C++ Modules are a crucial feature added in C++20 that aims to improve compile times and simplify dependencies in C++ codebases. With C++ Modules, developers can create self-contained units of code that can be compiled independently and imported as needed, eliminating the need for traditional header files.

In this blog post, we will explore how three popular build systems - CMake, Bazel, and Meson - support building and integrating C++ Modules into your projects.

CMake

CMake, one of the most widely used build systems, has been working on adding support for C++ Modules. As of now, CMake version 3.21 is required to compile C++ Modules.

To enable C++ Modules in CMake, you need to set the appropriate compiler flags. For Clang, the flag is -std=c++20 and for GCC, it is -std=gnu++20. Additionally, you need to specify the module file extension using the CMAKE_MODULE_FILE_EXTENSION variable.

To define a module target, you can use the add_library or add_executable commands, just like with regular libraries or executables. CMake will automatically handle the module compilation and linking.

cmake_minimum_required(VERSION 3.21)
project(MyProject)

set(CMAKE_MODULE_FILE_EXTENSION .cppm)

add_executable(MyApp main.cpp)
target_sources(MyApp PRIVATE mymodule.cppm)

Bazel

Bazel, a powerful build system mainly used by Google, also provides support for C++ Modules. Bazel automatically detects module dependencies and handles the compilation and linking process.

To enable C++ Modules in Bazel, you need to set the --c++14 or --c++17 flag in your bazelrc file to specify the C++ standard used by your project. Bazel will automatically detect and build C++ Modules present in your project.

Bazel also provides the module_cc_library rule to define a C++ module library target. Bazel handles the module compilation and linking transparently when you include the module in your build targets.

build --c++17

load("@rules_cc//cc:defs.bzl", "module_cc_library")

module_cc_library(
    name = "mymodule",
    srcs = ["mymodule.cppm"],
    deps = [
        ":mymodule_dep1",
        "//path/to:external_module",
    ],
)

Meson

Meson, a modern and user-friendly build system, also supports building C++ Modules. Meson aims to simplify the build process by providing an easy-to-use API and intuitive syntax.

To enable C++ Modules in Meson, you need to set the cpp_std option to the desired C++ version in your meson.build file.

project('myproject', 'cpp',
        version: '1.0',
        default_options: ['cpp_std=c++20'])

executable('myapp', 
           'main.cpp',
           include_directories: ['include'],
           sources: ['mymodule.cppm'])

Meson automatically recognizes C++ Modules and handles the compilation and linking seamlessly.

Conclusion

In this blog post, we explored how three popular build systems - CMake, Bazel, and Meson - support building and integrating C++ Modules into your projects. With the support for C++ Modules in these build systems, developers can take advantage of this powerful feature introduced in C++20 to improve build times and simplify dependencies in their C++ codebases.

#C++ #C++Modules #BuildSystems