PostScript (13 KB)
Word (6 KB)Adding Parameter Types to MSDL-S
Contribution M1055, 35th MPEG Meeting, Tampere, Finland, July 1996
Introduction
We propose the addition of parameter types in MSDL-S, to rectify problems of object data member references that are frequently encountered when describing complex bitstreams using MSDL-S. These references are associated with context (flags, conditions, etc.) that is needed in order to make parsing decisions within objects, and require that information from other objects is available. In keeping with the declarative nature of MSDL-S, such referenced objects are not instantiated at the time the syntax is declared, and hence it is possible to create inconsistent code (referencing an object that has not been instantiated yet). Furthermore, current practice makes it difficult to utilize multiple instances of similar objects when both have to be used in context-sensitive parsing of multiple instances of another object. The concept of parameter types provides a mechanism with which all these issues are resolved without altering the current design of MSDL-S. Very simply, parameter types can be thought of as class parameters (similar to the arguments or parameters of a function) that have to be specified when an object is instantiated. An equivalent mechanism using C++ as an example would be the definition of a class constructor which has a mandatory (and fixed) set of arguments.