Re: improve <boost/config.hpp>

From: John Maddock (jz.maddock_at_[hidden])
Date: 2023-03-07 18:11:03

> Not every compiler defines these macros. Let alone, not every compiler
> properly implements what it advertises.
I think it depends on the compiler - the current crop of the latest are 
pretty good at getting these right.
> 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.
Sort of, you know that Boost.Config uses <version> wherever possible 
already?  It does so even in non-C++20 mode since the header is just a 
bunch of #defines and is perfectly usable back to C++98 if it exists.
>
> So no, feature macros do not replace Boost.Config. In fact, I'd prefer
> to continue using Boost.Config in new libraries.

There are still plenty of things Boost.Config is good for: particularly 
where getting the configuration correct is tricky - for example if you 
want to use <header> then you need to know both that it exists via 
__has_include, but also that the current language standard is sufficient 
to parse the header, and that it has been been fully implemented.

On the other hand, for the suggested BOOST_NO_CXX20_CONSTEXPR, 
Boost.Config would implement this trivially as (__cpp_constexpr < 
201907L) which is a less obvious win - except perhaps to save folks 
having to look up the value - which now I come to think about it, might 
be a win after all given that it's value is not 202002L ;)

John.