Operator Overloading

1
Operator Overloading
Reading Chapter 8
2
Why overload operators?

• We can define new data types using classes.
• Member functions are invoked by sending messages.
• For many classes, this notation is cumbersome.
• For some classes, especially mathematical ones,
  it would be nice to be able to use operators with
  instances of these classes.

3
Example

• Suppose we have
• a class (called Time) for representing time of
  the day
• two Time objects time1 and time2
• We want to be able to do things like
• compare times
• if (time1 lt time2)
• print times to an output stream
• cout ltlt "The first time is " ltlt time1 ltlt endl

4
Simple example in C

• The division operator / is used for
• integer division
• floating point division
• Actually performs two different types of
  division, but we use the same operator, i.e., it
  is overloaded.
• We will talk about how to overload or redefine
  our own operators for classes

5
How to overload operators?

• An overloaded operator is nothing but a
• function
• The name of the function is the keyword operator
  followed by the symbol for the operator being
  overloaded.
• Example
• the function name operator would be used to
  overload the operator.

6
Operators That Can Be Overloaded
- / ! lt gt - /
ltlt gtgt gtgt ltlt ! lt gt -- -gt -gt
( ) new delete new delete
7
Details

• Overloading an operator like allows statements
  like
• object1 object2
• This does not allow statements like
• object1 object2
• The operator must be overloaded separately.
• The "aritiy" (number of operands) of the operator
  cannot change.

8
Implementing operator overloading

• The functions must have access to the private
  member data of the class.
• Two ways to write operator overloading functions
• Member functions
• Friend functions

9
Using member functions

• If the first operand of the operator is a class
  instance or a reference to a class instance, the
  overloaded operator can be implemented as a
  member function.
• Example invocation
• object object1 object2
• Translated by compiler to
• object object1.operator(object2)
• When overloading (), , -gt or any of the
  assignment operators, the operator overloading
  function must be declared as a class member.

10
Example - Rational Numbers
class Rational public Rational(int
n 0, int d 1) Rational
operator(const Rational a) const
Rational operator-(const Rational a) const
Rational operator(const Rational a) const
private int numerator int
denominator
11
Overloaded multiplication
Rational Rationaloperator(const Rational
a) int n (this).numerator
a.numerator int d (this).denominator
a.denominator return Rational(n,d)
12
Assignment

• Write the 2 other overloaded operators for the
  Rational class.

13
Rational Rationaloperator(const Rational
a) int commonD denominator
a.denominator int n denominator a.
numerator numerator a.
denominator return Rational(n,commonD)
14
Using friend functions

• If the first operand of a binary operator is not
  a class instance nor a reference to a class
  instance, the overloaded operator can be
  implemented as a friend function
• Both operands are function arguments.
• Example invocation
• cout ltlt object1
• Translated by compiler to
• operatorltlt(cout,object1)

15
Example Overloadingltlt and gtgt

• We often want to overload the insertion (gtgt) and
  extraction (ltlt) operators so that objects can be
  written and read using these operators.
• Phone number example want to be able to read
  and write in format like
• [phone]
• Input statements might look like
• cin gtgt phone1
• Output statements might look like
• cout ltlt "My phone number is " ltlt phone1 ltlt
  endl

16
Overloading ltlt and gtgt

• class PhoneNum
• friend ostream operatorltlt (ostream, const
  PhoneNum)
• friend istream operatorgtgt (istream,
  PhoneNum)
• private
• int areaCode
• int prefix
• int number

17
Overloading ltlt and gtgt

• ostream operatorltlt (ostream output, const
  PhoneNum num)
• output ltlt "(" ltlt num.areaCode ltlt ") " ltlt
  num.prefix ltlt
• "-" ltlt num.number
• return output
• istream operatorgtgt (istream input, PhoneNum
  num)
• input gtgt num.areaCode gtgt num.prefix gtgt
  num.number
• return input

18
Overloading ltlt and gtgt

• void main
• PhoneNum phone
• cout ltlt "Enter phone number like 999 325
  0007" ltlt endl
• cin gtgt phone
• cout ltlt "The number is" ltlt phone ltlt endl

19
Assignment

• Rewrite the Time class to include overloaded
  operators for insertion, extraction, and
  comparison (gtgt, ltlt, and )

class Time public Time( )
void SetTime( int, int, int) void
PrintMilitary( ) void printStandard( )
private int hour // 0-23
int minute // 0-59 int second //
0-59