Chapter 4 Inheritance

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Chapter 4Inheritance

• Saurav Karmakar
• Spring 2007

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Objective

• IS-A relationships and the allowable changes for
  derived classes
• The concept of polymorphism
• Public, private, and protected members and
  inheritance
• Static vs. dynamic binding
• Default constructor, copy constructor, and copy
  assignment operator
• Abstract classes
• Tricky stuff

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4.1 What is Inheritance?

• Another mechanism for code reuse.
• A process of deriving classes from a base class
  w/o disturbing the implementation of the base
  class.
• Ex Vehicle is a class
• Car is a Vehicle gt Car derived from Vehicle

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Figure 4.2
istringstream
istream
fstream
ifstream
IOS
iostream
stringstream
ostream
ostringstream
ofstream
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Polymorphism

• Variables can reference objects of several type.
  When operation are applied to the variable, the
  operation appropriate to the referenced object is
  automatically selected.
• Derived class is a new class which inherits all
  public protected properties of a base class.
• Derived class is type-compatible with its base
  class.

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4.2 Inheritance Basics

• Public inheritance all public members of the
  base class remain public in the derived class
  gtmodels IS-A relationship gtmostly used.
• Private inheritance all the public members of
  the base class remain hidden gt models HAS-A
  relationship.

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General layout of public inheritance

• class Derived public Base
• //any member are not listed are inherited
• //except constructor, destructor, copy,
  constructor and operator
• public
• //new constructor and destructor if necessary
• //modify base member function
• //new function
• private
• //new data member or private function
• //base members should be disable

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Inheritance cont. . .

• Derived class inherits all member functions from
  the base class. It may accept, disallow, or
  redefine them.
• Derived class can define new functions.

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Inheritence Ways to Implement

• Public access to a base class allows access to
  other classes in addition to derived classes.
• Protected Class member is is private to every
  class except a derived class.

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Access rules
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Constructor and Base class initialization

• If there is no constructor in a derived class, a
  default zero-parameter constructor will be
  generated
• Base class constructors can be explicitly called
  by name in the initializer list.
• Default constructor for a derived class Derived()
  Base()

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Constructor for the new exception class Underflow

• class Underflow public underflow_error
• public
• underflow(const string msg )
• underflow_error(msg.c_str())

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Overriding a method(member function)

• Overriding base class methods Derived class
  methods must have the same signature and
  compatible return types with the base class
  methods.
• Partial overriding overriding base class methods
  by adding more functionality.

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Example of partial overriding

• class Workaholic public Worker
• public
• void doWork() //overriding base method
• WorkerdoWork()// call base method
• drinkCoffee() // new method
• WorkerdoWork()// call base method

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Bindings

• Static binding the decision about which
  function/type to use to resolve an overload is
  made at compile time.
• Dynamic binding the decision must be made at run
  time.

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Examples

• Worker w
• Workaholic wh
• . . .
• w.doWork()
• wh.doWork()
• figure 4.8 static binding

• Worker wptr
• cingtgtx
• if(x ! 0)
• wptr new Workaholic()
• else
• wptr new Worker()
• . . .
• //which doWork is used ?
• wptr -gt doWork()
• Figure 4.9 dynamic binding

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Virtual

• If a function is redefined in a derived class,
  and the overloading is resolved in run time, it
  should be declared virtual in the base class.
• Example
• class Worker
• public
• virtual void doWork()

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Virtual Cont

• Constructors are NEVER virtual
• Destructors should be virtual for base class gt
  Let the computer know which one to call for
  deallocation.

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Abstract

• Abstract method, pure virtual function, has no
  meaningful definition in the base class and is
  defined in derived classes.
• Abstract class a class containing any abstract
  method gt cant be instantiated and must be
  inherited.
• Example shape class

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The abstract base class shape

• class Shape
• public
• Shape( const string shapeName "" )
• name( shapeName )
• virtual Shape( )
• virtual double area( ) const 0
• //abstract method
• bool operatorlt ( const Shape rhs ) const
• return area( ) lt rhs.area( )
• virtual void print( ostream out cout const
  
• out ltlt name ltlt " of area "ltlt area( )
• private
• string name
• //figure 4.13

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General Rules

• Nonvirtual functions Overloading is resolved at
  compile time.
• Virtual functions Overloading is resolved at
  run-time (Base class provide a default
  implementation)
• Pure virtual functions Overloading is resolved
  at run-time. (Base class provide no default
  implementation)

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4.4 Tricky C Details

• Changing the default value in a derived class is
  unsafe because doing so can create an
  inconsistency with virtual functions.

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Cont

• When a method is declared in a derived class, it
  hides all methods of the same name in the base
  class gt get away by partial overriding or using
  using directive

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Illustration of hiding

• class Base
• public
• virtual void bar()
• class Derived
• public
• void bar(int x)
• Void test( Base arg1, Derived arg2)
• arg1.bar() // call Basebar() gt OK
• arg1.bar(4) //call Basebar(int) gt fails for
  not arg2.bar(4) // call Derivedbar(int) gt OK
• arg2.bar() // fails b/c Derivedbar(int) has
  hidden
• Basebar()

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Two ways to solve Hiding

• Override the zero-parameter bar in Derived
• void bar() Basebar()
• Use keyword using in derived class
• using Basebar

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4.5 Multiple Inheritance

• A mechanism for deriving a class from several
  base classes.
• Ex Student and Employee are derived classes of
  UniversityPerson.
• class Student virtual public UniversityPerson().
  . .
• class Employeevirtual public UniversityPerson()
  
• class StudentEmployee public Student,
• public Employee. . .

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Common errors (page 151)

• Inheritance is private by default. A common error
  is to omit the keyword public, which is needed to
  specify public inheritance.
• Objects of abstract classes can NEVER be
  initiated.
• Constructors can never be declared virtual.
• More errors defined in page 151..

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Summary

• Inheritance a powerful way for code reuse
• Virtual Dynamic binding, binding at execution
  time.
• Abstract must be defined in derived classes.
  Abstract classes cant be instantiated.