Chapter 13 ObjectOriented Programming: Polymorphism

1
Chapter 13Object-Oriented Programming
Polymorphism

• C, How to Program
• Deitel Deitel

2
Relationships Among Objects in an Inheritance
Hierarchy

• Key concept
• An object of a derived class can be treated as an
  object of its base class
• is-a relationship A object is a B object.

3
Which Function Is Invoked with Pointer?

• Invoked functionality depends on type of the
  handle (type of the pointer) used to invoke the
  function, not type of the object to which the
  handle points

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Example

• BankAccount ba(12,John,200.00)
• BankAccount baPtr 0
• Base-class pointer points to base-class object
• Invoke base-class functionality
• baPtr ba baPtr-gtprint( )

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Example

• SavingsAccount sa(13, Tom, 300.00)
• SavingsAccount saPtr0
• Derived-class pointer points to derived-class
  object
• Invoke derived-class functionality
• saPtr sa saPtr-gtprint( )

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Invoking Base-Class Functions from Derived-Class
Objects

• Base-class pointer points to derived-class object
  because derived-class object is an object of base
  class
• Invoke base-class functionality because the type
  of pointer is Base-class
• baPtr sa baPtr-gtprint( )

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Aiming Derived-Class Pointers at Base-Class
Objects

• Derived-class pointer points to a base-class
  object
• Example saPtr ba ( Wrong!!!)
• C compiler generates error
• BankAccount (base-class object) is not a
  SavingsAccount (derived-class object)

BankAccount ba
BankAccount baPtr
12, John 200.00
1024
1024
13, Tom 300.00, 0.00
1048
1024
SavingsAccount sa
SavingsAccount saPtr
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Derived-Class Member-Function Calls via
Base-Class Pointers

• Base-class pointer points to derived-class object
• Calling functions that exist in base class causes
  base-class functionality to be invoked
• baPtr-gtwithdraw(30)
• Calling functions that do not exist in base class
  (may exist in derived class) will result in
  error
• baPtr-gtgetInterest( ) (Wrong!!! )
• Derived-class members cannot be accessed from
  base-class pointers
• However, they can be accomplished using
  downcasting (Section 13.8)

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virtual Functions

• Which classs function to invoke
• Normally
• Handle determines which classs functionality to
  invoke
• With virtual functions
• Type of the object being pointed to, not type of
  the handle, determines which version of a virtual
  function to invoke
• Allows program to dynamically (at runtime rather
  than compile time) determine which function to
  use
• Called dynamic binding or late binding

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virtual Functions

• Declared by preceding the functions prototype
  with the keyword virtual in base class
• Derived classes override function as appropriate
• An overridden function in a derived class has the
  same signature and return type (i.e. prototype)
  as the function it overrides in its base class.
• virtual -gtoverride
• non-virtual -gt redefine

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Example

• BankAccount.h
• virtual void print( )
• //print info. about bank account
• SavingsAccount.h
• virtual void print( )
• //print info. about savings account
• No change in .cpp files.

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virtual Functions in Inheritance Hierarchy

• virtual function is overridden.
• Once virtual, always virtual (even not explicitly
  declared).
• virtual function is not overridden.
• Inherits from its base class

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Static Binding/Dynamic Binding

• Static binding
• When calling a virtual function using specific
  object with dot operator, function invocation
  resolved at compile time
• Dynamic binding
• Choosing the appropriate function to call at
  execution time
• Dynamic binding occurs only off pointer

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Polymorphism with inheritance hierarchies

• Program in the general vs. program in the
  specific
• Process objects of classes that are part of the
  same hierarchy as if they are all objects of the
  base class
• Each object performs the correct tasks for that
  objects type
• Different actions occur depending on the type of
  object

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Summary
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Reference

• Reproduced from the Cyber Classroom for C, How
  to Program, 5/e by Deitel Deitel.
• Reproduced by permission of Pearson Education,
  Inc.