An extension of object-oriented programming languages enables the use of replaceable
classes and virtual constructors, thereby allowing existing code that creates objects
of a base class to be reusable for creating instances of a new class derived from
the base class. A base class intended to be replaceable later is declared to be
replaceable. In the case of a compiled language, the programming code defining
and using the base class is compiled into a reusable module. Later, a new class
derived from the base class is identified as a replacement for the base class and
is used in new programming code that uses the existing reusable module. The replacement
relationship between the old base class and the new class is registered. During
execution of the program, when the old code of the reusable module indicates the
creation of an object of the base class, the registered replacement information
is referenced. If the old class is to be replaced with the new class, the creation
information for the new class is passed instead so that an object of the new class
is created. The class replacement may be implemented with compiled and interpreted
programming languages. When a compiled language is used, the class replacement
may be implemented with new compiler-supported features that emit data for class
replacement into the .obj files, but may also be implemented without requiring
changes to existing compilers.
