How Do Virtual Functions and Vtables Enable Polymorphism in C ?

How Virtual Functions and Vtables Work in C

Virtual functions are a fundamental aspect of object-oriented programming, allowing objects of derived classes to override methods inherited from base classes. But how are virtual functions implemented beneath the surface?

Vtable Implementation

When a class defines a virtual function, the compiler generates a vtable (virtual function table). This table stores pointers to all virtual functions for that class. Each object instance has a vptr (virtual pointer) that points to the base address of its class's vtable.

Vtable Access and Modification

Accessing or modifying the vtable at runtime is generally considered unsafe in C . The language definition does not mandate the use of vtables, and different implementations may have varying behaviors. In most cases, the vtable exists only for classes with virtual functions and cannot be directly modified.

Vtables and Abstract Classes

Abstract classes have pure virtual functions, which are required to be overridden by derived classes. In some implementations, the vtable for abstract classes may contain NULL function pointers for pure virtual functions. This ensures that calling a pure virtual function on an abstract class results in undefined behavior.

Performance Impact

Having virtual functions adds a performance overhead due to the need for dynamic binding at runtime. This overhead only affects the execution of virtual functions, not the entire class. Overriding virtual functions does not change the execution speed. However, creating separate vtables for derived classes can result in additional space overhead.

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