VPTR and VTABLE

In C++, virtual function calls are typically implemented using two key concepts: the Virtual Pointer (VPTR) and the Virtual Table (VTABLE). These concepts play a crucial role in enabling dynamic binding and runtime polymorphism. Here's an explanation of VPTR and VTABLE:

1. Virtual Pointer (VPTR):

   • The Virtual Pointer, often abbreviated as VPTR, is a hidden pointer variable that exists as a member of a class with one or more virtual functions.

   • Each instance of a class with virtual functions (i.e., an object) contains a VPTR that points to the Virtual Table associated with that class.

   • The VPTR is used to determine which version of a virtual function to call when that function is invoked on an object.

2. Virtual Table (VTABLE):

   • The Virtual Table, often abbreviated as VTABLE or vtable, is a data structure associated with a class that contains pointers to the virtual functions of that class.

   • Each class with virtual functions has its own VTABLE, and these tables are typically stored in a read-only section of memory.

   • The VTABLE entries contain addresses of the actual implementations of the virtual functions for that class.

   • When a virtual function is called on an object, the VPTR points to the appropriate VTABLE, and the function's address is retrieved from the VTABLE for execution.

Here's a simplified example of how VPTR and VTABLE work:

class Base {
public:
    virtual void VirtualFunction() {
        // Base class implementation
    }
};

class Derived : public Base {
public:
    void VirtualFunction() override {
        // Derived class implementation
    }
};

int main() {
    Base* ptr = new Derived(); // Create a Derived object and assign it to a Base pointer
    ptr->VirtualFunction();   // Calls the Derived class's implementation at runtime

    delete ptr;
    return 0;
}

In this example, when ptr->VirtualFunction() is called, the program uses the VPTR to determine that the object is of type Derived. It then looks up the corresponding VTABLE (associated with Derived) to find the address of Derived::VirtualFunction() for execution. This mechanism enables dynamic binding and runtime polymorphism.

Keep in mind that the exact implementation of VPTR and VTABLE may vary between different C++ compilers and platforms, and they are part of the internal workings of C++ virtual function mechanism. Developers typically do not need to interact directly with VPTR and VTABLE but should be aware of their role in enabling polymorphism in C++.