CROS Dispatching

• Features of C++ classes are used to create reference and implementation models and hide the technical details of this dispatching mechanism, making it natural, efficient, and easy to use.
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• An object reference consists of the following private data, which must not change for binary compatibility:
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  • Pointer to the start of the combined object (for easy object identity, garbage collection, etc.).  The object pointer is null if this is a class reference, in which case the identity of the reference is null.
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  • Pointer to the dispatch table of the reference, which should never be null or dispatching will fail.
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• The dispatch table maps an implementation class to a reference class, with an entry for each dispatched method.  Each entry contains the following fields, which must not change for binary compatibility:
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  • Pointer to the method.  In addition to its primary function, this method is responsible for setting up its execution context.  For a fixed cast, this will be null.
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  • Pointer to the class context.  This contains including the nested dispatch table, the offset of the data within the object, and any additional class information required.  For a fixed cast, this will be the dispatch table of the resultant reference.
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• The first entry in any dispatch table contains the class implementation's _dispatch method, which accepts a case-insensitive method name, a canonical signature, the dispatch entry of the most derived class, and a consistency pointer (to determine the scope of the compilation unit within which no casting of reference arguments or returns is required) and returns the entry of the desired method's dispatch entry, rejecting the request if the signature is incompatible.  The class context of the _dispatch entry is also used to uniquely identify the class.
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• Fixed casts are precomputed by the reference, appearing as entries with no method since all that is required is the replacement dispatch table, which can be stored as the class context of the cast.
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• Dynamic casts are a function of the target reference that looks up a new dispatch table in a local cache. If this reference type has never been mapped to the specific reference class, a dispatch table is created using the _dispatch method of the class.
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• The natural object scope for object methods is constructed from a reference to the aggregate object type being defined, which is fixed up via a fixed cast specified in the dispatch table before invoking a given method. This scope also makes the superclass methods and private data available.
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• An array with entries of a primative type such as long is a simple class implementation. An array of CROS objects (not references) is a generic array class implementation which parameterizes the contained object type.  Each of these is available in the core CROS library.
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• Object instance classes provide static _sizeof and _alignment methods which are used to allocate the object and set up object offsets in dispatch tables.  If the object implementation inherits from other object implementations, these methods report the aggregate size and alignment.  An _aggregate method constructs a class definition where the data is offset, for purposes of aggragation.
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• Initialization, implementation inheritance, and other standard class methods can be created as required without breaking compatibility, since they are all robustly supplied by the _dispatch method.
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Created:  January 8, 2000
Last updated:  January 15, 2000