What is a vptr in C++?

Table of Contents

• Introduction:
• What is a Virtual Table Pointer (vptr) in C++?
  • Purpose of the Virtual Table Pointer
  • How the Virtual Table Pointer Works
  • Vptr and Inheritance
• Conclusion:

Introduction:

In C++, a virtual table pointer (vptr) is a fundamental concept that supports runtime polymorphism and dynamic method dispatch. It works in conjunction with virtual tables (vtables) to enable objects to call the correct method implementation based on their actual type at runtime. This guide will explain what a vptr is, how it functions, and its role in C++ programming.

What is a Virtual Table Pointer (vptr) in C++?

A virtual table pointer (vptr) is a hidden member in C++ classes that supports dynamic dispatch of virtual functions. It is crucial for enabling polymorphism in C++ object-oriented programming. Each object of a class that contains virtual functions has its own vptr, which points to the vtable for that class.

Purpose of the Virtual Table Pointer

The vptr enables runtime polymorphism by allowing objects to dynamically select the appropriate function implementation based on their actual type. This is achieved through the vptr pointing to the virtual table (vtable) that contains function pointers to the actual implementations of virtual functions.

Example:

In this example, b->display() uses the vptr to call the display method of the actual object type (Base or Derived), thanks to the vtable.

How the Virtual Table Pointer Works

• Vtable Creation: The compiler creates a vtable for each class with virtual functions. The vtable contains pointers to the actual implementations of these functions.
• Vptr Initialization: Each object of a class with virtual functions has a vptr that points to the class’s vtable. The vptr is set when the object is created and remains fixed throughout the object's lifetime.

Example of Vtable and Vptr Relationship:

Consider the following class hierarchy:

• Class A’s Vtable:
  • foo -> Address of A::foo()
• Class B’s Vtable:
  • foo -> Address of B::foo()

Objects of A and B will each have a vptr pointing to their respective vtables. For an object of type A, the vptr points to A's vtable, while for an object of type B, the vptr points to B's vtable.

Vptr and Inheritance

When dealing with inheritance, each derived class has its own vtable. An object of a derived class has a vptr pointing to the vtable of the most-derived class. This allows the correct method implementations to be called based on the object's type.

Example with Multiple Inheritance:

In this example, a->foo() calls the foo method from class C because the vptr of a points to C's vtable.

Conclusion:

The virtual table pointer (vptr) is essential for dynamic method dispatch and runtime polymorphism in C++. By pointing to the virtual table (vtable) of a class, the vptr enables objects to call the appropriate method implementations based on their actual types. Understanding the role and functionality of the vptr helps in mastering C++'s object-oriented programming and effectively implementing polymorphism in your code.