Pure virtual function

In object-oriented programming, a pure virtual function or pure virtual method is a virtual function that has a declaration (signature), but no definition (implementation). This may be used where it does not make sense to provide a default implementation of a method.

In C++, pure virtual functions only make sense (and are only legal) in an inheritance tree. Classes that have one or more pure virtual functions cannot be instantiated. This is because the compiler would not have any code to execute if the user of the class tried to call a pure virtual function. Thus, in C++, pure virtual functions force deriving classes to implement a piece of functionality to fulfil the pure virtual function.

A real world example may be a class of type account. The designer may deem it undesirable to provide a default implementation of a "Calculate Interest" method, instead leaving the actual implementation to the designers of the derived classes "Current Account" and "Savings Account."