Example of using C++ modules with Clang and CMake on Linux

An experiment with Ubuntu 24.04, CMake 3.30, and Clang 18 showing:

1. import std;
2. export module and import module for hello_lib

According to cmake-cxxmodules(7), import std Support, using Clang 18.1.2 or later is necessary (g++-14 does not support import std;).

Conventions

execute as sudo	execute as a user
# command output	$ command output

Installing Ninja

Ninja 1.11 or newer is required according to cmake-cxxmodules(7), Generator Support. GNU make is not supported.

# apt-get install ninja-build

Installing CMake 3.30.0

# snap install cmake
$ cmake --version
cmake version 3.30.0
...

Installing Clang and libc++

# apt-get install clang libc++-dev
# clang++ --version
Ubuntu clang version 18.1.3 (1ubuntu1)
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/bin

Configuring toolchain

It is important to set -stdlib=libc++ for both the compiler and the linker so that GNU libstdc++ is not used. Otherwise the following error occurs:

The "CXX_MODULE_STD" property on the target "hello_lib" requires that the
   "__CMAKE::CXX23" target exist, but it was not provided by the toolchain.
   Reason:
 
     Only `libc++` is supported

The compiler and linker flags can be set in a toolchain file clang-toolchain.cmake.

Building from command line

Configure

$ cd hello-cxx-modules
$ cmake --toolchain clang-toolchain.cmake \
        -G Ninja \
        -DCMAKE_BUILD_TYPE=MinSizeRel \
        -S . \
        -B build
-- The CXX compiler identification is Clang 18.1.3
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Check for working CXX compiler: /usr/bin/clang++ - skipped
-- Detecting CXX compile features
CMake Warning (dev) at /snap/cmake/1403/share/cmake-3.30/Modules/Compiler/CMakeCommonCompilerMacros.cmake:248 (cmake_language):
  CMake's support for `import std;` in C++23 and newer is experimental.  It
  is meant only for experimentation and feedback to CMake developers.
Call Stack (most recent call first):
  /snap/cmake/1403/share/cmake-3.30/Modules/CMakeDetermineCompilerSupport.cmake:113 (cmake_create_cxx_import_std)
  /snap/cmake/1403/share/cmake-3.30/Modules/CMakeTestCXXCompiler.cmake:83 (CMAKE_DETERMINE_COMPILER_SUPPORT)
  CMakeLists.txt:13 (project)
This warning is for project developers.  Use -Wno-dev to suppress it.

-- Detecting CXX compile features - done
-- Configuring done (0.3s)
-- Generating done (0.0s)
-- Build files have been written to: /.../hello-cxx-modules/build

Build

$ cmake --build build --target all
[4/14   7% :: 0.058] Scanning /.../hello-cxx-modules/hello.cxx for CXX dependencies
[4/14  14% :: 0.060] Scanning /.../hello-cxx-modules/hello_lib.cxx for CXX dependencies
[4/14  21% :: 0.089] Scanning /usr/lib/llvm-18/share/libc++/v1/std.compat.cppm for CXX dependencies
[4/14  28% :: 0.188] Scanning /usr/lib/llvm-18/share/libc++/v1/std.cppm for CXX dependencies
[5/14  35% :: 0.195] Generating CXX dyndep file CMakeFiles/__cmake_cxx23.dir/CXX.dd
[7/14  42% :: 0.202] Generating CXX dyndep file CMakeFiles/hello_lib.dir/CXX.dd
[8/14  50% :: 0.208] Generating CXX dyndep file CMakeFiles/hello.dir/CXX.dd
[8/14  57% :: 3.679] Building CXX object CMakeFiles/__cmake_cxx23.dir/usr/lib/llvm-18/share/libc++/v1/std.cppm.o
[10/14  64% :: 3.913] Building CXX object CMakeFiles/hello_lib.dir/hello_lib.cxx.o
[11/14  71% :: 3.980] Building CXX object CMakeFiles/hello.dir/hello.cxx.o
[11/14  78% :: 4.075] Building CXX object CMakeFiles/__cmake_cxx23.dir/usr/lib/llvm-18/share/libc++/v1/std.compat.cppm.o
[12/14  85% :: 4.136] Linking CXX static library lib__cmake_cxx23.a
[13/14  92% :: 4.196] Linking CXX static library libhello_lib.a
[14/14 100% :: 4.268] Linking CXX executable hello

Execute

$ build/hello 
Hello World! My name is Michał

Check dependencies

$ ldd build/hello
	linux-vdso.so.1 (0x00007fff96cdb000)
	libc++.so.1 => /lib/x86_64-linux-gnu/libc++.so.1 (0x00007f51ea2b4000)
	libc++abi.so.1 => /lib/x86_64-linux-gnu/libc++abi.so.1 (0x00007f51ea278000)
	libunwind.so.1 => /lib/x86_64-linux-gnu/libunwind.so.1 (0x00007f51ea26a000)
	libm.so.6 => /lib/x86_64-linux-gnu/libm.so.6 (0x00007f51ea181000)
	libgcc_s.so.1 => /lib/x86_64-linux-gnu/libgcc_s.so.1 (0x00007f51ea154000)
	libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007f51e9f40000)
	/lib64/ld-linux-x86-64.so.2 (0x00007f51ea3cb000)

Building from VSCode

Install CMake and CMake Tools extensions

VSCode does not work with snap version of CMake out-of-the box. Set CMake Path to /snap/cmake/current/bin/cmake in "CMake Tools Extension Settings" or in settings.json:

"cmake.cmakePath": "/snap/cmake/current/bin/cmake"

Configure the Clang kit to use the toolchain file

See .vscode/cmake-kits.json.

Configure and build

Ensure the kit Clang 18.1.3 x86_64-pc-linux-gnu is selected in Configure

Build as usual.

References

• import CMake; the Experiment is Over!
• import std in CMake 3.30