# CMP0128

New in version 3.22.

When this policy is set to `NEW`:

- `<LANG>_EXTENSIONS` is initialized to `CMAKE_<LANG>_EXTENSIONS` if set, otherwise falling back to `CMAKE_<LANG>_EXTENSIONS_DEFAULT`.

- Extensions are correctly enabled/disabled if `<LANG>_STANDARD` is unset or satisfied by the default.

- Standard mode-affecting flags aren't added unless necessary to achieve the specified mode.

The `OLD` behavior:

- Initializes `<LANG>_EXTENSIONS` to `CMAKE_<LANG>_EXTENSIONS` if set, otherwise falling back to `ON`.

- Always adds a flag if `<LANG>_STANDARD` is set and `<LANG>_STANDARD_REQUIRED` is `OFF`.

- If `<LANG>_STANDARD` is unset:

  - Doesn't disable extensions even if `<LANG>_EXTENSIONS` is `OFF`.

  - Fails to enable extensions if `<LANG>_EXTENSIONS` is `ON` except for the `IAR` compiler.

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:

  - Use `<LANG>_STANDARD` and `<LANG>_EXTENSIONS` instead of manually adding flags.

  - Or ensure the manually-specified flags are used during compiler detection.

- 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 3.25.3 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.

Note

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