Re: Interest for C++20 modules support of boost officially

From: Peter Dimov (pdimov_at_[hidden])
Date: 2024-12-22 12:10:16

Ruben Perez wrote:
> 3. Some libraries support configuration macros. For instance,
> Boost::throw_exception supports BOOST_NO_EXCEPTIONS. This is used by
> other Boost libraries to support builds that disable exceptions. In the current
> world, the end user defines BOOST_NO_EXCEPTIONS in their code, includes
> the library, and everything works. Using a scheme like the one I suggested
> requires the user to build all the Boost modules using
> BOOST_NO_EXCEPTIONS, which is more involved than what we have today.

BOOST_NO_EXCEPTIONS is not defined by the user. It's defined by
Boost.Config when -fno-exceptions or an equivalent is used.

Differing -std levels pose similar issues. E.g. mp11 defines some things only
when -std=c++17 or higher.

Not exported: https://github.com/boostorg/mp11/blob/43fd186a8eb5d2b3dcc04cde49092b5be2bc4085/include/boost/mp11/list.hpp#L61-L68

Would be exported: https://github.com/boostorg/mp11/blob/43fd186a8eb5d2b3dcc04cde49092b5be2bc4085/include/boost/mp11/list.hpp#L292

The user doesn't define any macros to enable this; as with -fno-exceptions,
it's detected automatically.

https://github.com/boostorg/mp11/blob/43fd186a8eb5d2b3dcc04cde49092b5be2bc4085/include/boost/mp11/detail/config.hpp#L129