In object-oriented programming, a virtual function or virtual method is a function or method whose behaviour can be overridden within an inheriting class by a function with the same signature. This concept is a very important part of the polymorphism portion of object-oriented programming (OOP).
The concept of the virtual function solves the following problem:
In OOP when a derived class inherits a base class, an object of the derived class may be referred to (or cast) as either being the base class type or the derived class type. If there are base class methods overridden by the derived class, the method call behavior is ambiguous.
The distinction between virtual and non-virtual resolves this ambiguity. If the function in question is designated virtual in the base class then the derived class' function would be called (if it exists). If it is not virtual, the base class' function would be called.
Virtual functions overcome the problems with the type-field solution by allowing the programmer to declare functions in a base class that can be redefined in each derived class.
In C++ virtual methods are declared by prepending the virtual keyword to the function's declaration.
Source: Wikipedia (Virtual function)
