INHERITANCE AND COMPOSITION

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Chapter 14

• INHERITANCE AND COMPOSITION

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Objectives

• In this chapter you will
• Learn about inheritance
• Learn about derived and base classes
• Explore how to redefine the member functions of a
  base class
• Examine how the constructors of base and derived
  classes work
• Learn how to construct the header file of a
  derived class

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Objectives

• Explore three types of inheritance public,
  protected, and private
• Learn about composition
• Become familiar with the three basic principles
  of object-oriented design

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Inheritance and Composition

• The two common ways to relate two classes in a
  meaningful way are
• 1. Inheritance (is-a relationship)
• 2. Composition (has-a relationship)

5
Inheritance

• Design a class, partTimeEmployee, to implement
  and process the characteristics of a part-time
  employee
• Every part-time employee has a name, pay rate,
  and number of hours worked
• Every part-time employee is a person
• Rather than design the class partTimeEmployee
  from scratch, we want to be able to extend the
  definition of the class personType by adding
  additional members (data and/or function)

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Inheritance

• We do not want to make the necessary changes
  directly to the class personTypethat is, edit
  the class personType, and add and/or delete
  members
• We want to create the class partTimeEmployee
  without making any physical changes to the class
  personType, by adding only the members that are
  necessary
• The new class that we create from existing
  classes is called the derived class and the
  existing classes are called the base classes

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Inheritance

• Single inheritance- the derived class is derived
  from a single base class
• Multiple inheritance- the derived class is
  derived from more than one base class
• Inheritance can be viewed as a tree-like, or
  hierarchical, structure wherein a base class is
  shown with its derived classes
• The word public in a heading, called the member
  access specifier, specifies that all public
  members of the class shape are inherited as
  public members by the class circle

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Inheritance
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Facts

• The following facts about the base and the
  derived classes should be kept in mind
• 1. The private members of the base class are
  private to the base class hence, the members of
  the derived class cannot directly access them
• 2. The public members of a base class can be
  inherited either as public members or as private
  members by the derived class
• 3. The derived class can include additional data
  and/or function members

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Facts

• 4. The derived class can redefine the public
  member functions of the base class. However, this
  redefinition applies only to the objects of the
  derived class, not to the objects of the base
  class
• 5. All data members of the base class are also
  data members of the derived class. Similarly, the
  member functions of the base class (unless
  redefined) are also the member functions of the
  derived class

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Constructors

• A derived class can have its own private data
  members
• Member functions of the derived class cannot
  directly access the private members of the base
  class
• The constructors of the derived class can
  (directly) initialize only the private data
  members of the derived class
• When a derived class object is declared, it must
  also automatically execute one of the
  constructors of the base class

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Constructors

• The execution of a derived classs constructor
  must trigger the execution of one of the base
  classs constructors
• A call to the base classs constructor is
  specified in the heading part of the derived
  class constructors definition

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Multiple Inclusions

• Suppose that the header file testA.h includes the
  file test.h to make use of the identifiers one
  and two
• The preprocessor first includes the header file
  test.h and then the header file testA.h
• When the header file testA.h is included, because
  it contains the preprocessor directive include
  "test.h", the header file test.h will be included
  twice in the program
• The second inclusion of the header file test.h
  will result in compile-time errors, such as
  identifier one already being declared
• To avoid multiple inclusion of a file in a
  program we use certain preprocessor commands in
  the header file

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Stream Classes
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Stream Classes

• The class ios is the base class for all stream
  classes
• Classes istream and ostream are directly derived
  from the class ios
• The class ifstream is derived from the class
  istream, and the class ofstream is derived from
  the class ostream
• The class ios contains formatting flags and
  member functions to access and/or modify the
  setting of these flags

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Stream Classes

• To identify the I/O status, the class ios
  contains an integer status word
• This integer status word provides a continuous
  update reporting the status of the stream
• The classes istream and ostream are responsible
  for providing the operations for the data
  transfer between memory and devices

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Stream Classes

• The class istream defines the extraction
  operator, gtgt, and functions such as get and
  ignore
• The class ostream defines the insertion operator,
  ltlt, which is used by the object cout
• The class ifstream is derived from the class
  istream to provide the file input operations
• The class ofstream is derived from the class
  ostream to provide the file output operations

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Stream Classes

• Objects of the type ifstream are used for file
  input
• Objects of the type ofstream are used for file
  output
• The header file fstream contains the definition
  of the classes ifstream and ofstream

