Operator Overloading

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Operator Overloading
Department of Computer and Information
Science,School of Science, IUPUI
Dale Roberts, Lecturer Computer Science,
IUPUI E-mail droberts_at_cs.iupui.edu
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Operator Overloading

• Function-call notation is cumbersome for certain
  kinds of classes, especially mathematical classes
  
• Allows extendable design
• Most appropriate for math classes. eg. Matrix,
  Vector, etc.
• Gives Operators Class-Specific Functionality
• In-built or Standard Overloading for Basic
  Numerical Data Types -- can be used with int,
  float, doubles
• Analogous to Function Overloading -- operator_at_ is
  Used as the Function Name
• 40 Operators can be Overloaded to Give
  Class-Specific Functionality
• C enables programmers to overload operators to
  be sensitive to the context in which they are
  used.  The compiler generates appropriate code
• Easier to read

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Requirements of Overloaded Operators

• Their Meaning Should be Intuitive -- Should
  Mean Addition
• When Appropriate, they Should be Associative -- a
  b Should Result in an Object, c of the Same
  Class
• If these Conditions are Not Satisfied then it is
  Better to Use Member Functions and Not Operator
  Overloading
• To use an operator on class objects, that
  operator must be overloaded - with two exceptions
  - the assignment operator (), which performs a
  member wise copy, and the address () operator

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Forms of Overloaded Operators

• Member Functions
• Friend Functions
• Free-Standing or Global Functions

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Operator Functions

• When to make class members, friends or global
  functions?
• If member function, then this is implicitly
  available for one of the arguments
• When overloading ( ), , -gt, or , the operator
  overloading function must be declared as a class
  member.  For other operators, the overloading
  functions can be non-members
• When an operator function is implemented as a
  member function, the left most (or only in the
  case of unary operators) operand must be a class
  object (or a reference to a class object) of
  operator's class
• If the left operand must be an object of a
  different class or a built-in type, this operator
  must be implemented as a non-class member. eg.
  ltlt, gtgt operators
• An operator function implemented as a non-member
  must be a friend if it needs to access non-public
  data members of that class.
• The overloaded ltlt operator must have a left
  operand of type ostream.  Therefore, it must be a
  non-member function.  Also, it may require access
  to the private data members of the class.  Thus,
  it needs to be a friend function for that class.
• Similar observation holds for gtgt operator which
  has a left operand of type istream.
• Operator member functions are classed only when
  the left operand of a binary operator is
  specifically an object of that class or when the
  single operand of a unary operator is an object
  of that class.
• If the operator needs to be commutative (a b
  b a), then making it a non-member function is
  necessary.

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Restrictions of Overloaded Operators

• New Operators CANNOT be Created
• Fundamental Data Types (e.g. int) CANNOT be
  Overloaded
• Operator Priority CANNOT be Changed
• Operator Associativity CANNOT be Changed
• The arity of CANNOT be changed -- can Take ONLY
  One or TWO Arguments
• Two Separate Overloaded Functions (With Different
  Signatures) can be Created for Operators Which
  Exist in Pre-fix and Post-fix Form --
• Overloaded Operators are NOT IMPLICITLY
  Associative or Commutative, Even if the Original
  Operators were Associative or Commutative --
  Associativity and Commutativity MUST be
  EXPLICITLY IMPLEMENTED
• Overloading the operator does not automatically
  overload related operators (, , etc).  If
  needed, these related operators must be
  explicitly overloaded

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Unary Overloaded Operators -- Member Functions

• Invocation in Two Ways -- Object_at_ (Direct) or
  Object.operator_at_() (As a Function)
• class number
• int n
• public
• number(int x 0)n(x)
• number operator-()return number (-n)
• main()
• number a(1), b(2), c, d
• //Invocation of "-" Operator -- direct
• d -b //d.n -2
• //Invocation of "-" Operator -- Function
• c a.operator-() //c.n -1

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Binary Overloaded Operators -- Member Functions

• Invocation in Two Ways -- ObjectA _at_ ObjectB
  (direct) or ObjectA.operator_at_(ObjectB) (As a
  Function)
• class number
• int n
• public
• number(int x 0)n(x)
• number operator(number ip)
• return number (ip.n n)
• main()
• number a(1), b(2), c, d
• //Invocation of "" Operator -- direct
• d a b //d.n 3
• //Invocation of "" Operator -- Function
• c d.operator(b) //c.n d.n b.n 5

