Operator Overloading

1
Operator Overloading

• Fundamentals
• Methods (class members friend functions)
• Binary operators
• Stream operators
• Unary operators
• Pre post increment decrement

2
Fundamentals

• Operator overloading using the same operator
  for different data types
• For example, is used to add int, float,
  double, etc.
• Function definition is written the same as any
  other function
• Except function name becomes operator followed
  by the symbol that you are overloading
• operator

3
Built-in Operators

• A few operators dont have to be explicitly
  overloaded (done automatically)
• Assignment operator ()
• Memberwise assignment of class data members
• Address operator ()
• Returns the address of the object in memory
• Both can be explicitly overloaded

4
Restrictions

• For a list of operators that can be overloaded
• See D D, p. 670
• Five operators cannot be overloaded
• ., ., , ?, sizeof
• These properties cannot be changed
• Precedence has higher precedence than
• Associativity (a b) c a (b c)
• Arity number of operands that the operator takes
  (unary binary operators)

5
Restrictions

• Only existing operators can be overloaded
• Overloading an assignment operator and an
  addition operator will not automatically overload
  the operator
• F1 F2 F3
• F1 F2
• Misuse
• Overloading the operator to subtract

6
Method 1

• Class Members overloading an operator by adding
  another method (member function) to the class
  itself
• Must use this method for overloading (), , -gt
  or any of the assignment operators
• Can use this method when the leftmost operand is
  a class object
• This works fine with f1 f2 2 which converts
  2 to a fraction (via a conversion constructor)
  and adds it to f2
• But will not work with f1 2 f2 since 2 is
  not of class fraction

7
Method 1

• class Fraction
• int num, den
• public
• Fraction(int n0, int d1)
• num(n),den(d)
• Fraction operator
• (const Fraction f)
• Fraction temp
• temp.numnumf.denf.numden
• temp.dendenf.den
• return temp
• void print() const
• coutltltnumltlt'/'ltltdenltltendl
• Note cab is interpreted as
• (see method1.txt) c.operator(a.operator(b))

void main() Fraction a(1,3) Fraction b(2,3)
Fraction c c a b c.print()
8
Method 2

• Friend Functions overloading an operator as a
  friend function (non-member function) of the
  class
• Can use this method when the leftmost or
  rightmost operand is a class object
• This will fix our problem when we have a
  different class object on the left side of an
  operator
• Must use this method when the leftmost operand is
  NOT a class object (for example, coutltltf)

9
Method 2
void main() Fraction a(1,3) Fraction b(2,3)
Fraction c c a b c.print()

• class Fraction
• int num, den
• public
• Fraction(int n0, int d1)
• num(n),den(d)
• friend Fraction operator
• (const Fraction a, const Fraction b)
• Fraction temp
• temp.numa.numb.denb.numa.den
• temp.dena.denb.den
• return temp
• void print() const
• coutltltnumltlt'/'ltltdenltltendl
• Note cab is interpreted as
• (See method2.txt) c.operator(operator(a, b))

10
Why Does This Program Run?

• class Fraction
• int num, den
• public
• Fraction(int n0, int d1)
• num(n),den(d)
• friend Fraction operator
• (const Fraction a,
• const Fraction b)
• Fraction temp
• temp.numa.numb.denb.numa.den
• temp.dena.denb.den
• return temp
• void print() const
• coutltltnumltlt'/'ltltdenltltendl

void main() Fraction a, b a a 1 b 2.5
b a.print() //1/1 b.print()
//2/1 (See run.cpp)
11
Because

• C will try and convert types to perform
  operations requested
• Asks Can I convert an int to a Fraction?
• Yes, we have a constructor that takes and integer
  and returns the equivalent Fraction
• System invokes constructor to build Fraction from
  int
• What about Fraction double Fraction?
• Affirmative. Can convert double to int (truncate)
• Can then convert int to Fraction (constructor)

12
Conversion Constructor

• A constructor that transforms objects of one type
  into objects of another type
• Write a constructor that will convert a double
  into a fraction (exercise1.txt)

13
Overloading Stream Operators

• cin object
• An instance of the istream class
• operatorgtgt
• So cingtgtagtgtb becomes
• operatorgtgt(cin,a) operatorgtgt(cin,b)
• cout object
• An instance of the ostream class
• operatorltlt
• So coutltltaltltb becomes
• operatorltlt(cout,a) operatorltlt(cout,b)

14
Stream Operators

• class Fraction
• . . .
• friend istream operatorgtgt(istream in, Fraction
  f)
• char ch
• ingtgtf.numgtgtchgtgtf.den
• return in
• friend ostream operatorltlt(ostream out,
• const Fraction f)
• outltltf.numltlt'/'ltltf.den
• return out
• int main()
• Fraction a,b
• cingtgtagtgtb
• coutltltaltlt" "ltltbltltendl
• return 0
• (See stream.txt)

15
Overloading Unary Operators

• Can be overloaded as a class member or as a
  friend function
• For example, the logical not operator (!)
• Will change true to false or false to true
• Returns either true (non-zero) or false (zero)
• Use bool (boolean) data type

16
Overloading Unary Operators

• As a class member
• bool operator!() const
• bool Fractionoperator!()const
• if(num) return false
• return true
• As a friend function
• friend bool operator!(const Fraction )
• bool operator!(const Fraction f)
• if(f.num) return false
• return true
• (See unary.txt)

17
Overloading --

• Preincrement
• Original code f1
• Compiler will generate f1.operator()
• Function prototype Fraction operator()
• Returns a reference to itself

18
Overloading --

• Postincrement
• Original code f1
• Compiler will generate f1.operator(0)
• This is a dummy value to distinguish it from
  preincrement
• Function prototype Fraction operator(int)
• Value return, not a reference return

19
Unary Operators

• class Fraction
• . . .
• Fraction operator()//preincrement
• numden
• return this
• Fraction operator(int)//postincrement
• Fraction temp this
• numden
• return temp
• void main()
• Fraction a,b,c
• b a //b.operator(a.operator())
• c a //c.operator(a.operator(0))
• coutltltaltlt" "ltltbltlt" "ltltcltltendl
• (see prepost.txt)

20
Use of const

• Why use const at the beginning of a function?
• When that function returns a reference, the user
  may have direct access to the objects data
  members
• See const.txt

21
Class Exercise

• What is the output of the following program
  (exercise2.txt)?