Provided by: cmake-data_4.1.1+really3.31.6-2ubuntu1_all 
NAME
cmake-cxxmodules - CMake C++ Modules Support Reference
Added 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.
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 <https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2022/p1689r5.html> 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.
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 CXX_SCAN_FOR_MODULES <#prop_sf:CXX_SCAN_FOR_MODULES> source file property is set, its value will
be used.
• If the CXX_SCAN_FOR_MODULES <#prop_tgt:CXX_SCAN_FOR_MODULES> target property is set, its value will be
used. Set the CMAKE_CXX_SCAN_FOR_MODULES <#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
CMP0155 <#policy:CMP0155>.
Note that any scanned source will be excluded from any unity build (see UNITY_BUILD <#
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 CMAKE_CXX_COMPILER_IMPORT_STD <#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:
• Ninja <#generator:Ninja>
• Ninja Multi-Config <#generator:Ninja Multi-Config>
• Visual Studio 17 2022 <#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 Visual Studio Generators <#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.
Copyright
2000-2024 Kitware, Inc. and Contributors
3.31.6 December 01, 2025 CMAKE-CXXMODULES(7)