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Operator Overloading Using a Friend Function

• Number of Parameters Accepted by an Overloaded
  Friend Operator Function Depend Upon the Operator
  Type -- One for Unary Operators and Two for
  Binary Operators
• class complex
• int re, im
• public
• complex(int ip1 0, int ip2 0)
• re(ip1), im(ip2)
• friend complex operator(complex,
  complex)
• //Friend Operator Function
• complex operator(complex a, complex b)
• return complex(a.reb.re, a.imb.im)
• main()
• complex one(1,1), two(2,2), three
• three operator(one, two) //three one
  two

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Operator Functions as Class Members vs. as friend
Functions

• Non-member overloaded operator functions
• Enable the operator to be commutative
• HugeInteger bigInteger
• int integer
• bigInteger integer bigInteger
• or
• bigInteger biginteger integer

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Global Operator Overloading

• Similar to friend Function Overloading, Except
  the Keyword friend is Omitted and Global
  Functions CANNOT ACCESS private Members
• class complex //All Public Members!
• public
• int re, im
• complex(int ip1 0, int ip2 0)
• re(ip1), im(ip2)
• void operator!(complex a)
• int temp a.re a.re a.im a.im temp
• cout ltlt "Real " ltlt a.re ltlt endl
• cout ltlt "Imaginary " ltlt a.im ltlt endl
• main()
• complex one(1,2)
• operator!(one)

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Overloading of Operators Having a Variable Arity

• Operators Such as and - Can be Unary or Binary
• Overloading of Such Operators Involves Creating a
  Unary Function (One Operand) and a Binary
  Function (Two Operands)
• Only if Both the Forms are Used, They Need to be
  Implemented
• class number
• int n
• public
• number(int x 0)n(x)
• number operator-()n -n return this
• number operator-(number ip)
  return(n-ip.n)
• main()
• number one(1), two(2), three
• one -one //unary operator
• three one - two //three.n -3

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Operators with Prefix and Postfix Forms

• Separate Functions for Each -- Prefix and Postfix
  -- Forms are Needed
• Prefix Form is Treated as an Unary Operator
• Postfix Form is Treated as a Binary Operator

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Prefix Overloaded Function -- Example

• class number
• int n
• public
• number(int x)n(x) //Constructor
• //prefix operator -- unary
• number operator()
• number numberoperator()
• n return this
• main()
• number one(10) //one.n 10
• one //one.n 11

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Postfix Overloaded Function -- Example

• Postfix Operator is Implemented as a Binary
  Operator with an int Argument with a Default
  Value of 0
• class number
• int n
• public
• number(int x)n(x) //Constructor
• //postfix operator -- binary -- int
  argument
• number operator(int)
• number numberoperator(int y)
• n y return this
• main()
• number one(10) //one.n 10
• one.operator(2) //one.n 12

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Special Overloading Forms

• A Few Operators Require Special Treatments During
  Overloading
• Conversion Operator
• const Array Operator
• Function Call -- Parenthesis Operator
• Stream Insertion -- ltlt Operator
• Stream Extraction -- gtgt Operator
• Pointer to Member -- -gt Operator
• Assignment Operator
• new Operator
• delete Operator

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Overloading Stream-Insertion and
Stream-Extraction Operators

• Overloaded ltlt and gtgt operators
• Must have left operand of types ostream ,
  istream respectively
• It must be a non-member function (left operand
  not an object of the class)
• It must be a friend function if it accesses
  private data members

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Converting between Types

• Cast operator
• Convert objects into built-in types or other
  objects
• Conversion operator must be a non-static member
  function.
• Cannot be a friend function
• Do not specify return type
• For user-defined class A
• Aoperator char () const // A to char
• Aoperator int() const //A to int
• Aoperator otherClass() const //A to
  otherClass
• When compiler sees (char ) s it calls
• s.operator char()

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Converting between Types (cont)

• The compiler can call these functions to create
  temporary objects.
• If s is not of type char
• Calls Aoperator char () const for
• cout ltlt s

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Special overloading forms - Example

• Special Forms Example

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Acknowledgements

• These slides were originally development by Dr.
  Uday Murthy and Dr. Rajeev Raje.
• Some contents comes from the Deitel slides that
  accompany your text.