How to declare and call virtual functions in C++?

Virtual function is a polymorphism mechanism that allows a derived class to override the member function of its base class: Declaration: Add the keyword virtual before the function name. Call: Using a base class pointer or reference, the compiler will dynamically bind to the appropriate implementation of the derived class. Practical case: By defining the base class Shape and its derived classes Rectangle and Circle, the application of virtual functions in polymorphism is demonstrated to calculate area and draw shapes.

Virtual function in C

Virtual function is a polymorphism mechanism that allows a derived class to override the properties of its base class Member functions. This enables programmers to define common behavior in a base class while still allowing derived classes to provide its instance-specific implementation of that behavior.

Declaring virtual functions

To declare a virtual function, place the keyword virtual at the beginning of the function name. For example:

class Base {
public:
    virtual void print() const;
};

Call virtual function

Call virtual function, use base class pointer or reference. The compiler will dynamically bind to the appropriate implementation of the derived class. For example:

void doSomething(Base* base) {
    base->print();
}

Practical case

The following is an example of using virtual functions:

#include <iostream>

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

class Rectangle : public Shape {
public:
    Rectangle(double width, double height) : width_(width), height_(height) {}
    
    double area() const override { return width_ * height_; }
    void draw() const override { std::cout << "Drawing rectangle" << std::endl; }
    
private:
    double width_;
    double height_;
};

class Circle : public Shape {
public:
    Circle(double radius) : radius_(radius) {}
    
    double area() const override { return 3.14 * radius_ * radius_; }
    void draw() const override { std::cout << "Drawing circle" << std::endl; }
    
private:
    double radius_;
};

int main() {
    Shape* rectangle = new Rectangle(5, 10);
    Shape* circle = new Circle(5);
    
    std::cout << rectangle->area() << std::endl; // Output: 50
    std::cout << circle->area() << std::endl;   // Output: 78.5
    
    rectangle->draw();   // Output: Drawing rectangle
    circle->draw();     // Output: Drawing circle
    
    return 0;
}

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