C++ Polymorphism

What is Polymorphism?

Polymorphism means "many forms". A single interface (base class pointer) can call different implementations depending on the actual object type at runtime.

Virtual Functions

Declare a method virtual in the base class to enable runtime polymorphism:

#include <iostream>
using namespace std;

class Shape {
  public:
    virtual void draw() {
        cout << "Drawing a shape" << endl;
    }
};

class Circle : public Shape {
  public:
    void draw() override {
        cout << "Drawing a Circle" << endl;
    }
};

class Triangle : public Shape {
  public:
    void draw() override {
        cout << "Drawing a Triangle" << endl;
    }
};

int main() {
    Shape* shapes[3];
    shapes[0] = new Shape();
    shapes[1] = new Circle();
    shapes[2] = new Triangle();

    for (int i = 0; i < 3; i++) {
        shapes[i]->draw();   // correct method called for each type
    }

    // cleanup
    for (int i = 0; i < 3; i++) delete shapes[i];
    return 0;
}

Output:

Drawing a shape
Drawing a Circle
Drawing a Triangle

#include <iostream>
using namespace std;

class Animal {
  public:
    virtual void speak() { cout << "..." << endl; }
    virtual ~Animal() { }
};

class Dog   : public Animal { public: void speak() override { cout << "Woof!" << endl; } };
class Cat   : public Animal { public: void speak() override { cout << "Meow!" << endl; } };
class Bird  : public Animal { public: void speak() override { cout << "Tweet!" << endl; } };

int main() {
    Animal* animals[] = { new Dog(), new Cat(), new Bird() };
    for (Animal* a : animals) { a->speak(); delete a; }
    return 0;
}

Summary

• Polymorphism allows different objects to respond to the same interface differently.
• Use virtual in the base class and override in derived classes.
• Base class pointers/references can hold derived objects.
• Add a virtual destructor to base classes when using polymorphism.