NAME

       cmake-cxxmodules - CMake C++ Modules Support Reference

       New in version 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.

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 CXX_SCAN_FOR_MODULES source file property is set, its value will be used.

       • If the CXX_SCAN_FOR_MODULES target property is set, its value will  be  used.   Set  the
         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 CMP0155.

       Note  that  any  scanned  source  will  be excluded from any unity build (see 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

GENERATOR SUPPORT

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

       • Ninja

       • Ninja Multi-Config

       • 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 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.

       • No diagnosis of using modules provided by PRIVATE sources from PUBLIC module sources.

COPYRIGHT

       2000-2024 Kitware, Inc. and Contributors