Simple usage of C++20 modules

In my previous post I showed how to compile {fmt} as a C++20 module with clang. Although taking only two commands, ideally it’s not something you should be doing manually. So in this post, I’ll talk about module support in CMake, everyone’s favorite not a build system.

My first attempt was to use the CMake’s built-in functionality advertised in “import CMake; C++20 Modules”. And after some struggle I made it to work with clang but unfortunately it was very limited. Here are some of the problems and limitations that I discovered:

  1. It only worked with ninja and while I don’t have anything against this build system it’s an extra hassle to get this additional dependency installed while make is usually available by default. This restriction also likely means that you cannot use such a CMake config with IDEs.

  2. Native CMake support only worked with clang 16 while the fmt module can be built manually with clang 15.

  3. It required the latest version of CMake and a lot of ceremony to set up, including some incomprehensible things like

  4. There were issues in dynamic dependency extraction both in clang-scan-deps and CMake itself.

In general I think that it’s a noble goal to try addressing the general case of extracting dependency information from source files and some progress has been made in that direction. However, in the {fmt} case this is completely unnecessary because there is a single static module.

Then I remembered that back in 2018 I wrote, a simle CMake library for using modules in clang. It is amazing that clang had module support (somewhat different from C++20 modules) 5 years ago! I brought it up to date, simplified a bit and added gcc support.

Here’s an example of using this library: (module):


#include <cstdio>

export module hello;

export void hello() { std::printf("Hello, modules!\n"); } (module consumer):

import hello;

int main() { hello(); }

CMakeLists.txt (build config):

cmake_minimum_required(VERSION 3.11)
project(HELLO CXX)



target_link_libraries(main hello)

The main difference from the usual CMake code is that add_library is replaced with add_module_library defined in modules.cmake which adds a library together with module-specific build rules such as the ones for building .pcm files in clang.

And the usage is trivial:

$ CXX=clang++ cmake .
$ make
$ ./main
Hello, modules!

It works with clang 15+, gcc 12+ (but note that module support in gcc is incomplete), pretty much any version of CMake and any generator. The only limitation is that it assumes a static mapping between sources and modules.

I used add_module_library to add C++20 module support for clang in {fmt}. It is possible to build the fmt module with gcc as well but unfortunatley the result is not particularly usable yet. Hopefully the situation will improve in upcoming releases of gcc. But building simpler code such as the above example already works in gcc. MSVC has partial module support and thanks to Daniela Engert {fmt} already contains all the necessary workarounds for MSVC bugs. The only thing that is missing is integration into add_module_library which should be relatively straightforward.

Here’s an example of using {fmt} as a C++20 module with CMake.

import fmt;

int main() {
  fmt::print("Hello, modules!\n");


cmake_minimum_required(VERSION 3.11)
project(HELLO CXX)

target_link_libraries(hello fmt)

Building with clang:

CXX=clang++ cmake -DFMT_MODULE=ON .




Hello, modules!

As you can see using modules is very simple. The only minor wrinkle is that you need to disable clang extensions by setting CMAKE_CXX_EXTENSIONS to OFF. Otherwise everything just works.

Happy modularizing!

Last modified on 2023-04-17