C++20 Modules with CLion and CMake

I’ve been working through Crafting Interpreters in C++20, so I tried out using the new modules. There were a couple of unintuitive things with using modules with CLion and CMake, so I’ll document them here so that it may help future me (and maybe other people too).

Skip to finished CMakeLists.txt

Creating a module

Create a CLion C++ Executable project with language standard C++ 20.

Create a new module by selecting New -> C++ Module Interface Unit. Give it a name, and leave “Add to targets” checked.

Then, fill out your module:

// carrot.ixx
export module carrot;

export class Carrot {
public:
    static constexpr int VITAMIN_D = 10'191;
};

Import your module from main.cpp and use it:

// main.cpp
#include <iostream>

import carrot;

int main() {
    std::cout << Carrot::VITAMIN_D << std::endl;
    return 0;
}

When you run the executable, you’ll see that the build errors! You’ll get a message like:

CMake Error: Output CMakeFiles/demo.dir/carrot.ixx.o provides the `carrot` module but it is not found in a `FILE_SET` of type `CXX_MODULES`

Even though we had CLion add our file to the CMake build target, CMake isn’t satisfied. Because we created a module interface unit (a file that exports a module), we’ll need to add it to a special file set.

A modules file set

Here is what our project’s CMakeLists.txt has by default:

cmake_minimum_required(VERSION 3.30)
project(demo)

set(CMAKE_CXX_STANDARD 20)

add_executable(demo main.cpp
        carrot.ixx)

Underneath that, add a file set of type CXX_MODULES and add the module interface unit (carrot.ixx) to it:

target_sources(demo
        PUBLIC
        FILE_SET modules TYPE CXX_MODULES FILES
        carrot.ixx)

Run the executable again, and the build should succeed.

Creating a file glob for module interface units

This configuration allowed us to add one module interface unit to our build, but each time we create a new one, we’ll need to manually add it, which would get annoying.

To resolve this, we can create a file glob for module interface units:

file(GLOB_RECURSE MODULE_INTERFACE_UNITS "*.ixx")

With this, we can replace the hard-coded list of files with the file glob:

target_sources(demo
        PUBLIC
        FILE_SET modules TYPE CXX_MODULES FILES
        ${MODULE_INTERFACE_UNITS})

Finished CMakeLists.txt

Here is what my finished CMakeLists.txt looks like (I’ve added an apple.ixx module to verify that it works):

cmake_minimum_required(VERSION 3.30)
project(demo)

set(CMAKE_CXX_STANDARD 20)

add_executable(demo main.cpp
        carrot.ixx
        apple.ixx)

file(GLOB_RECURSE MODULE_INTERFACE_UNITS "*.ixx")
target_sources(demo
        PUBLIC
        FILE_SET modules TYPE CXX_MODULES FILES
        ${MODULE_INTERFACE_UNITS})