What is a vtable in C?

Table of Contents

• Introduction:
• What is a Virtual Table (vtable) in C++?
  • Purpose of the Virtual Table
  • Structure of the Virtual Table
  • Vtable and Inheritance
• Why C Does Not Have a Vtable
• Conclusion:

Introduction:

In C++, a virtual table (vtable) is essential for supporting runtime polymorphism and dynamic dispatch. However, this mechanism is absent in C. This guide will explain what a vtable is in C++, why it does not exist in C, and how C handles dynamic behavior differently.

What is a Virtual Table (vtable) in C++?

A virtual table (vtable) is a data structure used in C++ to enable runtime polymorphism through virtual functions. Each class with virtual functions has a corresponding vtable, which contains pointers to the virtual functions defined in that class. Each object of such a class has a virtual table pointer (vptr) pointing to the vtable of its class. This setup allows the program to dynamically dispatch method calls to the correct function implementations based on the actual object type.

Purpose of the Virtual Table

The vtable supports dynamic dispatch by enabling objects to call the appropriate virtual function implementations. When a virtual function is called, the object’s vptr points to the correct vtable, which in turn points to the function implementations.

Example in C++:

In this example, obj->show() uses the vtable to dynamically call the appropriate show method depending on whether the object is of type Base or Derived.

Structure of the Virtual Table

• Vtable Creation: The compiler generates a vtable for each class with virtual functions. The vtable includes function pointers to the implementations of virtual functions.
• Vtable and Vptr: Each object of a class with virtual functions contains a vptr that points to its class's vtable. This allows the object to use the vtable for dynamic method dispatch.

Example Structure:

For the following class hierarchy:

• Class A’s Vtable: Contains a pointer to A::func().
• Class B’s Vtable: Contains a pointer to B::func().

Objects of A and B have vptrs pointing to their respective vtables.

Vtable and Inheritance

Derived classes inherit the vtable mechanism from base classes. The derived class’s vtable will include pointers to its own implementations of virtual functions, overriding the base class’s vtable entries.

Example with Inheritance:

Here, a->display() calls B's implementation because the vptr of a points to B's vtable.

Why C Does Not Have a Vtable

C does not support classes, inheritance, or virtual functions, and therefore lacks vtables. Instead, C uses function pointers to achieve dynamic behavior.

Function Pointers in C:

Function pointers allow dynamic function calls but do not provide the same level of abstraction as C++’s vtables. Function pointers can be used to implement basic dynamic dispatch, but without the structure of vtables.

Example of Function Pointers in C:

In this example, function pointers are used for dynamic behavior, mimicking some aspects of virtual functions without using vtables.

Conclusion:

A virtual table (vtable) is a mechanism in C++ that supports runtime polymorphism and dynamic method dispatch through virtual functions. C, lacking classes and virtual functions, does not have vtables. Instead, C relies on function pointers and other techniques to achieve some level of dynamic behavior. Understanding these differences helps in leveraging each language’s features effectively.