CS304: Lecture 5

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CS304 Lecture 5

• Operator Overloading, OOP
• Deitel, Chs. 11 12
• http//www.cis.uab.edu/cs304

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Contents

• Introduction
• Unary operator overloading
• Binary operator overloading
• Conversion operator and constructor
• Overloading and --

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What is Operator Overloading?

• Treating the words separately
• Operator A symbol that has some action on
  surrounding operands
• Overload To allow different meanings to exist
  based on context of use
• Operator Overloading Defining new meanings for
  existing operators in C

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Why would we want Operator Overloading?

• class matrix
• int main() matrix a, b, cc.times(a,
  b)c.print()

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Why would we want Operator Overloading?

• class matrix
• int main() matrix a, b, cc a bcout ltlt
  c

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What Operators Can Be Overloaded?

• - /
• ! lt gt -
• / ltlt gtgt gtgt
• ltlt ! lt gt
• -- -gt , -gt ()
• new delete new delete

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How Does Overloading Change Things?

• Short answer Only interpretation changes
• Long answer Precedence, Associativity, and
  Number of Operands are not changeable (and you
  wouldnt want to).

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What Other Restrictions?

• You cannot create your own operators.
• You cannot overload operators of fundamental
  types. Overloading is only intended for classes.
• You can, however, create operators that take a
  fundamental type and a class
• Related operators are not necessarily overloaded
  ( overloading wont change the meaning of )

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How are these defined?

• There are two ways As class members, and as
  global functions.
• Class Members
• Certain operators have to be designated within
  the class, such as (), , -gt, or any assignment
  operator
• The left operand must be of the type of the class
  for the operator to be put into a class
• Global Functions (normally friends of the class)
• Use this if the left operand is not of the class
  you want to put it in (example ltlt and gtgt)

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Overloading Unary Operators

• As a global function, one argument
• bool operator! (const String )
• As a class member, the argument is provided via
  this, so it does not require any parameters
• bool operator!() const

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Overloading Binary Operators

• Once again, within a class, one fewer parameters
  is required. The left operand is the one
  indicated by this
• Within a class
• bool operatorlt( const obj )
• Ouside of a class
• bool operatorlt(const obj1 , const obj2 )

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ltlt and gtgt example

• operatorgtgt and operatorltlt are the stream
  extraction and insertion operators
• They take the argument on the right and an
  istream or ostream reference on the left, then
  return an istream or ostream reference
• We cant alter the ostream or istream classes, so
  we have to make these into global functions

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Overloading gtgt and ltlt

• Imagine we have a class called phone number,
  which inputs and outputs a phone number as
  (xxx)xxx-xxxx, but stores the phone number as
  areacode, exchange, and line
• We want a gtgt operator that takes stuff off the
  stream and automatically parses it, and
  reconstitutes it in the ltlt operator

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• class PhoneNumber friend ostream
  operatorltlt(ostream , const PhoneNumber
  )friend istream operatorgtgt(istream ,
  PhoneNumber )public string getAreaCode()
  return areaCode private string areaCode,
  exchange, line

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• ostream operatorltlt(ostream output, const
  PhoneNumber number) output ltlt ( ltlt
  number.areaCode ltlt ) ltlt number.exchange ltlt
  - ltlt number.line return output

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• istream operatorgtgt(istream input, PhoneNumber
  number) input.ignore() //skip ( input gtgt
  setw(3) gtgt number.areaCode input.ignore(2)
  //skip ) and space input gtgt setw(3) gtgt
  number.exchange input.ignore() //skip
  dash input gtgt setw(4) gtgt number.line return
  input

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Conversion Constructors and Operators

• Casting for the masses!
• For class Type
• Type(Anothertype at) AnotherType
  gt Type
• operator AnotherType () Type gt
  AnotherType
• Convert a PhoneNumber into a String?
• class PhoneNumber operator string ()
  return ( areaCode ) exchange
  - line
• For converting other things to a string, just
  define a conversion operator It works as
  intended.
• Javas toString() ???

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Problems with Conversion Operators

• The compiler may use the conversion operators you
  provide to implicitly cast things
• This could become problematic, leading to bugs
  that are hard to track down.
• Use the explicit keyword to prevent these
  troubles.
• Dont forget constructors can be used as casting
  operators These can be designated as explicit
  as well.

