Chapter 15 What is Inheritance?

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Chapter 15 What is Inheritance?

• Inheritance allows a new class to be based on an
  existing class.
• The new class inherits all the member variables
  and functions of the class it is based on.
• Note difference between is-a and has-a.
• A station-wagon is-a car, but a car has-a wheel.

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Inherited Class
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Inheriting from Base Class
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test.h - Program

• ifndef TEST_H
• define TEST_H
• include "grade.h"
• class Test public Grade
• private
• int numQuestions
• float pointsEach
• int numMissed
• public
• Test ( int, int )
• // Test
• endif

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Why would you want?

• In our set example, we could derive sets with
  various differences
• traits, prints differently
• new methods, isKind
• keep track of number of traits
• Animals, add/replace features. Logical
  categorization
• Cars, add common characteristics, options
• Allows customization, reuse of base class.
• Keeps design consistent. All cars have
  dealerRetail. All cars have taxAmount.

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Protected Members and Class Access

• Protected members of a base class are like
  private members, but they may be accessed by
  derived classes.
• The base class access specification determines
  how private, protected, and public base class
  members may be accessed by derived classes.

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Base Class Access
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Constructors and Destructors

• The base classs constructor is called before the
  derived classs constructor.
• Since constructors have arguments, we are often
  not content with an argument-less call (the
  default).
• The destructors are called in reverse order, with
  the derived classs destructor being called first.

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Passing Arguments to Base Class Constructors

• Assume a class called Cube is derived from a
  class called Rect. Here is how the constructor
  looks in Rect
• RectRect ( float w, float l )
• width w
• length l
• area length width
• // RectRect

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Cube Constructor

• CubeCube( float wide, float long, float high )
  
• Rect(wide, long)
• height high
• volume area high
• // CubeCube

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Overriding Base Class Functions

• A member function of a derived class may have the
  same name as a member function of a base class.

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Polymorphism and Virtual Member Functions

• The term polymorphism means the ability to take
  many form.
• A virtual member function in a base class expects
  to be overridden in a derived class

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Need for Virtual Member Functions
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Abstract Base Classes and Pure Virtual Functions

• An abstract base class is not instantiated, but
  other classes are derived from it.
• A pure virtual function is a virtual member
  function of a base class that must be overridden.

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Abstract Base Classes and Pure Virtual Functions
(cont)

• A class becomes an abstract base class when it
  contains one or more pure virtual functions.
• A pure virtual function is a virtual member
  function declared in a manner similar to the
  followingvirtual void showInfo(void) 0

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Base Class Pointers

• Pointers to a base class may be assigned the
  address of a derived class object.
• The pointer, however, ignores any overrides the
  derived class performs

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Classes Derived from Derived Classes

• A base class can also be derived from another
  class.
• This gives us a chain of inheritance where one
  class is derived from a second, which in turn is
  derived from a third.

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Chain of Inheritance
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Multiple Inheritance

• Multiple inheritance is when a derived class has
  two or more base classes.

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Multiple Inheritance (cont)
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Examples

• Could have a fruit which falls under two
  categories both fruit and vegetable. A tomato
  is sometimes referred to as a fruit and sometimes
  as a vegetable.
• fruit - the ripened reproductive body of a seed
  plant
• vegetable - edible seeds or roots or stems or
  leaves or bulbs or tubers or nonsweet fruits of
  any of numerous herbaceous plant
• Cross-categorization. An apple is a fruit and an
  apple is a breakfast food.

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Given the following, organize them into
inheritance or composition

• A test for 1720, a student in 1720
• An electronic device, a computer, a calculating
  tool
• A woman, an employee, a person
• A course, a course evaluation
• Clothing, scarf, winter clothes, boots
• Sesame Street Character, Big Bird, Oscar the
  Grouch

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Concerns on multiple inheritance

• Multiple inheritance is generally thought to be a
  bad idea because a common base class (B and C
  inherit from A, and D inherits from both B and C)
  cause problems with both B and C using the common
  variables differently.
• Java handles with interface function use is
  specified, but no implementation.