Operator overload

Overloaded operators are functions with special names: the keyword operator followed by the symbol for the operator being defined. Like any other function, an overloaded operator has a return type and a parameter list.

#include <iostream>

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
};

int main(void) {
   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: We can use + sign with class
   Box3 = Box1 + Box2;

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

   return 0;
}

Output:

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

Operators that can be overloaded

Binary Arithmetic                               ->     +, -, *, /, %
Unary Arithmetic                                ->     +, -, ++, —
Assignment                                      ->     =, +=,*=, /=,-=, %=
Bit- wise                                       ->     & , | , << , >> , ~ , ^
De-referencing                                  ->     (->)
Dynamic memory allocation and De-allocation     ->     New, delete 
Subscript                                       ->     [ ]
Function call                                   ->     ()
Logical                                         ->     &,  | |, !
Relational                                      ->     >, < , = =, <=, >=

But, among them, there are some operators that cannot be overloaded. They are

Scope resolution operator                       ->     ::
Member selection operator                       ->     .
Member selection through                        ->     *
Conditional operator                            ->     ?:
Sizeof operator                                 ->     sizeof()