Re: [boost] [config] Configuring for C++14 and later library features.

Subject: Re: [boost] [config] Configuring for C++14 and later library features.
From: Andrey Semashev (andrey.semashev_at_[hidden])
Date: 2015-04-24 08:41:48

On Friday 24 April 2015 13:32:36 John Maddock wrote:
> Folks,
> 
> I have an open PR for adding BOOST_NO_CXX14_INTEGER_SEQUENCE
> (https://github.com/boostorg/config/pull/58).
> 
> Unfortunately, the correct way to check for std::integer_sequence is to
> include <utility> and then check for __cpp_lib_integer_sequence.  The
> problem is that we went to some lengths to remove dependencies on
> <utility> in https://github.com/boostorg/config/pull/40.  Further, if we
> get requests for more such feature-detection macros, we would end up
> including the whole std lib (see
> http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2013/n3745#recs.cpp14).
> 
> This doesn't look acceptable to me, but what to do?  The only solution I
> can see is to simply not support these features in Boost.Config, but
> rather rely on users needing these features checking the std detection
> macros directly.  Which will work up until someone implements a feature
> without defining the appropriate macro :(
> 
> Thoughts?

You could just define the macro based on compiler version. That could be 
backed by a test which not only checks for __cpp_lib_integer_sequence but for 
std::integer_sequence itself.

PS: I thought it was enough to include any STL header (e.g. cstddef) to get 
all these macros defined. Isn't it the case? How would one test for a feature 
that is implemented in a new header?