Re: On Boost and modules

From: Boris Kolpackov (boris_at_[hidden])
Date: 2024-04-09 18:07:55

Andrey Semashev via Boost <boost_at_[hidden]> writes:

> On 4/9/24 17:33, Boris Kolpackov wrote:
> 
> > export namespace n
> > {
> >   __symexport void f ();
> > 
> >   class __symexport C
> >   {
> >     ...
> >   };
> > }
> 
> Thanks for the example. So, if I understood correctly, this is the same
> as what we do now: define a macro for symbol markup that expands to
> either dllexport or dllimport, depending on whether the library is being
> compiled or consumed.

It is the same mechanism (a macro) but you only need to ever define
it to dllexport, never to dllimport. This allows a build system to
define this macro automatically. In the above example, you as the
library author never need to worry about defining __symexport or
any other macros that would ordinarily be defined during the build
to distinguish between static/shared and building/consuming. In
other words, you can treat __symexport as a pseudo-keyword for
exporting symbols from shared libraries that "just works".

 
> I'm assuming this is ok to have `f()` and members of `C` not defined in
> the module?

Yes, that doesn't matter. Here is an example:

https://github.com/build2/cxx20-modules-examples/blob/named-only-import-std/hello-library-module/libhello-module/libhello/hello.mxx#L9-L10

https://github.com/build2/cxx20-modules-examples/blob/named-only-import-std/hello-library-module/libhello-module/libhello/hello.cxx#L5-L12

This works with both MSVC and Clang on Windows.

> Also, will this work if the compiled library itself is built without
> modules?

I haven't personally tried to dllexport a using-declaration but I assume
that's how the MSVC folks modularized their standard library so presumably
it works, at least in MSVC. Maybe Stephan can confirm?