Re: [boost] [config] Positive feature macros, r2

Subject: Re: [boost] [config] Positive feature macros, r2
From: Peter Dimov (lists_at_[hidden])
Date: 2018-01-26 15:30:05

"P F" wrote:
> > #if defined(__cpp_foo) && __cpp_foo >= 201400
> > # define BOOST_CXX14_FOO
> > #endif
>
>
> I think it would be better to define the positive macros like this:
>
> #ifndef BOOST_CXX14_FOO
> #if defined(__cpp_foo) && __cpp_foo >= 201400
> # define BOOST_CXX14_FOO 1
> #else
> # define BOOST_CXX14_FOO 0
> #endif
> #endif
>
> This way:
>
> - We get a warning when checking the macro when it hasn’t been defined 
> properly
> - The user can easily override the value with `-DBOOST_CXX14_FOO=0` during 
> compilation because a feature turns out to be buggy.

As written, this does not allow the latter. The general case is:

// prefix.hpp:

#if defined(__cpp_foo) && __cpp_foo >= 201400
# define BOOST_CXX14_FOO
#endif

// compiler/bar.hpp:

#if __BAR_VERSION__ >= 12 && __BAR_VERSION__ < 18
// have foo, don't have feature macros yet
# define BOOST_CXX14_FOO
#endif

#if __BAR_VERSION__ == 21 && __cplusplus >= 201700
// bar++ version 21 has a bug in foo in -std=c++17 mode
# undef BOOST_CXX14_FOO
#endif

If you define to 0 or 1 in prefix.hpp, the information of whether the user 
overrode is lost, and the compiler-specific config can still stomp over the 
user define.

I think that Boost.Config already offers a few mechanisms for the user to 
override, they're not as simple as a define on the command line though.