User data: HarfBuzz Manual

## User data

To better integrate with client programs, HarfBuzz's objects offer a "user data" mechanism that can be used to attach arbitrary data to the object. User-data attachment can be useful for tying the lifecycles of various pieces of data together, or for creating language bindings.

Each object type has a `set_user_data()` method and a `get_user_data()` method. The `set_user_data()` methods take a client-provided `key` and a pointer, `user_data`, pointing to the data itself. Once the key-data pair has been attached to the object, the `get_user_data()` method can be called with the key, returning the `user_data` pointer.

The `set_user_data()` methods also support an optional `destroy` callback. Client programs can set the `destroy` callback and receive notification from HarfBuzz whenever the object is destructed.

Finally, each `set_user_data()` method allows the client program to set a `replace` Boolean indicating whether or not the function call should replace any existing `user_data` associated with the specified key.