# define_property

Define and document custom properties.

    define_property(<GLOBAL | DIRECTORY | TARGET | SOURCE |
                     TEST | VARIABLE | CACHED_VARIABLE>
                     PROPERTY <name> [INHERITED]
                     [BRIEF_DOCS <brief-doc> [docs...]]
                     [FULL_DOCS <full-doc> [docs...]]
                     [INITIALIZE_FROM_VARIABLE <variable>])

Defines one property in a scope for use with the `set_property()` and `get_property()` commands. It is mainly useful for defining the way a property is initialized or inherited. Historically, the command also associated documentation with a property, but that is no longer considered a primary use case.

The first argument determines the kind of scope in which the property should be used. It must be one of the following:

    GLOBAL    = associated with the global namespace
    DIRECTORY = associated with one directory
    TARGET    = associated with one target
    SOURCE    = associated with one source file
    TEST      = associated with a test named with add_test
    VARIABLE  = documents a CMake language variable
    CACHED_VARIABLE = documents a CMake cache variable

Note that unlike `set_property()` and `get_property()` no actual scope needs to be given; only the kind of scope is important.

The required `PROPERTY` option is immediately followed by the name of the property being defined.

If the `INHERITED` option is given, then the `get_property()` command will chain up to the next higher scope when the requested property is not set in the scope given to the command.

- `DIRECTORY` scope chains to its parent directory's scope, continuing the walk up parent directories until a directory has the property set or there are no more parents. If still not found at the top level directory, it chains to the `GLOBAL` scope.

- `TARGET`, `SOURCE` and `TEST` properties chain to `DIRECTORY` scope, including further chaining up the directories, etc. as needed.

Note that this scope chaining behavior only applies to calls to `get_property()`, `get_directory_property()`, `get_target_property()`, `get_source_file_property()` and `get_test_property()`. There is no inheriting behavior when setting properties, so using `APPEND` or `APPEND_STRING` with the `set_property()` command will not consider inherited values when working out the contents to append to.

The `BRIEF_DOCS` and `FULL_DOCS` options are followed by strings to be associated with the property as its brief and full documentation. CMake does not use this documentation other than making it available to the project via corresponding options to the `get_property()` command.

Changed in version 3.23: The `BRIEF_DOCS` and `FULL_DOCS` options are optional.

New in version 3.23: The `INITIALIZE_FROM_VARIABLE` option specifies a variable from which the property should be initialized. It can only be used with target properties. The `<variable>` name must end with the property name and must not begin with `CMAKE_` or `_CMAKE_`. The property name must contain at least one underscore. It is recommended that the property name have a prefix specific to the project.

## Property Redefinition

Once a property is defined for a particular type of scope, it cannot be redefined. Attempts to redefine an existing property by calling `define_property()` with the same scope type and property name will be silently ignored. Defining the same property name for two different kinds of scope is valid.

`get_property()` can be used to determine whether a property is already defined for a particular kind of scope, and if so, to examine its definition. For example:

    # Initial definition
    define_property(TARGET PROPERTY MY_NEW_PROP
      BRIEF_DOCS "My new custom property"
    )

    # Later examination
    get_property(my_new_prop_exists
      TARGET NONE
      PROPERTY MY_NEW_PROP
      DEFINED
    )

    if(my_new_prop_exists)
      get_property(my_new_prop_docs
        TARGET NONE
        PROPERTY MY_NEW_PROP
        BRIEF_DOCS
      )
      # ${my_new_prop_docs} is now set to "My new custom property"
    endif()

## See Also

- `get_property()`

- `set_property()`