cmake 3.28 and importing C++ headers in gcc

Code
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1

Hi,

I’m trying to create a CMakeLists.txt that more or less will do what the following Makefile does:

CXX=a path to gcc14 built from master 
MODULE_FLAGS = -fmodules-ts -flang-info-module-cmi

make_module_of_vector:
    $(CXX) -std=c++20 ${MODULE_FLAGS} -x c++-system-header vector

foo.o: foo.cxx
    $(CXX) -std=c++20 ${MODULE_FLAGS} -c foo.cxx

mystd.o: mystd.cxx
    $(CXX) -std=c++20 ${MODULE_FLAGS} -c mystd.cxx

all: main.cxx, mystd.o foo.o
    $(CXX) -std=c++20 ${MODULE_FLAGS} main.cxx foo.o mystd.o

The contents of each file are along the following lines:
mystd.cxx:

module;
export import <vector>;
export module mystd;

foo.cxx:

module;
#include <iostream>
export module foo;
// etc

main.cxx:

import foo;
import mystd;

int main() {
...
}

Although it is a toy example (and probably incorrect) it compiles and runs fine. I tried to achieve the same results by writing a CMakeLists.txt and use cmake 3.28, but I failed. Is this doable with cmake 3.28/gcc14 (master) ?

Thank you

2

It would be useful to know, what exactly has failed and with what error messages.
If you shared your current CMake project, it would be helpful too.

3

Did you read the recent blog post https://www.kitware.com/import-cmake-the-experiment-is-over/, about C++ module usage?

4

Hi Eric,

Thanks for taking an interest in my question. Yes I’ve read the post you mentioned and it mentions that only named modules are supported. I think it doesn’t support my use case although I may be wrong.

5

Hi retif,

initially I used the following CMakeLists.txt:

cmake_minimum_required(VERSION 3.28)
project(std_module_example CXX)

# Turning off extensions avoids and issue with the clang 16 compiler
# clang 17 and greater can avoid this setting
set(CMAKE_CXX_EXTENSIONS OFF)
# Set the version of C++ for the project
set(CMAKE_CXX_STANDARD 20)
# Create a library
add_library(foo)
# Add the module file to the library
target_sources(foo
  PUBLIC
    FILE_SET CXX_MODULES FILES
      foo.cxx
)
# Create an executable
add_executable(hello main.cxx)
# Link to the library foo
target_link_libraries(hello foo)

The contents are the same as in https://www.kitware.com/import-cmake-the-experiment-is-over/. Then, I created a file mystd.cxx with the following contents:

module;
export import <vector>
export module mystd;

and I added an import in main.cxx:

import foo;
import mystd;
...

I used the following commands to generate the appropriate vector.gcm and mystd.gcm:

g++ -std=c++20 -fmodules-ts -flang-info-module-cmi -x c++-system-header
g++ -std=c++20 -fmodules-ts -flang-info-module-cmi -c mystd.cxx

Then I updated the CMakeLists.txt to take into account mystd.o but when I run ninja I get the following error:

main.cxx:2:1:
mystd: error: failed to read compiled module: Unknown CMI mapping
mystd: note: imports must be built before being imported
mystd: fatal error: returning to the gate for a mechanical issue

6

Correct, there is zero support for importing header units at the moment.