Purpose
Further information: Dynamic dispatchThe concept of the virtual function solves the following problem:
In OOP when a derived class inherits from a base class, an object of the derived class may be referred to via a pointer or reference of either the base class type or the derived class type. If there are base class methods overridden by the derived class, the method actually called by such a reference or pointer can be bound either 'early' (by the compiler), according to the declared type of the pointer or reference, or 'late' (i.e. by the runtime system of the language), according to the actual type of the object referred to.
Virtual functions are resolved 'late'. If the function in question is 'virtual' in the base class, the most-derived class's implementation of the function is called according to the actual type of the object referred to, regardless of the declared type of the pointer or reference. If it is not 'virtual', the method is resolved 'early' and the function called is selected according to the declared type of the pointer or reference.
Virtual functions allow a program to call methods that don't necessarily even exist at the moment the code is compiled.
In C++, virtual methods are declared by prepending the virtual keyword to the function's declaration in the base class. This modifier is inherited by all implementations of that method in derived classes, meaning that they can continue to over-ride each other and be late-bound.
Read more about this topic: Virtual Function
Famous quotes containing the word purpose:
“Politics should share one purpose with religion: the steady emancipation of the individual through the education of his passions.”
—George F. Will (b. 1941)
“The United States is the only great nation whose government is operated without a budget. The fact is to be the more striking when it is considered that budgets and budget procedures are the outgrowth of democratic doctrines and have an important part in developing the modern constitutional rights.... The constitutional purpose of a budget is to make government responsive to public opinion and responsible for its acts.”
—William Howard Taft (1857–1930)
“Rule of religion: purpose breathes even in dirt and stones.”
—Mason Cooley (b. 1927)