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79 lines
2.8 KiB
79 lines
2.8 KiB
.. cmake-manual-description: CMake C++ Modules Support Reference
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cmake-cxxmodules(7)
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*******************
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.. versionadded:: 3.28
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C++ 20 introduced the concept of "modules" to the language. The design
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requires build systems to order compilations among each other to satisfy
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``import`` statements reliably. CMake's implementation asks the compiler
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to scan source files for module dependencies during the build, collates
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scanning results to infer ordering constraints, and tells the build tool
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how to dynamically update the build graph.
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Scanning Control
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================
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Whether or not sources get scanned for C++ module usage is dependent on the
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following queries. The first query that provides a yes/no answer is used.
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- If the source file belongs to a file set of type ``CXX_MODULES``, it will
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be scanned.
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- If the target does not use at least C++ 20, it will not be scanned.
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- If the source file is not the language ``CXX``, it will not be scanned.
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- If the :prop_sf:`CXX_SCAN_FOR_MODULES` source file property is set, its
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value will be used.
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- If the :prop_tgt:`CXX_SCAN_FOR_MODULES` target property is set, its value
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will be used. Set the :variable:`CMAKE_CXX_SCAN_FOR_MODULES` variable
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to initialize this property on all targets as they are created.
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- Otherwise, the source file will be scanned if the compiler and generator
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support scanning. See policy :policy:`CMP0155`.
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Note that any scanned source will be excluded from any unity build (see
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:prop_tgt:`UNITY_BUILD`) because module-related statements can only happen at
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one place within a C++ translation unit.
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Compiler Support
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================
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Compilers which CMake natively supports module dependency scanning include:
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* MSVC toolset 14.34 and newer (provided with Visual Studio 17.4 and newer)
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* LLVM/Clang 16.0 and newer
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* GCC 14 (for the in-development branch, after 2023-09-20) and newer
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Generator Support
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=================
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The list of generators which support scanning sources for C++ modules include:
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- :generator:`Ninja`
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- :generator:`Ninja Multi-Config`
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- :generator:`Visual Studio 17 2022`
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Limitations
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-----------
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There are a number of known limitations of the current C++ module support in
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CMake. This does not document known limitations or bugs in compilers as these
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can change over time.
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For all generators:
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- Header units are not supported.
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- No builtin support for ``import std;`` or other compiler-provided modules.
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For the Ninja Generators:
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- ``ninja`` 1.11 or newer is required.
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For the :ref:`Visual Studio Generators`:
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- Only Visual Studio 2022 and MSVC toolsets 14.34 (Visual Studio
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17.4) and newer.
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- No support for exporting or installing BMI or module information.
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- No support for compiling BMIs from ``IMPORTED`` targets with C++ modules.
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- No diagnosis of using modules provided by ``PRIVATE`` sources from
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``PUBLIC`` module sources.
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