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diff --git a/Help/policy/CMP0091.rst b/Help/policy/CMP0091.rst new file mode 100644 index 000000000..5b7c4e3d9 --- /dev/null +++ b/Help/policy/CMP0091.rst @@ -0,0 +1,47 @@ +CMP0091 +------- + +MSVC runtime library flags are selected by an abstraction. + +Compilers targeting the MSVC ABI have flags to select the MSVC runtime library. +Runtime library selection typically varies with build configuration because +there is a separate runtime library for Debug builds. + +In CMake 3.14 and below, MSVC runtime library selection flags are added to +the default :variable:`CMAKE_<LANG>_FLAGS_<CONFIG>` cache entries by CMake +automatically. This allows users to edit their cache entries to adjust the +flags. However, the presence of such default flags is problematic for +projects that want to choose a different runtime library programmatically. +In particular, it requires string editing of the +:variable:`CMAKE_<LANG>_FLAGS_<CONFIG>` variables with knowledge of the +CMake builtin defaults so they can be replaced. + +CMake 3.15 and above prefer to leave the MSVC runtime library selection flags +out of the default :variable:`CMAKE_<LANG>_FLAGS_<CONFIG>` values and instead +offer a first-class abstraction. The :variable:`CMAKE_MSVC_RUNTIME_LIBRARY` +variable and :prop_tgt:`MSVC_RUNTIME_LIBRARY` target property may be set to +select the MSVC runtime library. + +This policy provides compatibility with projects that have not been updated +to be aware of the abstraction. The policy setting takes effect as of the +first :command:`project` or :command:`enable_language` command that enables +a language whose compiler targets the MSVC ABI. + +.. note:: + + Once the policy has taken effect at the top of a project, that choice + must be used throughout the tree. In projects that have nested projects + in subdirectories, be sure to convert everything together. + +The ``OLD`` behavior for this policy is to place MSVC runtime library +flags in the default :variable:`CMAKE_<LANG>_FLAGS_<CONFIG>` cache +entries and ignore the :variable:`CMAKE_MSVC_RUNTIME_LIBRARY` abstraction. +The ``NEW`` behavior for this policy is to *not* place MSVC runtime +library flags in the default cache entries and use the abstraction instead. + +This policy was introduced in CMake version 3.15. Use the +:command:`cmake_policy` command to set it to ``OLD`` or ``NEW`` explicitly. +Unlike many policies, CMake version |release| does *not* warn +when this policy is not set and simply uses ``OLD`` behavior. + +.. include:: DEPRECATED.txt |