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C++20 module support #105

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C++20 module support #105

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fc52022
Install CXX_MODULES
anarthal Dec 22, 2024
3d6703b
Helper function for C++20 modules
anarthal Dec 22, 2024
ca5e12b
Warning suppression for MSVC
anarthal Dec 22, 2024
0f5e4ac
Install modules to datadir
anarthal Dec 30, 2024
f412580
Rename config macro to BOOST_USE_MODULES
anarthal Dec 30, 2024
17b3387
Remove common function for modules
anarthal Jan 10, 2025
6d4c13c
Install modules
anarthal Feb 26, 2026
cbbb162
Merge branch 'feature/cxx20-modules' of github.com:anarthal/boost-cma…
anarthal Feb 27, 2026
eef66a8
Merge branch 'develop' into feature/cxx20-modules
anarthal Mar 5, 2026
afbc1f8
Fix older cmake version
anarthal Mar 5, 2026
399ade9
Modules docs 1
anarthal Mar 5, 2026
f5763b8
Modules docs 2
anarthal Mar 5, 2026
29ef890
Running the test suite
anarthal Mar 5, 2026
0df3ae6
Compiled libs 1
anarthal Mar 7, 2026
b757c78
src headers
anarthal Mar 7, 2026
bece0c7
gcc15 workarounds
anarthal Mar 7, 2026
6acac98
Rephrasing
anarthal Mar 7, 2026
4aa1151
some links
anarthal Mar 7, 2026
13ce5d2
inlines
anarthal Mar 7, 2026
a5bf8c8
proper links
anarthal Mar 7, 2026
e29bc2a
expand on initializers
anarthal Mar 7, 2026
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7 changes: 7 additions & 0 deletions include/BoostInstall.cmake
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Expand Up @@ -337,13 +337,20 @@ function(boost_install_target)
string(APPEND CONFIG_INSTALL_DIR "-static")
endif()

# C++20 modules supported since CMake 3.28
set(__BOOST_INSTALL_FILE_SET_ARGS)
if(CMAKE_VERSION VERSION_GREATER_EQUAL 3.28)
set(__BOOST_INSTALL_FILE_SET_ARGS FILE_SET CXX_MODULES DESTINATION "${CMAKE_INSTALL_DATADIR}")
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@fdr400 fdr400 May 23, 2026

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Hi @anarthal ! I believe that it is also required to set CXX_MODULES_BMI DESTINATION "" to not install the BMI (because it is compiler/version specific and should be build from module interface unit and public headers)

as CMake documentation says

Any module files from C++ modules from PUBLIC sources in a file set of type CXX_MODULES will be installed to the given DESTINATION. All modules are placed directly in the destination as no directory structure is derived from the names of the modules. An empty DESTINATION may be used to suppress installing these files (for use in generic code).

I am not pretty sure whether BMI is installed with unset CXX_MODULES_BMI but explicit option specification seems more straightforward way

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@ClausKlein ClausKlein May 25, 2026

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The CXX_MODULES_BMI can be omitted, it is as wrong designed install option! The installed BMI files are never used from CMake!

endif()

install(TARGETS ${LIB} EXPORT ${LIB}-targets
# explicit destination specification required for 3.13, 3.14 no longer needs it
RUNTIME DESTINATION "${CMAKE_INSTALL_BINDIR}"
LIBRARY DESTINATION "${CMAKE_INSTALL_LIBDIR}"
ARCHIVE DESTINATION "${CMAKE_INSTALL_LIBDIR}"
PRIVATE_HEADER DESTINATION "${CMAKE_INSTALL_INCLUDEDIR}"
PUBLIC_HEADER DESTINATION "${CMAKE_INSTALL_INCLUDEDIR}"
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@fdr400 fdr400 May 24, 2026 •
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Does install actually work without specifying PUBLIC FILE_SET HEADERS with all library headers explicitly listed their and adding FILE_SET HEADERS DESTINATION ${CMAKE_INSTALL_INCLUDEDIR} in install command?
I am not very experienced in modules installation but I guess that it is required for approach when module may be imported after library headers preprocessed. Probably, FILE_SET HEADERS with specified only BASE_DIRS
@ClausKlein I saw your PRs with attempts of Boost libraries CMake installation with modules support and FILE_SET HEADERS was specified their. Can you please clarify how it actually should be done

Also such installation approach won't work for both headers and modules users simultaneously if Boost library is not header-only. Because installed .a or .so library will contain symbols attached to the target module (with @<module_name> suffix), so headers users (who did not specified BOOST_USE_MODULES) can't link with modules-aware compiled library, and vice versa modules users can't link with non-modules-aware compiled library.
And I can not actually imagine how such libraries should be provided for, for instance, Ubuntu debian packages to be available for header-based and modules-based users. Possibly, only if two packages are provided for each, but such a way all headers are duplicated in them.
Maybe, for CMake install both module and non-module library should be compiled and installed somehow?

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@ClausKlein ClausKlein May 25, 2026

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FILE_SET HEADERS are the modern way to build and install headers and should be used.

${__BOOST_INSTALL_FILE_SET_ARGS}
)

export(TARGETS ${LIB} NAMESPACE Boost:: FILE export/${LIB}-targets.cmake)
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3 changes: 3 additions & 0 deletions include/BoostRoot.cmake
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Expand Up @@ -303,6 +303,9 @@ macro(__boost_add_header_only lib)

endmacro()

# C++20 modules
option(BOOST_USE_MODULES "Build Boost as a collection of C++20 modules (unstable)" OFF)

#

file(GLOB __boost_libraries RELATIVE "${BOOST_SUPERPROJECT_SOURCE_DIR}/libs" "${BOOST_SUPERPROJECT_SOURCE_DIR}/libs/*/CMakeLists.txt" "${BOOST_SUPERPROJECT_SOURCE_DIR}/libs/numeric/*/CMakeLists.txt")
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