object oriented programming

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 < time2)
– print times to an output stream
cout << "The first time is " << time1 << 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
+ - * / % ^ & |
~ ! = < > += -= *=
/= %= ^= &= |= << >> >>=
<<= == != <= >= && || ++
-- ->* ‘ -> [] () 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 (), [], -> 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 Rational::operator*(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 Rational::operator+(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 << object1;
• Translated by compiler to
operator<<(cout,object1);

15. Example: Overloading
<< and >>
• We often want to overload the insertion (>>) and extraction
(<<) 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
(662) 325-7505
• Input statements might look like
cin >> phone1;
• Output statements might look like
cout << "My phone number is " << phone1 << endl;

16. Overloading << and >>
class PhoneNum
{
friend ostream& operator<< (ostream&, const PhoneNum&);
friend istream& operator>> (istream&, PhoneNum&);
private:
int areaCode;
int prefix;
int number;
};

17. Overloading << and >>
ostream& operator<< (ostream &output, const PhoneNum &num)
{
output << "(" << num.areaCode << ") " << num.prefix <<
"-" << num.number;
return output;
};
istream& operator>> (istream &input, PhoneNum &num)
{
input >> num.areaCode >> num.prefix >> num.number;
return input;
};

18. Overloading << and >>
void main
{
PhoneNum phone;
cout << "Enter phone number like 999 325 0007" << endl;
cin >> phone;
cout << "The number is:" << phone << endl;
};

19. Assignment
• Rewrite the Time class to include overloaded operators for
insertion, extraction, and comparison (>>, <<, 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
};