CPP Polymorphism and Virtual Methods

What is Polymorphism in C++?

• Poly means many, and morph means form: A polymorphic function is many-formed.
• Polymorphism works only with pointers or references.
• Building polymorphism is :
  • the capability to bind specific derived class objects to base class pointers at runtime.
  • the fundament to this is Use of virtual methods in derived classes.
• It is important to understand the differences between:

  Class	A Class is a specification for a specific type of objects (class example: Person, Car, Animal, ...). They are also referred as complex types. Example: class Person { // here comes the class specification };
  Object	An Object is an occurrence that is created with a Class specification. Example: Person * pObama = new Person(); The "new Person();" creates an object of the Person Class and returns a pointer reference to that object.
  Reference	A Reference tells us that it has an relation to an object created with a Class of the same type as the Reference or an object created with a Class that is a subclass of the Reference. Example: Person * pObama = new Employee(45, 65, 50000); In this case: pObama is a Person Reference that holds a pointer to an Employee Object, which is created by a drived Class, Employee, of the Person Class.

• With Polymorphism we want to execute methods for the created object of a Class even the Reference is of the same Class or a base Class of that Class.
  Example:

  shape.getArea()	length * width, if shape is a reference to a Rectangle object.
  shape.getArea()	radius * radius *PI, if shape is a reference to a Circle object.

  To achieve this kind of Polymorphism we need virtual methods.

What is Virtual Methods?

• In Java is Polymorphism by default built-in, but in C++ we can build the control of the Polymorphism as we want.
• To achieve the polymorphic functionality we make overridden methods virtual.
• A method is virtual if when we write the virtual keyword in front of the method.
• When a method in a class is made virtual all overridden methods in the subclasses are also virtual.
• It is not necessary to write virtual in front of a virtual methods in the derived class, but a good practice to do that.
• Note that the virtual function magic operates only on pointers and references.
  Polymorphism and virtual methods example:

  #include <iostream>
  
  class Person {
  public:
    Person (int age, int weight){
      this->age = age;
      this->weight = weight;
    }
    // this method is made virtual
    virtual void showInfo() {
      std::cout << "I am " << age << " years old " ;
      std::cout << "and weighs " << weight << " kilo.\n\n" ;
    }
    int getAge() { return age; }
    int getWeight() { return weight; }
  protected:
    int age;
    int weight;
  };
  
  class Employee : public Person {
  public:
    Employee (int age, int weight, int salary): Person(age,weight){
      this->salary = salary;
    }
    // The base has made this method virtual
    void showInfo() {
      std::cout << "I am " << age << " years old " ;
      std::cout << "and weighs " << weight << " kilo " ;
      std::cout << "and earns " << salary << " dollar.\n\n" ;
    }
    int getSalary() { return salary; }
  private:
    int salary;
  };
  int main()
  {
    Employee Rune(35, 95, 40000);
    Person & Bengt=Rune;
    // As the showInfo method is virtual 
    // the Employee showInfo() will be executed
    // as Bengt is a reference to an object of Employee
    Bengt.showInfo();
  
    Person * pRicard = new Person(40, 70);
    pRicard->showInfo();
  
    delete pRicard;
    Person * pObama = new Employee(45, 65, 50000);
    // As the showInfo method is virtual 
    // the Employee showInfo() will be executed
    // as pObama is a pointer reference to an object of Employee
    pObama->showInfo();
    delete pObama;
  
    return 0;
  }

  When we run this application the result will be:

  I am 35 years old and weighs 95 kilo and earns 40000 dollar.
  
  I am 40 years old and weighs 70 kilo.
  
  I am 45 years old and weighs 65 kilo and earns 50000 dollar.

  You can download this example here (needed tools can be found in the right menu on this page).