On 3/7/23 18:22, Jeff Garland via Boost wrote:

On Fri, Mar 3, 2023 at 8:04 AM Alexander Grund via Boost < boost@lists.boost.org> wrote:

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But I don't understand why you are against it. After all, there is BOOST_CXX14_CONSTEXPR (BOOST_CXX17/20_CONSTEXPR are still missing - but I already offered that). So we have an expressive scheme BOOST_CXXnn_CONSTEXPR that is unambiguous and readable.

C++14 lifted some major constraints on what can be in a constexpr function compared to C++11. Most notably support for more than just a return statement. Being able to differentiate that is useful in practice.

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For example, std::round in C++23 is constexpr

The CXXyy part in the Boost.Config macros refers to the language level support. While std::round is at library level. So a compiler supporting C++14 constexpr syntax might not have a std library where std::round is constexpr even if it claims C++23 support. So the better way here would be a more focused macro combining the language level of the constexpr feature and the library support of what that specific function is using.

Hence a possible BOOST_CXX23_CONSTEXPR is misleading: It doesn't refer to the language level anymore. And if you bind that to availability of constexpr std::round the next user may want to use it with another std:: function which is supposed to be constexpr in C++23 but due to a std library defect is not yet. Hence the distinction between library level and language level feature macros.

Please see: https://en.cppreference.com/w/cpp/feature_test for language level macros -- look for _cpp_constexpr. I believe you'll discover that every combination you're looking for going back to cxx 2011 is represented -- basically making a boost config macro extraneous.

Not every compiler defines these macros. Let alone, not every compiler properly implements what it advertises. Also, library-defined macros are worthless as they only allow one to know the feature is/isn't supported after including the header, i.e. after the fact. C++20 <version> fixes this, but C++20 is a too high minimum bar to target. So no, feature macros do not replace Boost.Config. In fact, I'd prefer to continue using Boost.Config in new libraries.