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In object-oriented programming such as is often used in C++ and Object Pascal, a virtual function or virtual method is an inheritable and overridable function or method that is dispatched dynamically.
A virtual call requires at least an extra indexed dereference and sometimes a "fixup" addition, compared to a non-virtual call, which is simply a jump to a compiled-in pointer. Therefore, calling virtual functions is inherently slower than calling non-virtual functions.
The default form of dispatch is static. To get dynamic dispatch the programmer must declare a method as virtual. C++ compilers typically implement dynamic dispatch with a data structure called a virtual function table (vtable) that defines the name-to-implementation mapping for a given class as a set of member function pointers. This is purely ...
In C++ implementations, this is commonly done using virtual function tables. If the object type is known, this may be bypassed by prepending a fully qualified class name before the function call, but in general calls to virtual functions are resolved at run time.
Non-virtual or static methods cannot be overridden. The overridden base method must be virtual, abstract, or override. In addition to the modifiers that are used for method overriding, C# allows the hiding of an inherited property or method. This is done using the same signature of a property or method but adding the modifier new in front of it ...
Virtual class; Virtual function (also called virtual method) Virtual function pointer (also called virtual method pointer) Virtual inheritance (Object Oriented Programming) Virtual method table (also called vtable, virtual function table or virtual method table) Viscosity (programming) Void type
[26] [27] In C++, an abstract class is a class having at least one abstract method given by the appropriate syntax in that language (a pure virtual function in C++ parlance). [25] A class consisting of only pure virtual methods is called a pure abstract base class (or pure ABC) in C++ and is also known as an interface by users of the language. [13]
Some languages require that method be specifically declared as virtual (e.g. C++), and in others, all methods are virtual (e.g. Java). An invocation of a non-virtual method will always be statically dispatched (i.e. the address of the function call is determined at compile-time).