Polymorphism in c++

The term “Polymorphism” is the combination of “poly” + “morphs” which means many forms. It is a greek word. In object-oriented programming, we use 3 main concepts: inheritance, encapsulation, and polymorphism.

In C++ polymorphism is mainly divided into two types:

• Compile time Polymorphism
• Runtime Polymorphism

Compile Time Polymorphism

This type of polymorphism is achieved by function overloading or operator overloading.

What is function overloading?

When there are multiple functions with the same name but different parameters then these functions are said to be overloaded. Functions can be overloaded by a change in a number of arguments or/and a change in the type of arguments.

#include <iostream>    
using namespace std;    
class Cal {    
    public:    
static int add(int a,int b){      
        return a + b;      
    }      
static int add(int a, int b, int c)      
    {      
        return a + b + c;      
    }      
};     
int main(void) {    
    Cal C;                                                    //     class object declaration.   
    cout<<C.add(10, 20)<<endl;      
    cout<<C.add(12, 20, 23);     
   return 0;    
}    

Output:

30
55

What is Operator Overloading?

Operator overloading is a compile-time polymorphism in which the operator is overloaded to provide the special meaning to the user-defined data type.

Operator that cannot be overloaded are as follows:

• Scope operator (::)
• Sizeof
• member selector(.)
• member pointer selector(*)
• ternary operator(?:)

Syntax of Operator Overloading

return_type class_name  : : operator op(argument_list)  
{  
     // body of the function.  
}  

Where the return type is the type of value returned by the function.

class_name is the name of the class.

operator op is an operator function where op is the operator being overloaded, and the operator is the keyword.

Rules for Operator Overloading

• Existing operators can only be overloaded, but the new operators cannot be overloaded.
• The overloaded operator contains at least one operand of the user-defined data type.
• We cannot use the friend function to overload certain operators. However, the member function can be used to overload those operators.
• When unary operators are overloaded through a member function take no explicit arguments, but, if they are overloaded by a friend function, take one argument.
• When binary operators are overloaded through a member function takes one explicit argument, and if they are overloaded through a friend function takes two explicit arguments.

C++ Operators Overloading Example

#include <iostream>    
using namespace std;    
class Test    
{    
   private:    
      int num;    
   public:    
       Test(): num(8){}    
       void operator ++()         {     
          num = num+2;     
       }    
       void Print() {     
           cout<<"The Count is: "<<num;     
       }    
};    
int main()    
{    
    Test tt;    
    ++tt;  // calling of a function "void operator ++()"    
    tt.Print();    
    return 0;    
}    

Output:

The Count is: 10

Let’s see a simple example of overloading the binary operators.

#include <iostream>  
using namespace std;  
class A  
{  
    
    int x;  
      public:  
      A(){}  
    A(int i)  
    {  
       x=i;  
    }  
    void operator+(A);  
    void display();  
};  
  
void A :: operator+(A a)  
{  
     
    int m = x+a.x;  
    cout<<"The result of the addition of two objects is : "<<m;  
  
}  
int main()  
{  
    A a1(5);  
    A a2(4);  
    a1+a2;  
    return 0;  
}

Output:

The result of the addition of two objects is : 9 

What is runtime polymorphism?

This type of polymorphism is achieved by Function Overriding.

What is function overriding?

Function overriding on the other hand occurs when a derived class has a definition for one of the member functions of the base class. That base function is said to be overridden.

// C++ program for function overriding
  
#include <bits/stdc++.h>
using namespace std;
  
class base
{
public:
    virtual void print ()
    { cout<< "print base class" <<endl; }
   
    void show ()
    { cout<< "show base class" <<endl; }
};
   
class derived:public base
{
public:
    void print () //print () is already virtual function in derived class, we could also declared as virtual void print () explicitly
    { cout<< "print derived class" <<endl; }
   
    void show ()
    { cout<< "show derived class" <<endl; }
};
  
//main function
int main() 
{
    base *bptr;
    derived d;
    bptr = &d;
       
    //virtual function, binded at runtime (Runtime polymorphism)
    bptr->print(); 
       
    // Non-virtual function, binded at compile time
    bptr->show(); 
  
    return 0;
} 

Output:

print derived class
show base class