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# cmake_policy

Manage CMake Policy settings. See the `cmake-policies(7)` manual for defined policies.

As CMake evolves it is sometimes necessary to change existing behavior in order to fix bugs or improve implementations of existing features. The CMake Policy mechanism is designed to help keep existing projects building as new versions of CMake introduce changes in behavior. Each new policy (behavioral change) is given an identifier of the form `CMP<NNNN>` where `<NNNN>` is an integer index. Documentation associated with each policy describes the `OLD` and `NEW` behavior and the reason the policy was introduced. Projects may set each policy to select the desired behavior. When CMake needs to know which behavior to use it checks for a setting specified by the project. If no setting is available the `OLD` behavior is assumed and a warning is produced requesting that the policy be set.

## Setting Policies by CMake Version

The `cmake_policy` command is used to set policies to `OLD` or `NEW` behavior. While setting policies individually is supported, we encourage projects to set policies based on CMake versions:

    cmake_policy(VERSION major.minor[.patch[.tweak]])

Specify that the current CMake code is written for the given version of CMake. All policies introduced in the specified version or earlier will be set to use `NEW` behavior. All policies introduced after the specified version will be unset (unless the `CMAKE_POLICY_DEFAULT_CMP<NNNN>` variable sets a default). This effectively requests behavior preferred as of a given CMake version and tells newer CMake versions to warn about their new policies. The policy version specified must be at least 2.4 or the command will report an error.

Note that the `cmake_minimum_required(VERSION)` command implicitly calls `cmake_policy(VERSION)` too.

## Setting Policies Explicitly

    cmake_policy(SET CMP<NNNN> NEW)
    cmake_policy(SET CMP<NNNN> OLD)

Tell CMake to use the `OLD` or `NEW` behavior for a given policy. Projects depending on the old behavior of a given policy may silence a policy warning by setting the policy state to `OLD`. Alternatively one may fix the project to work with the new behavior and set the policy state to `NEW`.

## Checking Policy Settings

    cmake_policy(GET CMP<NNNN> <variable>)

Check whether a given policy is set to `OLD` or `NEW` behavior. The output `<variable>` value will be `OLD` or `NEW` if the policy is set, and empty otherwise.

## CMake Policy Stack

CMake keeps policy settings on a stack, so changes made by the cmake_policy command affect only the top of the stack. A new entry on the policy stack is managed automatically for each subdirectory to protect its parents and siblings. CMake also manages a new entry for scripts loaded by `include()` and `find_package()` commands except when invoked with the `NO_POLICY_SCOPE` option (see also policy `CMP0011`). The `cmake_policy` command provides an interface to manage custom entries on the policy stack:

    cmake_policy(PUSH)
    cmake_policy(POP)

Each `PUSH` must have a matching `POP` to erase any changes. This is useful to make temporary changes to policy settings. Calls to the `cmake_minimum_required(VERSION)`, `cmake_policy(VERSION)`, or `cmake_policy(SET)` commands influence only the current top of the policy stack.

Commands created by the `function()` and `macro()` commands record policy settings when they are created and use the pre-record policies when they are invoked. If the function or macro implementation sets policies, the changes automatically propagate up through callers until they reach the closest nested policy stack entry.

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