Modules in libc++

Warning

Modules are an experimental feature. It has additional build requirements and not all libc++ configurations are supported yet.

The work is still in an early development state and not considered stable nor complete

This page contains information regarding C++23 module support in libc++. There are two kinds of modules available in Clang

• Clang specific modules

• C++ modules

This page mainly discusses the C++ modules. In C++20 there are also header units, these are not part of this document.

Overview

The module sources are stored in .cppm files. Modules need to be available as BMIs, which are .pcm files for Clang. BMIs are not portable, they depend on the compiler used and its compilation flags. Therefore there needs to be a way to distribute the .cppm files to the user and offer a way for them to build and use the .pcm files. It is expected this will be done by build systems in the future. To aid early adaptor and build system vendors libc++ currently ships a CMake project to aid building modules.

Note

This CMake file is intended to be a temporary solution and will be removed in the future. The timeline for the removal depends on the availability of build systems with proper module support.

What works

• Building BMIs

• Running tests using the std and std.compat module

• Using the std and std.compat module in external projects

• The following “parts disabled” configuration options are supported

  • LIBCXX_ENABLE_LOCALIZATION

  • LIBCXX_ENABLE_WIDE_CHARACTERS

  • LIBCXX_ENABLE_THREADS

  • LIBCXX_ENABLE_FILESYSTEM

  • LIBCXX_ENABLE_RANDOM_DEVICE

  • LIBCXX_ENABLE_UNICODE

  • LIBCXX_ENABLE_EXCEPTIONS [1]

• A C++20 based extension

Note

[1]

This configuration will probably not work on Windows due to hard-coded compilation flags.

Some of the current limitations

• There is no official build system support, libc++ has experimental CMake support

• Requires CMake 3.26 for C++20 support

• Requires CMake 3.26 for C++23 support

• Requires CMake 3.27 for C++26 support

• Requires Ninja 1.11

• Requires Clang 17

• The path to the compiler may not be a symlink, clang-scan-deps does not handle that case properly

• Libc++ is not tested with modules instead of headers

• Clang:

  • Including headers after importing the std module may fail. This is hard to solve and there is a work-around by first including all headers bug report.

Blockers

• libc++

  • Currently the tests only test with modules enabled, but do not import modules instead of headers. When converting tests to using modules there are still failures. These are under investigation.

  • It has not been determined how to fully test libc++ with modules instead of headers.

• Clang

  • Some concepts do not work properly bug report.

Using in external projects

Users need to be able to build their own BMI files.

Note

The requirements for users to build their own BMI files will remain true for the foreseeable future. For now this needs to be done manually. Once libc++’s implementation is more mature we will reach out to build system vendors, with the goal that building the BMI files is done by the build system.

Currently there are two ways to build modules

• Use a local build of modules from the build directory. This requires Clang 17 or later and CMake 3.26 or later.

• Use the installed modules. This requires Clang 18.1.2 or later and a recent build of CMake. The CMake changes will be part of CMake 3.30. This method requires you or your distribution to enable module installation.

Using the local build

$ git clone https://github.com/llvm/llvm-project.git
$ cd llvm-project
$ mkdir build
$ cmake -G Ninja -S runtimes -B build -DLLVM_ENABLE_RUNTIMES="libcxx;libcxxabi;libunwind"
$ ninja -C build

The above build directory will be referred to as <build> in the rest of these instructions.

This is a small sample program that uses the module std. It consists of a CMakeLists.txt and a main.cpp file.

import std; // When importing std.compat it's not needed to import std.
import std.compat;

int main() {
  std::cout << "Hello modular world\n";
  ::printf("Hello compat modular world\n");
}

cmake_minimum_required(VERSION 3.26.0 FATAL_ERROR)
project("example"
  LANGUAGES CXX
)

#
# Set language version used
#

set(CMAKE_CXX_STANDARD 23)
set(CMAKE_CXX_STANDARD_REQUIRED YES)
set(CMAKE_CXX_EXTENSIONS OFF)

