[SOLVED] C++20 Reuse already compiled modules?

Issue

My goal is to reuse some already generated c++20 modules in one project into another.

I tried the following rel="nofollow noreferrer">github repo and module usage worked perfectly.

First project generating the module files:

I decided to change the Cmake structure to this:

cmake_minimum_required(VERSION 3.26)

set(APP_NAME CppModules)

project(${APP_NAME} LANGUAGES CXX)

set(CMAKE_EXPERIMENTAL_CXX_MODULE_CMAKE_API "2182bf5c-ef0d-489a-91da-49dbc3090d2a")

set(CMAKE_EXPERIMENTAL_CXX_MODULE_DYNDEP 1)

set(CMAKE_CXX_STANDARD 23)
set(CMAKE_CXX_EXTENSIONS Off)


add_executable(${APP_NAME} main.cpp)

add_library(my_lib2)
target_sources(my_lib2
  PUBLIC
    FILE_SET cxx_modules TYPE CXX_MODULES FILES
    src/my_lib/my_lib2.cpp
)
add_library(my_lib)
target_sources(my_lib
  PUBLIC
    FILE_SET cxx_modules TYPE CXX_MODULES FILES
    src/my_lib/my_lib.cpp
)

target_link_libraries(${APP_NAME} PRIVATE my_lib my_lib2 )

set_target_properties(${APP_NAME} PROPERTIES LINKER_LANGUAGE CXX)

Now, I have some .pcm files and some {my_lib, mylib2}.a files that have been generated.

Second project trying to include:

This is the structure of the second project trying to import the module:

import my_lib;

int main() {
    foo f;
    f.helloworld();
    return 0;
}

But that program complains with

fatal error: module 'my_lib' not found

even if I have the following cmake file:

cmake_minimum_required(VERSION 3.26)

set(APP_NAME Importer)

project(${APP_NAME} LANGUAGES CXX)

set(CMAKE_EXPERIMENTAL_CXX_MODULE_CMAKE_API "2182bf5c-ef0d-489a-91da-49dbc3090d2a")

set(CMAKE_EXPERIMENTAL_CXX_MODULE_DYNDEP 1)

set(CMAKE_CXX_STANDARD 23)
set(CMAKE_CXX_EXTENSIONS Off)


add_executable(${APP_NAME} main.cpp)

target_link_libraries(${APP_NAME} PRIVATE /absolute/path/to/libmy_lib.a)

set_target_properties(${APP_NAME} PROPERTIES LINKER_LANGUAGE CXX)

so I miss a way to inject the previously generated module into that project.

Question is, how may I do ? What is the missing bit ?

Solution

See /reference compiler parameter for MSVC (https://learn.microsoft.com/en-us/cpp/build/reference/module-reference?view=msvc-170). I don't think there is something special in CMake, so you have to set up it manually, like this:

target_compile_options(${PROJECT_NAME} PUBLIC /reference "mymodule.ifc")

As far as I know, module ABI is not stable - it differs for debug and release and for different compilers, maybe even for different compiler versions / MSVC toolkit versions

Answered By - Perl99

Answer Checked By - Gilberto Lyons (WPSolving Admin)