[C++] Virtual Function

Let's think about OOP literally while reading the code below.

 

#include <iostream>
using namespace std;
 
class AAA
{
public:
    void ABC(){
        cout<<"AAA is called"<<endl;
    }
};
  
class BBB : public AAA
{
public:
    void ABC(){
        cout<<"BBB is called"<<endl;
    }
};
 
class CCC  : public BBB
{
public:
    void ABC(){
        cout<<"CCC is called"<<endl;
    }
};
 
int main(void)
{
    CCC* c = new CCC;
    BBB* b = c;
    AAA* a = b;
 
    a->ABC();
    b->ABC();
    c->ABC();
 
    delete c;
 
    return 0;
}

 

 

Pure Virtual Function

 

A virtual function that have nothing implemented inside.

 

#include <iostream>
using namespace std;
 
class Person
{
private:
    virtual void Move() = 0;
public:
    void Do()
        {
            cout<<"Let's ";
            Move();
        }
};
 
class Artist: public Person
{
private:
    virtual void Move()
    {
        cout<<"Create"<<endl;
    }
};
 
class Athlete: public Person
{
private:
    virtual void Move()
    {
        cout<<"Win"<<endl;
    }
};
 
int main()
{
    Artist Twice;
    Athlete Son;
 
    Twice.Do();
    Son.Do();
}

 

As you see pure virtual function is a fuction, that needs an overriding by derived class.

 

Every classes that have declaration of pure virtual function is called Abstract Class.

 

Which means that it's not a perfect class so it's impossible to make an object.

 

If every member functions of a class is pure virtual, we call the class an Interface.

 

 

Polymorphism

 

Polymorphism means having many forms.

 

And from the perspective of OOP, Polymorphism means

 

"A call to a member function will cause a different function to be executed depending on the type of object that invokes the function"

 

And when you explain OOP theory Polymorphism is one of most important ideas with Abstraction.

 

#include <iostream>
using namespace std;
 
class Shape {
   protected:
      int width, height;
 
   public:
      Shape(int a, int b) : width(a), height(b)
      virtual int area() = 0;
};
class Rectangle: public Shape {
   public:
      Rectangle(int a, int b) : Shape(a, b) { }
      
      int area () {
         cout <<"Rectangle class area :"<<width * height<<endl;
         return 0;
      }
};
 
class Triangle: public Shape {
   public:
      Triangle( int a = 0, int b = 0):Shape(a, b) { }
 
      int area () {
         cout <<"Triangle class area :"<<width * height / 2<<endl;
         return 0;
      }
};
 
int main() {
   Shape *shape;
   Rectangle rec(12,6);
   Triangle  tri(6,10);
 
   shape = &rec;
   shape->area();
   shape = &tri;
   shape->area();
 
   return 0;
}

Rectangle class area :72
Triangle class area :30