CMake only build lib when compiler supports C 20 or higher-CodePudding

in our project we are using the highest available C and CXX standard by setting

  set(CMAKE_C_STANDARD 17)
  set(CMAKE_CXX_STANDARD 20)

However the project is also build with some old compilers that do not support C 20.

Some libs on the other hand require C 20. How can i configure my project so that these libs are only build if the compiler supports C 20? (I know i could use #ifdef __cplusplus >= ... but I'm trying to avoid #ifdefs ;))

Thx for your help :)

CodePudding user response：

Well, the variable name speaks for itself. https://cmake.org/cmake/help/latest/variable/CMAKE_CXX_STANDARD_REQUIRED.html

set(CMAKE_CXX_STANDARD_REQUIRED YES)

You should prefer set_target_properties.

My expectation is that old compilers automatically skip building the c 20 lib

So do not add target if we don't have C 20.

if(NOT "cxx_std_20" IN_LIST CMAKE_CXX_COMPILE_FEATURES)
   add_library(the_lib_to_skip ...)
endif()

CodePudding user response：

CMakeLists of libraries should only specify usage requirements.

For example if your library requires at least C 11 standard at build time & consume time:

target_compile_features(mylib PUBLIC cxx_std_11)

With this compile feature, by default CMake takes care to inject proper compilation flags so that this target is compiled with a C standard greater or equal than C 11 (it depends on compiler and its version).

Do not hardcode CMAKE_CXX_STANDARD in CMakeLists of a library, because it prevents users to compile with a different compatible C standard (and it's not uncommon to want to compile all libraries of a dependency graph with the same C standard).

Then if you want to build this lib with a very specific standard, you can inject CMAKE_CXX_STANDARD externally during CMake configuration:

cmake -S <source_dir> -B <build_dir> -DCMAKE_CXX_STANDARD=20

=============================================

And now that you have clarified your question:

add_library(mylib1 ...)

# only build lib2 if compiler supports C  20 standard
if("cxx_std_20" IN_LIST CMAKE_CXX_COMPILE_FEATURES)
    add_library(mylib2 ...)
    target_compile_features(mylib2 PUBLIC cxx_std_2O)
endif()

I would suggest to display a summary at the end of your configuration, because it can be quite surprising for users to silently disable the build of several libs.