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Determining-the-actual-class-of-an-object-at-run-time

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Details

Inventors: Ball, Michael S.; Ebnesajjad, Soroor;
Assignee: Sun Microsystems, Inc. (Palo Alto, CA)
Primary Examiner: Chaki; Kakali
Assistant Examiner:
Attorney, Agent or Firm: Beyer Weaver & Thomas, LLP

A dynamic down cast or cross cast is performed at run time within an object-oriented programming language such as C++. The compiler generates data structures during a compile; at run time a routine in the language support library accesses the generated data structures in order to perform the dynamic cast. Class structure information is provided in a size that varies linearly as the total number of classes in the class hierarchy. Cryptographic hashing of class names is used to provide uniqueness. The dynamic cast addresses complications in C++ due to "virtual" and "nonvirtual" inheritance. A run time type identification (RTTI) data structure is created at compile time. An offset within an object description is calculated for each sub-object of the object. Those virtual base classes that are duplicates are skipped producing a data structure that may be searched linearly as the number of classes in the hierarchy. An identifier for each sub-object is stored in the data structure along with its offset. Each virtual table also stores the offset of its corresponding sub-object. A cast operation is performed at run time using the RTTI data structure. The static type object pointer is used to access the object through a particular sub-object. The virtual table associated with the sub-object is used to access the RTTI data structure. The data structure is searched for the target type, and the offset associated with the target type is combined with the offset from the virtual table to adjust the object pointer to cast it to the target type.

DETAILED DESCRIPTION To achieve the foregoing, and in accordance with the purpose of the present invention, dynamic cast techniques are disclosed that performs a down cast or cross cast simply and efficiently within an object-oriented programming language.
In one aspect of the invention, the dynamic cast technique is embodied within an object-oriented compiler and run time system.
The compiler generates data structures during a compile, while at run time a routine in the language support library is called to access the generated data structures to perform the dynamic cast.
The present invention provides class structure information in a size that varies linearly as the total number of classes in the class hierarchy to reduce the time necessary to perform a cast operation.
Also, cryptographic hashing of class names is used to provide uniqueness in another aspect of the invention.
Efficient algorithms are provided for modifying pointers to point to different sub-objects of an object to perform a cast operation.
The dynamic cast technique of the present invention addresses the complications in C++ due to the two kinds of inheritance in that language.
The present invention more quickly and more simply performs cast operations than the directed acyclical graph (DAG) of C++.
The present invention is useful not only to an object-oriented language such as C++, but to any object-oriented language that implements cast operations that are either too complex or too slow.
In particular, the present invention is applicable to other object-oriented programming language such as Eiffel, Java, and CLOS.
In one specific embodiment, a run time type identification (RTTI) data structure is created at compile time and linked to each object description.
An offset within the object description is calculated for each sub-object of the object.
Those virtual base classes that are duplicates of virtual base classes already encountered are skipped, thereby producing a data structure that may be searched using an algorithm that varies linearly with the number of classes in the hierarchy

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