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

I was not able to find Cmake support for modules. Here is an example how to use modules using clang. I am using Mac and this example works ok on my system. It took me quite a while to figure this out so unsure how general this is across linux or Windows.

Source code in file driver.cxx

import hello;
int main() { say_hello("Modules"); } 

Source code in file hello.cxx

#include 
module hello;
void say_hello(const char *n) {
  std::cout << "Hello, " << n << "!" << std::endl;
}

Source code in file hello.mxx

export module hello;
export void say_hello (const char* name);

And to compile the code with above source files, here are command lines on terminal

clang++ \
  -std=c++2a                        \
  -fmodules-ts                      \
  --precompile                      \
  -x c++-module                     \
  -Xclang -fmodules-embed-all-files \
  -Xclang -fmodules-codegen         \
  -Xclang -fmodules-debuginfo       \
  -o hello.pcm hello.mxx

clang++ -std=c++2a -fmodules-ts -o hello.pcm.o -c hello.pcm

clang++ -std=c++2a -fmodules-ts -x c++ -o hello.o \
  -fmodule-file=hello.pcm -c hello.cxx

clang++ -std=c++2a -fmodules-ts -x c++ -o driver.o \
  -fmodule-file=hello=hello.pcm -c driver.cxx

clang++ -o hello hello.pcm.o driver.o hello.o

and to get clean start on next compile

rm -f *.o
rm -f hello
rm -f hello.pcm

expected output

./hello
Hello, Modules!

Hope this helps, all the best.