Abstract Classes and Pure Virtual Functions
A pure virtual function or pure virtual method is a virtual function that is required to be implemented by a derived class, if that class is not abstract. Classes containing pure virtual methods are termed "abstract"; they cannot be instantiated directly. A subclass of an abstract class can only be instantiated directly if all inherited pure virtual methods have been implemented by that class or a parent class. Pure virtual methods typically have a declaration (signature) and no definition (implementation).
As an example, an abstract base class MathSymbol may provide a pure virtual function doOperation, and derived classes Plus and Minus implement doOperation to provide concrete implementations. Implementing doOperation would not make sense in the MathSymbol class, as MathSymbol is an abstract concept whose behaviour is defined solely for each given kind (subclass) of MathSymbol. Similarly, a given subclass of MathSymbol would not be complete without an implementation of doOperation.
Although pure virtual methods typically have no implementation in the class that declares them, pure virtual methods in C++ are permitted to contain an implementation in their declaring class, providing fallback or default behaviour that a derived class can delegate to, if appropriate.
Pure virtual functions can also be used where the method declarations are being used to define an 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 OO languages do not support multiple inheritance, they often provide a separate interface mechanism. An example is the Java programming language.
Read more about this topic: Virtual Function
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—Ralph Waldo Emerson (1803–1882)