How to use c++20 modules with CMake?

Question

Clang and MSVC already supports Modules TS from unfinished C++20 standard. Can I build my modules based project with CMake or other build system and how?

I tried bui

Answer 1

This works on Linux Manjaro (same as Arch), but should work on any Unix OS. Of course, you need to build with new clang (tested with clang-10).

helloworld.cpp:

export module helloworld;
import ;
export void hello() { puts("Hello world!"); }

main.cpp:

import helloworld;  // import declaration

int main() {
    hello();
}

CMakeLists.txt:

cmake_minimum_required(VERSION 3.16)
project(main)

set(CMAKE_CXX_STANDARD 20)
set(CMAKE_CXX_STANDARD_REQUIRED ON)
set(CMAKE_CXX_EXTENSIONS OFF)
set(PREBUILT_MODULE_PATH ${CMAKE_BINARY_DIR}/modules)

function(add_module name)
    file(MAKE_DIRECTORY ${PREBUILT_MODULE_PATH})
    add_custom_target(${name}.pcm
            COMMAND
                ${CMAKE_CXX_COMPILER}
                -std=c++20
                -stdlib=libc++
                -fmodules
                -c
                ${CMAKE_CURRENT_SOURCE_DIR}/${ARGN}
                -Xclang -emit-module-interface
                -o ${PREBUILT_MODULE_PATH}/${name}.pcm

            )
endfunction()


add_compile_options(-fmodules)
add_compile_options(-stdlib=libc++)
add_compile_options(-fbuiltin-module-map)
add_compile_options(-fimplicit-module-maps)
add_compile_options(-fprebuilt-module-path=${PREBUILT_MODULE_PATH})

add_module(helloworld helloworld.cpp)
add_executable(main
        main.cpp
        helloworld.cpp
        )
add_dependencies(main helloworld.pcm)