asked to narrow down the actual question, so I'll stick to my main reason to use C++20:

The main feature I'm interested in are the concepts. I've experimented with concepts on GCC with the -fconcepts flag. As I understand, this should correspond to the Concepts TS. But what's the state of concepts in the current standard? I've noticed that there are some minor syntactical differences between the TS and some other sources I've found on C++20. Is it realistic to use the current GCC's implementation (or maybe other compiler, that does it better) in a way that will be (at least with a high probability) valid in the actual finalized standard? Are there any reliable sources to keep track of the current agreed upon specification of concepts and other features?

The original questions:

What's the state of C++20 standard? When can I expect it to be complete, or at least in such a state that I can use it safely without worrying about my code not being valid in the final standard? I use cppreference as my primary source of information on language details. When it says since C++20, does that mean, that it is a finalized version that will stay in the standard?

What's the state of C++20 support? When can I expect it to be fully implemented (or at least the most important parts) in GCC, Clang, or maybe MSVC? In particular, what's the state of concepts and modules? I know that GCC has experimental support for concepts with -fconcepts (though cppreference says, that it supports "TS only") and there's a branch of GCC that supports modules with -fmodules (but doesn't work with concepts).

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1 Answers

The C++20 standard, baring catastrophic circumstances, will be complete in... 2020. This ain't rocket science ;)

The C++20 draft was designated feature complete at the last standards meeting, so new things will generally not be added. The likelihood of features being removed or having significant alterations is also low, but non-zero.

As for support for various C++20 features, that will take time. Not only that, it will take further time for said support to reach maturity. If you just want to play around with C++20 features, odds are good that you can do so in some compiler for many C++20 features sometime in 2020. But if you want to actually produce a product that's stable, it would be better to wait for compiler/library maturity until 2021 or 2022.

Visual Studio has a tendency to take longer to implement features than the other compilers. But generally, they take less time to implement library features, and will typically do so immediately upon shipping any dependent language features. By contrast, libc++ and libstdc++ tend to be much slower about getting library features done than their respective compilers about getting language features done.

Also for C++20, Microsoft has been pushing coroutines and modules hard, and they have the most mature implementations of both at present. So if that's what you're looking for, VS will likely have you covered more than the others.

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