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Protected Members of a Class

• The private members of a class are private to the
  class and cannot be directly accessed outside the
  class
• The derived class cannot access private members
  of a class
• It is sometimes necessary for a derived class to
  access a private member of a base class
• If you make a private member become public, then
  anyone can access that member

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Protected Members of a Class

• For a base class to give access to a private
  member to its derived class and still prevent its
  direct access outside the class, you must declare
  that member under the memberAccessSpecifier
  protected
• The accessibility of a protected member of a
  class is in between public and private
• A derived class can directly access the protected
  member of a base class

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Inheritance

• 1. If memberAccessSpecifier is public, then
• a. The public members of A are public members of
  B and they can be directly accessed in class B
• b. The protected members of A are protected
  members of B and they can be directly accessed by
  the member functions (and friend functions) of B
• c. The private members of A are hidden in B and
  they can be accessed by the member functions (and
  friend functions) of B through the public or
  protected members of A

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Inheritance

• 2. If memberAccessSpecifier is protected, then
• a. The public members of A are protected members
  of B and they can be accessed by the member
  functions (and friend functions) of B
• b. The protected members of A are protected
  members of B and they can be accessed by the
  member functions (and friend functions) of B
• c. The private members of A are hidden in B and
  they can be accessed by the member functions (and
  friend functions) of B through the public or
  protected members of A

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Inheritance

• 3. If memberAccessSpecifier is private, then
• a. The public members of A are private members of
  B and they can be accessed by the member
  functions (and friend functions) of B
• b. The protected members of A are private members
  of B and they can be accessed by the member
  functions (and friend functions) of B
• c. The private members of A are hidden in B and
  they can be accessed by the member functions (and
  friend functions) of B through the public or
  protected members of A

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Composition

• In composition one or more member(s) of a class
  is an object of another class type
• Composition is a has-a relation
• The arguments to the constructor of a
  member-object are specified in the heading part
  of the definition of the constructor of the class
• Member-objects of a class are constructed in the
  order they are declared, not in the order they
  are listed in the constructors member
  initialization list, and before the enclosing
  class objects are constructed

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OOD and OOP

• The fundamentals of Object-Oriented Design (OOD)
  are
• 1. Encapsulationcombine data and operations on
  data in a single unit
• 2. Inheritancecreate new objects from existing
  objects
• 3. Polymorphismthe ability to use the same
  expression to denote different operations
• In OOD an object is a fundamental entity, while
  in structured programming a function is a
  fundamental entity

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OOD and OOP

• In OOD we debug objects, while in structured
  programming we debug functions
• In structured programming a program is a
  collection of interacting functions, while in OOD
  a program is a collection of interacting objects
• OOD encourages code reuse
• In structured programming the programmer is
  action-oriented, while in OOD the programmer is
  object-oriented

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OOD and OOP

• Object-oriented programming (OOP) implements OOD
• C supports OOP through the use of classes
• A polymorphic function or operator has many forms
• A function name and operators can be overloaded
• Templates provide parametric polymorphism
• C provides virtual functions as a means to
  implement polymorphism in an inheritance hierarchy

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OOD and OOP

• Objects are created when class variables are
  declared
• Objects interact with each other via function
  calls
• Every object has an internal state and external
  state
• The private members form the internal state and
  the public members form the external state
• Only the object can manipulate its internal state

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Classes, Objects, Operations

• We begin with a description of the problem and
  then identify all of the nouns and verbs
• From the list of nouns we choose our classes, and
  from the list of verbs we choose our operations
• Suppose that we want to write a program that
  calculates and prints the volume and surface area
  of a cylinder

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Classes, Objects, Operations

• We can state this problem as follows
• Write a program to input the dimensions of a
  cylinder and calculate and print the surface area
  and volume
• The nouns are bold and the verbs are italic
• From the list of nounsprogram, dimensions,
  cylinder, surface area, and volumewe can easily
  visualize cylinder to be a classsay,
  cylinderTypefrom which we can create many
  cylinder objects of various dimensions

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Classes, Objects, Operations

• The nounsdimensions, surface area, and
  volumeare characteristics of a cylinder
• After we identify a class, the next step is to
  determine three pieces of information
• Operations that an object of that class type can
  perform
• Operations that can be performed on an object of
  that class type
• Information that an object of that class type
  must maintain

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Classes, Objects, Operations

