Feature/attach to gmf

[DanielaE]

No description provided.

[vitaut]

This overlaps with #3386. Did you mean to include only the first commit in this PR?

[DanielaE]

I pushed only the second as a PR from the Github UI. I.e. the changes to fmt.cc.

[vitaut]

Please rebase.

[DanielaE]

Done.

[vitaut]

Please provide some details why this change is needed.

[DanielaE]

If you build {fmt} as a module, you're basically compiling it as a static library that happens to export its interface in a binary form called BMI. All declarations and definitions are attached to to the module, all signatures and symbols carry an additional part that says so. In msvc this is a suffix <!fmt>, clang and gcc have something similar now that they both are following msvc's example and have switched over to the so-called 'strong module ownership' model.

Suppose you have a program in transition: parts are using {fmt} in module form, the rest is still using it through #includes. This means you have one set of declarations and symbols (all of them!) attached to the module, and another, additional set (brought in by the #includes) attached to the so-called global module. Both can coexist in the same program because their signatures and symbols are different. But you pay for all those duplications.

If you define macro FMT_ATTACH_TO_GLOBAL_MODULE, all declarations are subjected to explicit language linkage specification "extern C++". This detaches them from the module, the additional symbol name parts don't exist and the module behaves like a static library build in the traditional sense. The private module partition provides all the definitions of entitites declared out-of-line.

[DanielaE]

I have this feature in my Asio module to support exactly this kind of scenario.

[vitaut]

Makes sense, thanks for the explanation. Please add a short comment clarifying this before the FMT_ATTACH_TO_GLOBAL_MODULE check.

[vitaut]

It looks like this comment applies to FMT_ATTACH_TO_GLOBAL_MODULE which is not right. Should it be moved some place else?

[vitaut]

Merged, thanks!