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

From: Ruben Perez (rubenperez038_at_[hidden])
Date: 2025-01-13 19:18:18

On Mon, 13 Jan 2025 at 14:01, Peter Dimov <pdimov_at_[hidden]> wrote:
>
> Ruben Perez wrote:
> >       In the general case (when a header also defines macros), the public
> >       header will contain both
> >
> >       import boost.mp11;
> >
> >       and the part that defines the macros, which in Mp11's case is
> >
> >       #include <boost/mp11/version.hpp>
> >
> >       User code wouldn't need to know that in addition to the import,
> >       it also needs to include a separate macro header.
> >
> >
> >
> > This assumes that macros are "standalone" and don't require including any
> > stdlib headers. This is the case for mp11/version.hpp and lightweight test, but
> > is not for Boost.Config or Boost.Charconv headers, for instance. These may
> > require including <cstddef> or <cstdint> for macros, which need to happen
> > before any import std. Wouldn't this cause trouble with your scheme?
>
> Possibly, yes. If we want to keep the public headers usable in a module unit,
> we can't include <cstddef> there because it will introduce declarations.
>
> Although whether we need to keep the headers usable in a module unit
> is up for debate.
>
> Fixing this properly can only be done on the stdlib/implementation side.
>
>

I think this applies for non module units, too. I've just tested that,
under MSVC, the following errors:

// File: main.cpp
import std;
#include <cstddef>
int main() {}

This does not seem to happen with all the headers, though.
* version, cstdint, climits, cassert, cfloat, cstdio, cstdlib, cerrno
seem to work fine
* cmath and cstddef seem to error.

So if we have in a program:

// File: main.cpp
#include <boost/mp11.hpp>
#include <boost/charconv.hpp>
int main() {}

Under modular builds, this would be roughly equivalent to:

// From boost/mp11.hpp
#include <cassert> // insert here any headers required by mp11 macros
#include <boost/mp11/detail/config.hpp> // the actual macro definitions
import std; // generated by the standard header proxies you propose
import boost.mp11; // generated by mp11 own headers

// From boost/charconv.hpp
#include <cmath> // insert here any headers required by charconv
macros - this errors
#include <boost/charconv/detail/config.hpp> // the actual macro definitions
import std; // generated by the standard header proxies you propose
import boost.charconv; // generated by charconv own headers

int main() {}

We can probably trim down most of the required stdlib headers so this
is not a problem. Or just ifdef-out standard headers without the proxy
headers that expand to import std.