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• class Array friend void outputArray(const
  Array)public Array( int 10
  )private //Array data
• void outputArray(const Array x)

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• class Array friend void outputArray(const
  Array)public explicit Array( int 10
  )private //Array data
• void outputArray(const Array x)

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What would happen here?

• int main() Array integers1( 7)outputArray(inte
  gers1)outputArray(3)outputArray(Array(3))ret
  urn 0

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Overloading and --, Prefix and Postfix

• Prefix looks like other unary operators
• Obj operator (Obj ) global function
• Obj operator() class member of Obj
• The post fix is more of a hack. In order to
  distinguish it with prefix one, by convention,
  the prototype looks like this
• Obj operator (Obj , int) global function
• Obj operator (int) class member of Obj

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Chapter 12

• Object Oriented Programming Inheritance

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Inheritance

• Inheritance is a form of software reuse in which
  the programmer creates a class that absorbs an
  existing classs data and behaviors and enhances
  them with new capabilities.
• Saves time by preventing rewritten code
• Reduces errors by encouraging reuse of debugged
  code

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The Inheritance Relationship
Base Class
Derived Class A
Derived Class B
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Vocabulary

• An existing class that is inherited from is the
  base class.
• A class that inherits from another class is a
  derived class.
• A direct base class is the base class from which
  a derived class explicitly inherits.
• An indirect base class is inherited from two or
  more levels up in the class hierarchy.

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Vocabulary (contd)

• Single Inheritance means that a class is derived
  from one base class. (This is Javas style of
  inheritance)
• Multiple Inheritance allows a derived class to
  inherit from multiple (possibly unrelated)
  classes. C allows both.

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Inheritance

• Inheritance exhibits an is-a relationship.
• This is in contrast to object composition, which
  was the has-a relationship
• As an example, consider a class rectangle and a
  class quadrilateral
• What is the relationship between the two?
• Other examples
• Students, Shapes, Employee Etc.

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Types of Inheritance

• Public
• Our main focus
• Private members of a base class are not
  accessible directly from a derived class, but
  these private members are still inherited
• Friends are not inherited
• Public members remain public
• Protected members are still protected

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Types of Inheritance (contd)

• Protected Inheritance
• Public and protected members are protected in
  derived class, private remains hidden
• Private Inheritance
• Public and protected members are private in
  derived class, private remains hidden
• Public is the most common type of inheritance

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Protected Members

• Protected is an access modifier (like public and
  private)
• Intermediate level of protection
• Protected members are unavailable to other
  classes (like private), but are accessible by
  derived classes

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Inheritance example

• class Petpublic Pet() weight(1), food("Pet
  Chow") Pet() void setWeight(int w)
  weight w int getWeight() return
  weight void setfood(string f) food
  f string getFood() return food void
  eat() void speak()protected int
  weight string food

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• void Peteat() cout ltlt "Eating " ltlt food ltlt
  endl
• void Petspeak()cout ltlt "Growl" ltlt endl

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Public Inheritance Example

• class Shape
• class TwoDimensionalShape public Shape

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Derived Class Rat

• class Rat public Pet public Rat() Rat()
  void sicken() cout ltlt "Speading Plague"
  ltlt endl

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Derived Class Cat

• class Cat public Pet public Cat()
  numberToes(5) Cat() void
  setNumberToes(int toes) numberToes
  toes int getNumberToes() return
  numberToesprivate int numberToes

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in action! (Inaction?)

• int main() Rat charles Cat fluffycharles.setW
  eight(25)cout ltlt "Charles weighs " ltlt
  charles.getWeight() ltlt " lbs. " ltlt
  endlcharles.speak() charles.eat()
  charles.sicken() fluffy.speak() fluffy.eat()
  cout ltlt "Fluffy has " ltlt fluffy.getNumberToes()
  ltlt " toes " ltlt endl return 0

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Construction/Destruction

• When constructing a derived class member, the
  constructor for the parent class is either
  explicitly or implictly called
• Explicit base-class member initializer
• Implicit default constructor
• Example If Pet(int weight) were another
  constructorRat(int weight) Pet(weight)