• From the list of verbs identified in the problem
  description, choose a list of possible operations
  that an object of that class can perform, or has
  performed, on itself
• From the list of verbs for the cylinder problem
  descriptionwrite, input, calculate, and
  printthe possible operations for a cylinder
  object are input, calculate, and print
• For the cylinderType class, the dimensions
  represent the data

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Classes, Objects, Operations

• The center of the base, radius of the base, and
  height of the cylinder are the characteristics of
  the dimensions
• The verb calculate applies to determining the
  volume and the surface area
• You can deduce the operations cylinderVolume and
  cylinderSurfaceArea
• The verb print applies to the display of the
  volume and the surface area on an output device

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Classes, Objects, Operations

• Identifying classes via the nouns and verbs from
  the descriptions to the problem is not the only
  technique possible
• There are several other OOD techniques in
  literature

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Programming Example

• This programming example further illustrates the
  concepts of inheritance and composition
• The mid-semester point at your local university
  is approaching
• The registrars office wants to prepare the grade
  reports as soon as the students grades are
  recorded

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Programming Example

• Some of the students enrolled have not yet paid
  their tuition, however,
• 1. If a student has paid the tuition, the grades
  are shown on the grade report together with the
  grade-point average (GPA)
• 2. If a student has not paid the tuition, the
  grades are not printed
• For these students, the grade report contains a
  message indicating that the grades have been held
  for nonpayment of the tuition
• The grade report also shows the billing amount

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The Data File

• The data are stored in a file in the following
  form
• 15000 345
• studentName studentID isTuitionPaid
  numberOfCourses
• courseName courseNumber creditHours grade
• courseName courseNumber creditHours grade
• .
• studentName studentID isTuitionPaid
  numberOfCourses
• courseName courseNumber creditHours grade
• courseName courseNumber creditHours grade
• .

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The Data File

• The first line indicates the number of students
  enrolled and the tuition rate per credit hour
• The students data is given thereafter
• A sample-input file is
• 3 345
• Lisa Miller 890238 Y 4
• Mathematics MTH345 4 A
• Physics PHY357 3 B
• ComputerSci CSC478 3 B
• History HIS356 3 A
• .

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Output

• The desired output for each student is of the
  form
• Student Name Lisa Miller
• Student ID 890238
• Number of courses enrolled 4
• Course No Course Name Credits Grade
• CSC478 ComputerSci 3 B
• HIS356 History 3 A
• MTH345 Mathematics 4 A
• PHY357 Physics 3 B
• Total number of credits 13
• Mid-Semester GPA 3.54

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Input and Output

• Input - a file containing data in the form given
  above
• For easy reference in the rest of the discussion,
  we assume that the name of the input file is
  "astData.txt"
• Output - a file containing output of the form
  given above

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Problem Analysis

• The two main components that we see are student
  and course
• Course
• the main characteristics of a course are the
  course name, course number, and number of credit
  hours
• although the grade a student receives is not
  really a characteristic of a course, to simplify
  the program this component also includes the
  students grade

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Problem Analysis

• Operations on an object of the course type are
• 1. Set the course information
• 2. Print the course information
• 3. Show the credit hours
• 4. Show the course number
• 5. Show the grade
• Student
• The main characteristics of a student are the
  student name, student ID, number of courses in
  which enrolled, courses in which enrolled, and
  grade for each course

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Problem Analysis

• Because every student has to pay tuition, we also
  include a member to indicate whether the student
  has paid the tuition
• Every student is a person, and every student
  takes courses
• We have already designed a class personType to
  process a persons first name and last name
• We can derive the class studentType to keep track
  of a students information from the class
  personType, and one member of this class is of
  the type courseType

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Algorithm Design

• The basic operations to be performed on an object
  of the type studentType are as follows
• 1. Set the student information
• 2. Print the student information
• 3. Calculate the number of credit hours taken
• 4. Calculate the GPA
• 5. Calculate the billing amount
• 6. Because the grade report will print the
  courses in ascending order, sort the courses
  according to the course number

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Main Program

• 1. Declare the variables
• 2. Open the input file
• 3. If the input file does not exist, exit the
  program
• 4. Open the output file
• 5. Get the number of students registered and the
  tuition rate
• 6. Load the students data
• 7. Print the grade reports

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Variables

• studentType studentListmaxNumberOfStudents
• //array to store the students data
• int noOfStudents //variable to store the
  //number of students
• double tuitionRate //variable to store the
  tuition rate
• ifstream infile //input stream variable
• ofstream outfile //output stream variable