OOC:en:2.7 Summary

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Given a pointer to an object, dynamic linkage lets us find type-specific functions: every object starts with a descriptor which contains pointers to functions applicable to the object. In particular, a descriptor contains a pointer to a constructor which initializes the memory area allocated for the object, and a pointer to a destructor which reclaims resources owned by an object before it is deleted.

We call all objects sharing the same descriptor a class. An object is an instance of a class, type-specific functions for an object are called methods, and messages are calls to such functions. We use selector functions to locate and call dynamically linked methods for an object.

Through selectors and dynamic linkage the same function name will take dif- ferent actions for different classes. Such a function is called polymorphic.

Polymorphic functions are quite useful. They provide a level of conceptual abstraction: differ() will compare any two objects — we need not remember which particular brand of differ() is applicable in a concrete situation. A cheap and very convenient debugging tool is a polymorphic function store() to display any object on a file descriptor.