Re: [boost] [config] Rethinking feature macros?

Subject: Re: [boost] [config] Rethinking feature macros?
From: Steven Watanabe (watanabesj_at_[hidden])
Date: 2017-11-06 01:48:24

AMDG

On 11/05/2017 06:15 PM, Peter Dimov via Boost wrote:
> Now that there are standard feature-testing macros
> (http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/p0096r5.html)
> 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?
> 

I think it's probably a bad idea for Boost.Config
to try to define the standard feature macros.
- #defining third-party macros in library code is a recipe for
  ODR violations.
- It's easy for those using compilers with support for these
  macros to forget to #include <boost/config.hpp>.  There's
  also a good chance that the automated tests will pass for
  compilers without such support in this case, as it's quite
  common for the fallback implementation to work even if the
  feature is actually present.  (Or the corresponding tests
  might just end up being disabled as well.)

> 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.)
> 

In Christ,
Steven Watanabe