How is virtual pointer implemented in C++?

The virtual pointer mechanism in C++ is implemented through a virtual table, which contains function pointers pointing to member functions of the class. When a base class pointer points to a derived class object, the virtual pointer stores the virtual table address, which is used by the compiler to find and call the correct virtual function. This mechanism allows polymorphism, that is, using base class pointers to operate derived class objects, improving the maintainability and scalability of the code. But it will increase memory overhead and reduce performance.

The implementation mechanism of virtual pointers in C++

Virtual pointers are the core mechanism for realizing polymorphism in object-oriented programming . It allows base class pointers to point to objects of derived classes and call methods in derived classes.

Virtual table

The virtual mechanism in C++ is implemented through virtual tables. Every class has a vtable, which is an array containing function pointers. Function pointers in the virtual table point to member functions of the class.

Virtual function

A virtual function is a function with a virtual table. When a base class pointer points to an object of a derived class, the compiler uses the vtable to find the correct method to call.

Virtual pointer

A virtual pointer is a pointer that stores the address of a virtual table. When the compiler needs to execute a virtual function, it uses a virtual pointer to look up the vtable.

Practical case

Consider the following code:

class Shape {
public:
    virtual double area() = 0;
};

class Rectangle : public Shape {
public:
    double width;
    double height;
    
    double area() override {
        return width * height;
    }
};

class Circle : public Shape {
public:
    double radius;
    
    double area() override {
        return 3.14 * radius * radius;
    }
};

int main() {
    Shape* shapes[] = {new Rectangle(5, 10), new Circle(5)};
    for (Shape* shape : shapes) {
        cout << "Area: " << shape->area() << endl;
    }
    return 0;
}

In this example, the area() function is a virtual function. When the compiler calls area() in the main function, it uses virtual pointers to find the correct version to call.

Implementation details

Virtual pointers and virtual tables are usually generated by the compiler at compile time. Virtual pointers are usually stored at the beginning of the object, while virtual tables are stored in a global data segment.

Advantages

• Allows polymorphism, that is, using base class pointers to operate derived class objects.
• Improves the maintainability and scalability of the code.
• Avoid the overhead of type conversion.

Disadvantages

• Increased memory overhead because each class has a virtual table.
• May reduce performance because the compiler needs to perform extra lookups when executing virtual functions.

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