I'm also not quite clear on how things will work on the CMake side. I can envisage the add_subdirectory case working, more or less, but what would (or needs to) happen when Boost is _installed_ via CMake, with module support enabled?
And how will it then be consumed on the user side, after find_package?
I think this may be the most difficult part right now. Last time I checked (as described in the cited articles), the workflow with CMake was something like: a) You add_library() and target_souces() using the new FILE_SET CXX_MODULES feature. This builds a library (e.g. libboost_asio.a) and a BMI. Note that even previously header-only libraries end up generating compiled libraries in the modules world. The compiled library contains code for global initializers, and is emitted even if there are no global initializers [1]. b) You install() the library with its module bindings. This means that you end up installing libboost_asio.a and any other files required to generate the BMI for the Asio module (in this case, this would be Asio headers). If the module contained implementation units (somehow akin to .cpp files in today's world - apologies if the comparison is not exact), these shouldn't get installed. c) The user calls find_package() on the installed package, and calls target_link_libraries() linking to the library installed in the previous step. When doing this, CMake rebuilds the BMI, but not the object files (libboost_asio.a). The BMI will be built with the same cxxstd (I don't know about other properties) as the original library. This is the point that generates questions for me, as it imposes many more restrictions than the traditional header model [2]. On the other hand, the CMake team seems to be working on enhancing this, but without an ETA [3].
Also, we're basically talking Clang-only here, right? GCC doesn't work and MSVC... may or may not work?
The approach where you #ifdef out standard includes and conditionally export entities applies to both MSVC and clang. gcc has had experimental support for modules for a couple of releases. libstdc++ has added support for the std module recently [4], but it hasn't been released yet. I'm going to experiment with Boost.CMake and modules to see if we could produce an installed Boost containing only the libraries that support modules. And see how much trouble point c) would cause us. I'm particularly worried about libraries like Asio, that radically change based on macros. [1] https://clang.llvm.org/docs/StandardCPlusPlusModules.html#module-initializer... [2] https://crascit.com/2024/04/04/cxx-modules-cmake-shared-libraries/ [3] https://discourse.cmake.org/t/advice-on-c-20-modules-boost/10641/5 [4] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106852