A Movie definition is defined by the gnash::movie_definition class. A Movie instance is defined by the gnash::movie_instance class.
A Movie instance exposes the ActionScript Object base interface (gnash::as_object), thus it can manage gnash::as_value members.
The implementation of SWF parsing for a Movie definition is found in gnash::movie_def_impl::read.
Note that movie_definition is also used as a base class for gnash::sprite_definition, which is a sub-movie defined in an SWF file. This seems to be the only reason to have a movie_def_impl class, being the top-level definition of a movie (the one with a CharacterDictionary in it).
Also note that gnash::movie_instance is a subclass of gnash::sprite_instance, being the instance of a gnash::sprite_definition.
1.5.4