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Subject: Re: [boost] [config] Rethinking feature macros?
From: Edward Diener (eldiener_at_[hidden])
Date: 2017-11-06 01:55:21

On 11/5/2017 8:15 PM, Peter Dimov via Boost wrote:
> 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.)

I brought up SD-6 for Boost Config some time ago, but the original
objection was that a great deal of the standard c++ header files would
have to be included by Boost Config to use most of SD-6. While I see in
your link that a number of the __cpp_... macros are predefined, like
your example __cpp_noexcept, a number of other __cpp_... macros require
header files to be included. So maybe Boost config can use macros which
are predefined without incurring the header file inclusion overhead of
those which require a header file to be included to determine if the
feature is present or not.

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