Stream I/O Operators and Friend Functions

1
Stream I/O OperatorsandFriend Functions
2
Stream I/O Background

• ltiostream.hgt contains declarations for classes
• istream
• ostream
• cin is an istream object
• cout is an ostream object

3
Stream I/O Background (continued)

• C implements stream I/O by overloading the gtgt
  and ltlt operators to allow integers, reals, and
  characters to be input from or output to a
  stream. (gtgt and ltlt are really bit shift
  operators.)
• Extraction operator (gtgt) use
• istream_object gtgt object1 gtgt object2 gtgt
  objectn
• cin gtgt x gtgt y
• Insertion operator (ltlt) use
• ostream_object ltlt object1 ltlt object2 ltlt
  objectn
• cout ltlt Number ltlt num ltlt endl
• gtgt and ltlt return objects of type
  istream_object and ostream_object, respectively.

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Overloading Stream I/O Operators

• How can we read data into or write out the values
  of data types other than integer, floating point,
  or character?
• Fraction
• Vector
• Set
• Options
• Class methods, such as
• Fraction FractionreadFraction()
• Fraction FractiondisplayFraction()
• Write functions to further overload gtgt and ltlt

5
Overloading Stream I/O Operators (continued)

• Syntax for overloading gtgt
• istream operator gtgt (istream is_name,
• 
  class_name object_name)
• is_name is passed by reference because its state
  will be changed
• object_name is passed by reference because data
  will be read into it
• Syntax for overloading ltlt
• ostream operator ltlt(ostream os_name,
• const
  class_name object_name)
• os_name is passed by reference because its state
  will be changed
• object_name is a const because it will not be
  changed

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Overloading Stream I/O Operators (continued)

• Fraction class example
• class Fraction
• public
• // constructors and other methods
• void setNumerator(int n)
• void setDenominator(int d)
• int getNumerator()
• int getDenominator()
• private
• int numerator, denominator
• istream operator gtgt(istream input, Fraction
  f)
• ostream operator ltlt(ostream output, const
  Fraction f)

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Overloading Stream I/O Operators (continued)

• istream operator gtgt(istream input, Fraction f)
• char c // to hold the / separator
• int n, d
• input gtgt n gtgt c gtgt d
• if (d 0)
• cerr ltlt Zero denominator invalid ltlt endl
• exit(1)
• f.setNumerator(n)
• f.setDenominator(d)
• return input

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Overloading Stream I/O Operators (continued)

• ostream operator ltlt(ostream output, const
  Fraction f)
• output ltlt f.getNumerator() ltlt / ltlt
  f.getDenominator()
• return output

9
Friend Functions

• A function can be defined to be a friend of a
  class
• friend function-name(arguments) // inside of
  class definition
• A friend function has direct access to the class
  private members, both data and methods
• Making a function a friend can be convenient --
  the class does not need get methods for the
  function to access its data
• Using a friend function violates the rules of
  encapsulation.
• Use friends only when absolutely necessary!

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Friend Functions (continued)

• Fraction class using friends
• class Fraction
• public
• // constructors and other methods
• friend istream operator gtgt(istream
  input, Fraction f)
• friend ostream opeator ltlt(ostream
  input, const Fraction f)
• // set and get methods
• private
• int numerator, denominator
• istream operator gtgt(istream input, Fraction
  f)
• ostream operator ltlt(ostream output, const
  Fraction f)

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Friend Functions (continued)

• istream operator gtgt(istream input, Fraction f)
• char c // to hold the / separator
• input gtgt f.numerator gtgt c gtgt f.denominator
  // direct access
• if (f.denominator 0)
• cerr ltlt Zero denominator invalid ltlt endl
• exit(1)
• return input

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Friend Functions (continued)

• ostream operator ltlt(ostream output, const
  Fraction f)
• output ltlt f.numerator ltlt / ltlt f.denominator
  // direct access
• return output