New in version 3.22.

When this policy is set to NEW:

The OLD behavior:

Code may need to be updated for the NEW behavior in the following cases:

  • If a standard mode flag previously overridden by CMake's and not used during compiler detection now takes effect due to CMake no longer adding one as the default detected is appropriate.

    Such code should be converted to either:

  • If extensions were disabled without <LANG>_STANDARD being set CMake previously wouldn't actually disable extensions.

    Such code should be updated to not disable extensions if they are required.

  • If extensions were enabled/disabled when <LANG>_STANDARD was satisfied by the compiler's default CMake previously wouldn't actually enable/disable extensions.

    Such code should be updated to set the correct extensions mode.

If compiler flags affecting the standard mode are used during compiler detection (for example in a toolchain file using CMAKE_<LANG>_FLAGS_INIT) then they will affect the detected default standard and extensions.

Unlike many policies, CMake version 3.28.6 does not warn when the policy is not set and simply uses the OLD behavior. Use the cmake_policy() command to set it to OLD or NEW explicitly. See documentation of the CMAKE_POLICY_WARNING_CMP0128 variable to control the warning.


The OLD behavior of a policy is deprecated by definition and may be removed in a future version of CMake.