Virtual Method Table (VMT)
A Virtual Method Table (VMT), a virtual function table or vtable, is a mechanism used in object-oriented programming languages, such as C++ and Java, to support dynamic dispatch or runtime polymorphism. The VMT is a data structure that holds pointers to the virtual methods (functions) of a class, allowing the appropriate implementation of a method to be called based on the actual type of the object at runtime, rather than its declared type at compile time.
Purpose and Role: The primary purpose of a Virtual Method Table is to enable runtime polymorphism, which is a key feature of object-oriented programming. Polymorphism allows objects of different classes to be treated as objects of a common superclass, and the appropriate method implementation is chosen dynamically based on the actual type of the object. This makes it possible to write more flexible and extensible code, as new classes can be added or modified without changing the code that uses these classes.
Components:
- Virtual methods: These are methods declared in a class with the "virtual" keyword (in C++) or non-final methods in Java. Virtual methods can be overridden by derived classes to provide specific implementations.
- Virtual Method Table: This is a data structure associated with each class that has virtual methods. The VMT contains pointers to the virtual methods of the class, with each entry corresponding to a specific method.
- VMT pointer: Each object instance of a class with virtual methods has a hidden pointer to the Virtual Method Table of its class. This pointer is used to look up the appropriate method implementation at runtime.
Importance: The Virtual Method Table is crucial for supporting runtime polymorphism in object-oriented programming languages. It enables more flexible and extensible code design by allowing objects of different classes to be treated as objects of a common superclass, with the correct method implementation being chosen dynamically based on the actual type of the object.
In summary, a Virtual Method Table is a data structure used in object-oriented programming languages to support dynamic dispatch or runtime polymorphism. It holds pointers to the virtual methods of a class, enabling the appropriate method implementation to be called based on the actual type of the object at runtime. This mechanism is essential for writing flexible and extensible code in object-oriented programming languages.
See Also