# CMP0126

Added in version 3.21.

When this policy is set to `NEW`, the `set(CACHE)` command does not remove any normal variable of the same name from the current scope. The `OLD` behavior removes any normal variable of the same name from the current scope in the following situations:

- No cache variable of that name existed previously.

- A cache variable of that name existed previously, but it had no type. This can occur when the variable was set on the command line using a form like `cmake -DMYVAR=blah` instead of `cmake -DMYVAR:STRING=blah`.

- The `FORCE` or `INTERNAL` keywords were used when setting the cache variable.

Note that the `NEW` behavior has an important difference to the similar `NEW` behavior of policy `CMP0077`. The `set(CACHE)` command always sets the cache variable if it did not exist previously, regardless of the `CMP0126` policy setting. The `option()` command will not set the cache variable if a non-cache variable of the same name already exists and `CMP0077` is set to `NEW`.

Policy `CMP0126` was introduced in CMake version 3.21. Use the `cmake_policy()` command to set it to `OLD` or `NEW` explicitly within a project. Use the `CMAKE_POLICY_DEFAULT_CMP0126` variable to set the policy for a third-party project in a subdirectory without modifying it. Unlike many policies, CMake version 3.28.3 does not warn when the policy is not set and simply uses `OLD` behavior. See documentation of the `CMAKE_POLICY_WARNING_CMP0126` 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.