METAWRAPPERS: Implementing Object Models of Nested Data Formats

Sometimes, we may want to modify, or just read (in a computer program) specific parts of a message received (as a stream of bytes) from a communications port or of data read (also as a stream of bytes) from a flat file. What would be convenient for this is a kind of API to locate any component of a message in order to modify or read it. Such an API could also be used to construct an entire message. If the format of the message is a nested one (such as those defined using the ASN.1 [ISO 1987] or BNF notations or those defined by the HL7 Standard [HL7 1997]), one possible style of such an API is to treat the components of a message (including the message itself) as objects and the constituent components of these components as objects nested within objects. In other words, direct programming with message component types (each message component type appears as a type in the computer program). This pattern deals with how to implement these types when their number is great or when new types are introduced and existing ones modified with great frequency.