Virtual method table

A virtual table is a mechanism used in Programming languages to support dynamic polymorphism, i.e. at run-time.

Suppose a program contains several classes in an inheritance heirarchy: a superclass, Cat, and two subclasses, Tiger and HouseCat. Class Cat defines a method named "speak" but provides no implementation. Instead, its subclasses must provide an appropriate implementation (i.e. either meow or roar). In a compiled language such as C++, the run-time environment must be able to determine which version of the speak method to call through a base class pointer. This is called polymorphism on the programmer's side, but is implemented using virtual tables (a.k.a. vtables).

When a class is compiled, it also includes data and function pointers from its parent classes. In this example, a Tiger also includes everything a Cat does when compiled. However, the actual function code is not duplicated: rather, a pointer to the parent class code is included. This helps reduce code bloat and redundancy. When one calls a method through a derived class, it knows exactly where to find the relevant code. When one calls a method through the superclass, it does not necessarily know what code to execute for e.g. a virtual method (C++) or a non-final method (Java). Instead, the compiler creates a virtual table that is, essentially, a table of pointers pointing to possible implementations.

If an object is referenced through a base-class pointer or reference at run-time, the program uses the virtual table to "look up" which method to call. This does incur a speed hit, however, as compilers mature they are better able to optimize virtual tables as well as the code that uses them.