Operator Overloading Member Functions

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Operator Overloading
Member Functions
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

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Operator Overloading (Cont)
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

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Operator Functions (Cont)

• 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

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Operator Functions (cont)

• 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.

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Operator Functions (Cont)

• 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
  TWO Arguments (there is no unary )
• Two Separate Overloaded Functions (With Different
  Signatures) can be Created for Operators Which
  Exist in Pre-fix and Post-fix Form --

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Restrictions of Overloaded Operators (Cont)

• Overloaded Operators are NOT IMPLICITLY
  Associative or Commutative, Even if the Original
  Operators were Associative or Commutative --
  Associativity and Commutativity must be
  explicitly implemented. For Associativity this
  means returning an instance of the class.
• 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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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.
• Some information regarding the postfix form the
  increment and decrement operators comes from MSDN.