39. 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 overloading is used to overload or redefines most of the operators available in C++. It is used to perform the operation on the user-defined data type. For example, C++ provides the ability to add the variables of the user-defined data type that is applied to the built-in data types.

The advantage of Operators overloading is to perform different operations on the same operand.

Overloadable/Non-overloadableOperators

Following is the list of operators which can be overloaded −

+	–	*	/	%	^
&	|	~	!	,	=
<	>	<=	>=	++	—
<<	>>	==	!=	&&	||
+=	-=	/=	%=	^=	&=
|=	*=	<<=	>>=	[]	()
->	->*	new	new []	delete	delete []

Following is the list of operators, which can not be overloaded −

::

.*

.

?:

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 atleast one operand of the user-defined data type.
• We cannot use 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, takes 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

Let’s see the simple example of operator overloading in C++. In this example, void operator ++ () operator function is defined (inside Test class).

// program to overload the unary operator ++.

#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

Following is the example to show the concept of operator overloading using a member function. Here an object is passed as an argument whose properties will be accessed using this object, the object which will call this operator can be accessed using this operator as explained below −

#include <iostream>
using namespace std;

class Box {
   public:
      double getVolume(void) {
         return length * breadth * height;
      }
      void setLength( double len ) {
         length = len;
      }
      void setBreadth( double bre ) {
         breadth = bre;
      }
      void setHeight( double hei ) {
         height = hei;
      }
      
      // Overload + operator to add two Box objects.
      Box operator+(const Box& b) {
         Box box;
         box.length = this->length + b.length;
         box.breadth = this->breadth + b.breadth;
         box.height = this->height + b.height;
         return box;
      }
      
   private:
      double length;      // Length of a box
      double breadth;     // Breadth of a box
      double height;      // Height of a box
};

// Main function for the program
int main() {
   Box Box1;                // Declare Box1 of type Box
   Box Box2;                // Declare Box2 of type Box
   Box Box3;                // Declare Box3 of type Box
   double volume = 0.0;     // Store the volume of a box here
 
   // box 1 specification
   Box1.setLength(6.0); 
   Box1.setBreadth(7.0); 
   Box1.setHeight(5.0);
 
   // box 2 specification
   Box2.setLength(12.0); 
   Box2.setBreadth(13.0); 
   Box2.setHeight(10.0);
 
   // volume of box 1
   volume = Box1.getVolume();
   cout << "Volume of Box1 : " << volume <<endl;
 
   // volume of box 2
   volume = Box2.getVolume();
   cout << "Volume of Box2 : " << volume <<endl;

   // Add two object as follows:
   Box3 = Box1 + Box2;

   // volume of box 3
   volume = Box3.getVolume();
   cout << "Volume of Box3 : " << volume <<endl;

   return 0;
}

When the above code is compiled and executed, it produces the following result −

Volume of Box1 : 210
Volume of Box2 : 1560
Volume of Box3 : 5400