Discussion: upgrading to C++17

[dongcarl]

UPDATE: Release plan summary from April 2nd, 2020 meeting:

	C++11 Compat	C++17 Compat	Gitian/Guix Release
v0.20	✔️	if easy	C++11
v0.21	✔️	✔️	C++17
v0.22	❌	✔️	C++17

Meeting logs can be found here: http://www.erisian.com.au/meetbot/bitcoin-core-dev/2020/bitcoin-core-dev.2020-04-02-19.01.log.html

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There were some discussion around upgrading to C++17. It seems to be able to give us library and language features that will simplify future code. For example, for library features, our current optional.h is just a wrapper around boost::optional, and can be replaced with std::optional. Other library features that might be useful include (non-exhaustively): std::variant, the file system library, and std::shared_mutex. @elichai has kindly provided a table summarizing features useful for Bitcoin Core here: #16684 (comment).

I want to apologize for misspeaking about C++17 requiring GCC8, that is untrue, and I believe all of the features we would want will be covered by GCC7, and some by GCC6.

For some background on the cons: #13356 (comment)

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Default GCC versions major distros (as of Aug 22nd):

	Operating System	release version	GCC version	Notes
✔️	Debian	Buster	8.3
✔️	Ubuntu	18.04 LTS	7.4
✔️	Fedora	30	9.1.1
✔️	RHEL	8	8.2
	CentOS	7	4.8.5	CentOS 8 (GCC 8.2) is being released as we speak

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In-depth code examples of

C++17 features: https://github.com/AnthonyCalandra/modern-cpp-features/blob/master/CPP17.md
C++14 features: https://github.com/AnthonyCalandra/modern-cpp-features/blob/master/CPP14.md

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Here are some compatibility tables from https://en.cppreference.com/w/cpp/compiler_support#cpp17

Library features:

Language features:

[practicalswift]

Some C++14 features worth mentioning as well: std::make_unique, more expressive constexpr functions allowed, return type deduction (auto foo() { … }), [[deprecated]], binary literals (0b10101110) and polymorphic lambdas ([](auto foo) { … }).

[elichai]

A table on features in C++14/17 which I think would be very useful (feel free to comment if you disagree or think other stuff should be added)

Name	Important	Nice to have	Boost Replacement
std::variant			✔️
std::make_unique		✅
Variable Templates		✅
Conditions in constexpr functions		✔️
auto in lambdas params		✅
Binary literals		✔️
std::filesystem	✔️		✔️
std::optional			✔️
[[nodiscard]]+[[fallthrough]]		✔️
de-structuring initialization		✅
shared_mutex + scoped_lock + shared_lock		✅	✔️

[elichai]

Another reason we might want to upgrade:
very powerful const expressions. both c++14 and 17 made constexpr way more powerfull.
such that we could start making a lot of functions constexpr which might maybe result in some performance increase (for functions that might be able to be evaluated at compile time)

[maflcko]

Is C++17 supported on Ubuntu Xenial, Debian 9, and the *BSD operating systems?

[ysangkok]

Debian 9 ships GCC 6.3 according to the development packages listing. GCC 6 is listed as supporting some features of C++17 experimentally.

Ubuntu Xenial ships with a default GCC 5.3, which is even older. A PPA offers newer GCC versions: ~ubuntu-toolchain-r

[laanwj]

Added 0.21 milestone optimistically

[sanjaykdragon]

I agree with moving to C++17, but when do we plan to move to the soon coming C++20?

[sipa]

@sanjaykdragon It's not that easy, as we need to remain compatible with various stable Linux distributions, which don't update their C++ compilers that frequently (or at all, after release of a particular distro version).

C++20 does not exist yet, so that seems entirely out of scope.

[sanjaykdragon]

@sanjaykdragon It's not that easy, as we need to remain compatible with various stable Linux distributions, which don't update their C++ compilers that frequently (or at all, after release of a particular distro version).

C++20 does not exist yet, so that seems entirely out of scope.

Are we just waiting for linux distributions to become compatible with C++17 for a migration?