• In my previous article, I had discussed about how private members of a class can be shared and accessed using “Friend Function”. In this article I am going to show you one another important features of C++ i.e Operator Overloading.
  • Recall that, I have already explained how a function can be overloaded to implement function polymorphism in my post “Function Overloading” and similar to normal function how a constructor can be overloaded with the same mechanism in the post titled “Constructor Overloading”. If you haven’t read yet, I strongly recommend to read that posts first as it will help you in understanding the concept a lot.
  • C++ has the ability to use operators with user-defined data type just like basic data type. C++ provides the special meaning to the operators.
  • By operator overloading we can provide the new definition to operator. In other words, an operator can be used to perform different task besides its traditional purpose by overloading it. A general definition of operator overloading can be given as below:

“Operator overloading is a technique to provide special meaning to an operator.”

  • Fore example if a screw driver can be used to unlock the door just like the key then a screw driver is said to be overloaded as we have used it for different purpose than it is designed for.

Rules For Overloading Operator

  1. Only existing operator in C++ can be overloaded. We cannot create our new operator for overloading.
  2. The overloaded operator must have at lease one operand that is user-defined type.
  3. We can overload all C++ operators except the following ones.

    1. Class member access operator ( . and .* )
    2. Scope resolution operator ( :: )
    3. Size operator ( sizeof )
    4. Conditional operator ( ?: )
  1. We must remember that the precedence of operators cannot be changed  by operator overloading. g. Multiplication operator has higher precedence than addition operator. We cannot change this meaning.
  2. We cannot use friend function to overload following operators. we can use only member function.
Operator Sign Operator Name
= Assignment operator
( ) Function call operator
[ ] Subscript or index operator
– > Class member access operator
  1. While overloading unary operator by using the friend function we have to pass one argument.
  2. While overloading unary operator by using the member function we are not passing any argument to the operator function.
  3. While overloading binary operator by using the friend function we are passing two arguments.
  4. While  overloading binary operator by using the member function we are passing only one argument.
  5. Binary arithmetic operators such as +, -, *, and / must return a value.

How to overload operator?

  • To overload an operator we are using special operator function. The format of operator function is as follow:
return-type  operator  op ( argument1, argument2, … ) ;
  • Where, return-type specifies return type of the function, operator is the keyword and op is the operator that we wish to overload.
  • Operator function can be implemented either by using member function or with the help of friend function.
  • A friend function has only one argument for unary operator and two arguments for binary operator. A member function does not take argument for unary operator but takes one argument for binary operation
  • Operator function can be called by the following two ways:
  1. Using Traditional Method

  • This method is similar to call function which we are using for the other normal function. For unary operation we can call the operator function by:
object-name . operator op(operand);  // where op=operator
  • For binary operation we can call the operator function by:
object-name . operator op(operand1, operand2);
  1. Using Expression Syntax

  • This method is just like the normal expression we use for various arithmetic operations such as addition, subtraction, multiplication, etc. (e.g. a + b)
  • For unary operation we can call the operator function.
op operand      // here op = operator
operand    op
  • For binary operation we can call the operator function by:
operand1  op  operator2