#
# Enable modules in CMake
#

# This is required to write your own modules in your project.
if(CMAKE_VERSION VERSION_LESS "3.28.0")
  if(CMAKE_VERSION VERSION_LESS "3.27.0")
    set(CMAKE_EXPERIMENTAL_CXX_MODULE_CMAKE_API "2182bf5c-ef0d-489a-91da-49dbc3090d2a")
  else()
    set(CMAKE_EXPERIMENTAL_CXX_MODULE_CMAKE_API "aa1f7df0-828a-4fcd-9afc-2dc80491aca7")
  endif()
  set(CMAKE_EXPERIMENTAL_CXX_MODULE_DYNDEP 1)
else()
  cmake_policy(VERSION 3.28)
endif()

#
# Import the modules from libc++
#

include(FetchContent)
FetchContent_Declare(
  std
  URL "file://${LIBCXX_BUILD}/modules/c++/v1/"
  DOWNLOAD_EXTRACT_TIMESTAMP TRUE
  SYSTEM
)
FetchContent_MakeAvailable(std)

#
# Add the project
#

add_executable(main)
add_dependencies(main std.compat)
target_link_libraries(main std.compat)
target_sources(main
  PRIVATE
    main.cpp
)

Building this project is done with the following steps, assuming the files main.cpp and CMakeLists.txt are copied in the current directory.

$ mkdir build
$ cmake -G Ninja -S . -B build -DCMAKE_CXX_COMPILER=<path-to-compiler> -DLIBCXX_BUILD=<build>
$ ninja -C build
$ build/main

Warning

<path-to-compiler> should point point to the real binary and not to a symlink.

Warning

When using these examples in your own projects make sure the compilation flags are the same for the std module and your project. Some flags will affect the generated code, when these are different the module cannot be used. For example using -pthread in your project and not in the module will give errors like

error: POSIX thread support was disabled in PCH file but is currently enabled

error: module file _deps/std-build/CMakeFiles/std.dir/std.pcm cannot be loaded due to a configuration mismatch with the current compilation [-Wmodule-file-config-mismatch]

Using the installed modules

CMake has added experimental support for importing the Standard modules. This is available in the current nightly builds and will be part of the 3.30 release. Currently CMake only supports importing the Standard modules in C++23 and later. Enabling this for C++20 is on the TODO list of the CMake developers.

The example uses the same main.cpp as above. It uses the following CMakeLists.txt:

# This requires a recent nightly build.
# This will be part of CMake 3.30.0.
cmake_minimum_required(VERSION 3.29.0 FATAL_ERROR)

# Enables the Standard module support. This needs to be done
# before selecting the languages.
set(CMAKE_EXPERIMENTAL_CXX_IMPORT_STD "0e5b6991-d74f-4b3d-a41c-cf096e0b2508")
set(CMAKE_CXX_MODULE_STD ON)

project("example"
  LANGUAGES CXX
)

#
# Set language version used
#

set(CMAKE_CXX_STANDARD 23)
set(CMAKE_CXX_STANDARD_REQUIRED YES)
# Currently CMake requires extensions enabled when using import std.
# https://gitlab.kitware.com/cmake/cmake/-/issues/25916
# https://gitlab.kitware.com/cmake/cmake/-/issues/25539
set(CMAKE_CXX_EXTENSIONS ON)

add_executable(main)
target_sources(main
  PRIVATE
    main.cpp
)

Building this project is done with the following steps, assuming the files main.cpp and CMakeLists.txt are copied in the current directory.

$ mkdir build
$ cmake -G Ninja -S . -B build -DCMAKE_CXX_COMPILER=<path-to-compiler> -DCMAKE_CXX_FLAGS=-stdlib=libc++
$ ninja -C build
$ build/main

Warning

<path-to-compiler> should point point to the real binary and not to a symlink.

If you have questions about modules feel free to ask them in the #libcxx channel on LLVM’s Discord server.

If you think you’ve found a bug please it using the LLVM bug tracker. Please make sure the issue you found is not one of the known bugs or limitations on this page.