What is vtable in C++

The vtable, or virtual table, is a mechanism used in C++ to support dynamic dispatch (or run-time method binding).

When a class defines a virtual function (or it inherits one), the compiler builds a vtable for that class. The vtable is essentially an array of function pointers that point to the virtual functions defined in the class. Each object of the class is then provided with a hidden pointer to its class's vtable (often called a vptr).

When a virtual function is called on an object, the actual function that gets called is determined by following the object's vptr to the vtable and then looking up the appropriate entry in the vtable. This allows the correct function to be called even if the exact type of the object is not known at compile time, enabling polymorphic behavior.

Here's a simple example:

class Base {
public:
    virtual void foo() { cout << "Base::foo" << endl; }
};

class Derived : public Base {
public:
    void foo() override { cout << "Derived::foo" << endl; }
};

int main() {
    Base* ptr = new Derived();
    ptr->foo();  // Outputs "Derived::foo"
    delete ptr;
    return 0;
}

In this example, ptr is a pointer to Base, but it actually points to an object of type Derived. When ptr->foo() is called, the vtable mechanism ensures that Derived::foo is called, even though the compile-time type of ptr is Base*.