.. cmake-manual-description: CMake C++ Modules Support Reference

cmake-cxxmodules(7)
*******************

.. versionadded:: 3.28

C++ 20 introduced the concept of "modules" to the language.  The design
requires build systems to order compilations among each other to satisfy
``import`` statements reliably.  CMake's implementation asks the compiler
to scan source files for module dependencies during the build, collates
scanning results to infer ordering constraints, and tells the build tool
how to dynamically update the build graph.

Compilation Strategy
====================

With C++ modules, compiling a set of C++ sources is no longer embarrassingly
parallel. That is, any given source may first require the compilation of
another source file first in order to provide a "CMI" (compiled module
interface) or "BMI" (binary module interface) that C++ compilers use to
satisfy ``import`` statements in other sources. With headers, sources could
share their declarations so that any consumers could compile independently.
With modules, declarations are now generated into these BMI files by the
compiler during compilation based on the contents of the source file and its
``export`` statements.

The order necessary for compilation requires build-time resolution of the
ordering because the order is controlled by the contents of the sources. This
means that the ordering needs extracted from the source during the build to
avoid regenerating the build graph via a configure and generate phase for
every source change to get a correct build.

The general strategy is to use a "scanner" to extract the ordering dependency
information and update the build graph with new edges between existing edges
by taking the per-source scan results (represented by `P1689R5`_ files) and
"collating" the dependencies within a target and to modules produced by
targets visible to the target. The primary task is to generate "module map"
files to pass to each compile rule with the paths to the BMIs needed to
satisfy ``import`` statements. The collator also has tasks to use the
build-time information to fill out information including ``install`` rules for
the module interface units, their BMIs, and properties for any exported
targets with C++ modules.

.. _`P1689R5`: https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2022/p1689r5.html

.. note::

   CMake is focusing on correct builds before looking at performance
   improvements. There are known tactics within the chosen strategy which may
   offer build performance improvements. However, they are being deferred
   until we have a working model against which to compare them. It is also
   important to note that a tactic useful in one situation (e.g., clean
   builds) may not be performant in a different situation (e.g., incremental
   builds). Finding a balance and offering controls to select the tactics is
   future work.

Scanning Control
================

Whether or not sources get scanned for C++ module usage is dependent on the
following queries. The first query that provides a yes/no answer is used.

- If the source file belongs to a file set of type ``CXX_MODULES``, it will
  be scanned.
- If the target does not use at least C++ 20, it will not be scanned.
- If the source file is not the language ``CXX``, it will not be scanned.
- If the :prop_sf:`CXX_SCAN_FOR_MODULES` source file property is set, its
  value will be used.
- If the :prop_tgt:`CXX_SCAN_FOR_MODULES` target property is set, its value
  will be used.  Set the :variable:`CMAKE_CXX_SCAN_FOR_MODULES` variable
  to initialize this property on all targets as they are created.
- Otherwise, the source file will be scanned if the compiler and generator
  support scanning.  See policy :policy:`CMP0155`.

Note that any scanned source will be excluded from any unity build (see
:prop_tgt:`UNITY_BUILD`) because module-related statements can only happen at
one place within a C++ translation unit.

Compiler Support
================

Compilers which CMake natively supports module dependency scanning include:

* MSVC toolset 14.34 and newer (provided with Visual Studio 17.4 and newer)
* LLVM/Clang 16.0 and newer
* GCC 14 (for the in-development branch, after 2023-09-20) and newer

``import std`` Support
======================

Support for ``import std`` is limited to the following toolchain and standard
library combinations:

* Clang 18.1.2 and newer with ``-stdlib=libc++``
* MSVC toolset 14.36 and newer (provided with Visual Studio 17.6 Preview 2 and
  newer)

The :variable:`CMAKE_CXX_COMPILER_IMPORT_STD` variable may be used to detect
support for a standard level with the active C++ toolchain.

.. note ::

   This support is provided only when experimental support for
   ``import std;`` has been enabled by the
   ``CMAKE_EXPERIMENTAL_CXX_IMPORT_STD`` gate.

Generator Support
=================

The list of generators which support scanning sources for C++ modules include:

- :generator:`Ninja`
- :generator:`Ninja Multi-Config`
- :generator:`Visual Studio 17 2022`

Limitations
-----------

There are a number of known limitations of the current C++ module support in
CMake.  This does not document known limitations or bugs in compilers as these
can change over time.

For all generators:

- Header units are not supported.
- No builtin support for ``import std;`` or other compiler-provided modules.

For the Ninja Generators:

- ``ninja`` 1.11 or newer is required.

For the :ref:`Visual Studio Generators`:

- Only Visual Studio 2022 and MSVC toolsets 14.34 (Visual Studio
  17.4) and newer.
- No support for exporting or installing BMI or module information.
- No support for compiling BMIs from ``IMPORTED`` targets with C++ modules
  (including ``import std``).
- No diagnosis of using modules provided by ``PRIVATE`` sources from
  ``PUBLIC`` module sources.
