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Subject: [boost] [config] Rethinking feature macros?
From: Peter Dimov (lists_at_[hidden])
Date: 2017-11-06 01:15:16

Now that there are standard feature-testing macros
( that
are being implemented by at least g++ and clang++, would it perhaps make
sense for us to reevaluate our decision to provide negative macros in
Boost.Config and start defining the standard feature macros instead on the
compilers that don't support them?

This would seem to require less maintenance, and the feature macro can be
used without waiting for Boost.Config to add it.

For a concrete example, let's take noexcept in function types. This is
__cpp_noexcept_function_type, and is implemented by g++ 7, clang 4, clang 5
(in C++17 mode), and apparently in the latest VS2017 preview.

noexcept function pointers break Boost.Bind, and to fix it, I need to add
overloads for them, but only if they are implemented, otherwise the
overloads would be an error.

With the feature macro, I can just #ifdef __cpp_noexcept_function_type and
it will immediately work on g++ and clang++ and all compilers that don't
support noexcept function types will still work. Only msvc would need to be
fixed in some way.

With our traditional approach, I would need to request the addition of
BOOST_NO_CXX17_NOEXCEPT_FUNCTION_TYPE, wait for it to be added and to be
present on every compiler except the latest ones (which requires changes
throughout Boost.Config), and only then be able to use #ifndef
BOOST_NO_CXX17_NOEXCEPT_FUNCTION_TYPE. (Then wait for it to be merged to
master before merging my changes to master.)

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