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→Abstract classes and pure virtual functions: noting that Python follows this pattern as well as C++ |
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<code>doOperation()</code>.
Although pure virtual methods typically have no implementation in the class that declares them, pure virtual methods in some languages (e.g. C++ and Python) are permitted to contain an implementation in their declaring class, providing fallback or default behaviour that a derived class can delegate to, if appropriate.<ref>[https://en.cppreference.com/w/cpp/language/destructor#Pure_virtual_destructors Pure virtual destructors - cppreference.com]</ref><ref>[https://docs.python.org/3/library/abc.html "abc — Abstract Base Classes: @abc.abstractmethod"]</ref>
Pure virtual functions can also be used where the method declarations are being used to define an [[interface (Java)|interface]] - similar to what the interface keyword in Java explicitly specifies. In such a use, derived classes will supply all implementations. In such a [[design pattern]], the abstract class which serves as an interface will contain ''only'' pure virtual functions, but no data members or ordinary methods. In C++, using such purely abstract classes as interfaces works because C++ supports [[multiple inheritance]]. However, because many OOP languages do not support multiple inheritance, they often provide a separate interface mechanism. An example is the [[Java (programming language)|Java programming language]].
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