Xmake uses .mpp as the default module extension, but also supports .ixx, .cppm, .mxx and other extensions.

At present, xmake has fully supported the C++20 Modules construction support of gcc11/clang/msvc, and can automatically analyze the dependencies between modules to maximize parallel compilation.

set_languages("c++20")
target("class")
    set_kind("binary")
    add_files("src/*.cpp", "src/*.mpp")

For more examples, see: C++ Modules

Cpp-only project

The v2.7.1 release has refactored and upgraded the C++20 module implementation to include support for Headerunits, which allows us to introduce Stl and user header modules into the module.

The relevant patch is available at: #2641.

Note: Normally we need to add at least one .mpp file to enable C++20 modules compilation, if we only have a cpp file, module compilation will not be enabled by default.

However, if we just want to use the module's Headerunits feature in the cpp file, e.g. by introducing some stl Headerunits into the cpp, then we can also set set_policy to .mpp. then we can also force C++ Modules compilation by setting set_policy("build.c++.modules", true), for example:

add_rules("mode.debug", "mode.release")

target("test")
    set_kind("binary")
    add_files("src/*.cpp")
    set_languages("c++20")
    set_policy("build.c++.modules", true)

C++ Module distribution

Many thanks to Arthapz for continuing to help improve xmake's support for C++ Modules in this new release.

We can now distribute C++ Modules as packages for quick integration and reuse in other projects.

This is a prototype implementation based on the draft design for module distribution in p2473r1.

Creating a C++ Modules package for distribution

We start by maintaining a build of the modules using xmake.lua and telling xmake which module files to install for external distribution by specifying ``{install = true}`''.

add_rules("mode.release", "mode.debug")
set_languages("c++20")

target("foo")
    set_kind("static")
    add_files("*.cpp")
    add_files("*.mpp", { install = true })

We then make it into a package that we can commit to the xmake-repo repository, or of course directly into a local package, or a private repository package.

Here, for testing purposes, we just make it a local package via set_sourcedir.

package("foo")
    set_sourcedir(path.join(os.scriptdir(), "src"))
    on_install(function(package)
        import("package.tools.xmake").install(package, {})
    end)

Integrating the C++ Modules package

We then quickly integrate the C++ Modules package for use via the package integration interface with add_requires("foo").

Since the modules packages for foo are defined in a private repository, we introduce our own package repository via add_repositories("my-repo my-repo").

If the package has already been committed to the official xmake-repo repository, there is no need to configure it additionally.

add_rules("mode.release", "mode.debug")
set_languages("c++20")

add_repositories("my-repo my-repo")
add_requires("foo", "bar")

target("packages")
    set_kind("binary")
    add_files("src/*.cpp")
    add_packages("foo", "bar")
    set_policy("build.c++.modules", true)

Once the packages are integrated, we can run the xmake command to download, compile and integrate the C++ Modules package for use with one click.

$ xmake
checking for platform ... linux
checking for architecture ... x86_64
note: install or modify (m) these packages (pass -y to skip confirm)?
in my-repo:
  -> foo latest
  -> bar latest
please input: y (y/n/m)

  => install bar latest ... ok
  => install foo latest ... ok
[ 0%]: generating.module.deps src/main.cpp
[ 0%]: generating.module.deps /mnt/xmake/tests/projects/c++/modules/packages/build/.packages/b/bar/latest/ 4e0143c97b65425b855ad5fd03038b6a/modules/bar/bar.mpp
[ 0%]: generating.module.deps /mnt/xmake/tests/projects/c++/modules/packages/build/.packages/f/foo/latest/ 4e0143c97b65425b855ad5fd03038b6a/modules/foo/foo.mpp
[ 14%]: compiling.module.release bar
[ 14%]: compiling.module.release foo
[ 57%]: compiling.release src/main.cpp
[ 71%]: linking.release packages
[ 100%]: build ok!
```''

Note: After each package is installed, a meta-info file for the maintenance module is stored in the package path, this is a format specification agreed in ``p2473r1.pdf``, it may not be the final standard, but this does not affect our ability to use the distribution of the module now.

```sh
$ cat . /build/.packages/f/f/foo/latest/4e0143c97b65425b855ad5fd03038b6a/modules/foo/foo.mpp.meta-info
{"_VENDOR_extension":{"xmake":{"name": "foo", "file": "foo.mpp"}}, "definitions":{}, "include_paths":{}}

The full example project is available at: C++ Modules package distribution example project

Support for C++23 Std Modules

Arthapz has also helped to improve support for C++23 Std Modules.

add_rules("mode.debug", "mode.release")
set_languages("c++latest")

target("mod")
    set_kind("static")
    add_files("src/*.cpp")
    add_files("src/*.mpp", {public = true})

target("stdmodules")
    set_kind("binary")
    add_files("test/*.cpp")
    add_deps("mod")

my_module.mpp

export module my_module;

import std;

export auto my_sum(std::size_t a, std::size_t b) -> std::size_t;

my_module.cpp

module my_module;

import std;

auto my_sum(std::size_t a, std::size_t b) -> std::size_t { return a + b; }