On Thu, 19 Dec 2024, 15:44 John Maddock, <jz.maddock@googlemail.com> wrote:

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If I understood the CMake developers and Chuanqi correctly (please let me know if I didn't), you do install a library with the generated object files, but you do not install the BMIs, as they're like pre-compiled headers.

Let me explain how I understood it with two concrete examples. Take Boost.Url as an example of a compiled library, and Boost.Regex as an example of a header-only library.

1. Boost.Url today installs: * libboost_url.a, containing function definitions. * Headers, containing declarations. 2. Boost.Url in the module world would install: * libboost_url.a, containing the same function definitions as above. * A boost_url.cppm file, akin to what headers are today. CMake would build a BMI from this file when a user needs to import boost.url. In practice, this cppm file will likely be implemented in terms of the header files that we have today. 3. Boost.Regex today installs: * Headers, containing declarations and definitions. 4. Boost.Regex in the module world would install: * A libboost_regex.a object, containing module initializers [1]. For instance, if you have a singleton defining a Boost.System error category, initialization code for the singleton would be emitted here. From the docs, it seems this object gets created even if no initialization code is required. * A boost_regex.cppm file, containing the bulk of the library, akin to what we provide using headers today.

My intention is doing some testing to see how much trouble would this new libboost_regex.a library cause.

I'm not quite convinced it's that simple, at it's most basic, consider an inline function that is part of the module interface, if I compile the module's object library for release, and then do a debug build of the bmi and my project what happens? Even if clang makes that example work out in msvc land where almost every option creates binary compatibility issues, I see users compiling the object library from source on a per-project basis.

I'd expect the inline function to be part of the BMI (which gets built by the user), and not part of the object file, thus not creating trouble. Of course, I could be wrong, so I'll try it. Regards, Ruben.