The filter class maintains all filter objects. A filter object defines the IO functions of an IO object. An IO object possesses only a primitive set of IO functions when it’s created and that set always returns MM_FUNCTION_NOT_SUPPORTED. After an IO object is created, a filter is registered and associated with an IO object. Filters can be added or removed on the fly to modify an IO object’s behavior.document.
The registration of filter objects is typically performed by classes and is currently beyond the scope of this document.