Operator Overloading

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Operator Overloading

• CS 308 Data Structures

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What is operator overloading?

• Changing the definition of an operator so it can
  be applied on the objects of a class is called
  operator overloading.
• To overload an operator, we need to write a
  function for the operator we are overloading.

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Special cases

• The assignment operator () may be used with
  every class without explicit overloading.
• The default behavior of the assignment operator
  is a memberwise assignment of the data members of
  the class.
• The address operator () may also be used with
  objects of any class without explicit
  overloading.
• It returns the address of the object in memory.

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Explicit overloading of the assignment operator

• The default overloading is not enough for classes
  with pointer members.
•  
• void operator(class_name)
•  
• class string
• private
• char s
• int size
• public
• string(char ) // constructor
• string() // destructor
• void operator(string)
• void print()
• void copy(char )

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• void stringoperator(string old_str)
• char tmp
• size old_str.size
• tmp new charsize1 // assign new memory
• strcpy(tmp, old_str.s)
• delete s // must release previously
  assigned memory
• s tmp
•  
• void main()
• string str1("George")
• string str2("Mary")
• string str3("John")
•  
• str1.print() // what is printed ?
• str2.print()

str2 str1 str3.copy("Ha ha")  
str1.print() // what is printed now ?
str2.print()  
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Differences between copy constructor and
assignment operator

• The copy constructor creates a new object.
• The assignment operator works on an already valid
  object.

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Another example overloading the operator

• class Array
• private
• int numElems
• int arr
• public
• Array(int) // constructor
• Array() // destructor
• int operator(int)
•  

ArrayArray(int n) numElems n arr new
intn   ArrayArray() delete arr
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• int Arrayoperator(int index)
• if ((index lt 0) (index gt numElems))
• cout ltlt "Out of bounds error !!" ltlt endl
• exit(0) // error invalid index !!
• else
• return(arrindex)
•  
• void main()
• int i
• Array A(10)
•  
• for(i0 ilt10 i) // i10 error !!
• Ai i

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Comments on operator overloading

• Attempting to create new operators via operator
  overloading is a syntax error.
• Attempting to change the "arity" of an operator
  via operator overloading is a syntax error.
• Overloading is allowed only if at least one
  operand is a class instance (e.g., you cannot
  overload an operator to take two integers as
  operands).