Tutorial Seven

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Tutorial Seven

• Inheritance

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Lesson A
Understanding Inheritance
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Lesson A Objectives

• To learn
• What inheritance is
• How to create a derived class
• How to choose the class access specifier
• How to override parent class functions

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What Is Inheritance?

• Inheritance
• Principle that knowledge of a general category
  can be applied to more specific objects
• Objects used in computer programs are easier to
  understand if you can place them within a
  hierarchy of inheritance

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What Is Inheritance?

• When a new class is inherited from a previous
  class, it is said to be derived from the previous
  class
• The previous class is called a parent class, base
  class, or superclass
• The new class is called a child class, derived
  class, or subclass

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What Is Inheritance?

• In many cases, a derived class may not possess
  all of its parents traits
• Members of the new class may require a different
  display format than members of the previous class
• A feature of inheritance is that the descendant
  class can override those attributes from the
  parent that are inappropriate

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What Is Inheritance?

• To be truly object-oriented, a programming
  language must allow inheritance
• One major feature of object-oriented programming
  is the ability to create new classes from one
  that already exists

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What Is Inheritance?

• Programs in which you create classes that are
  derived from existing classes offer several
  advantages
• Save time because much of the code needed for
  your class is already written
• Save time because the existing code has already
  been testedthat is, it is reliable
• Save time because you already understand how the
  base class works

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What Is Inheritance?

• Programs in which you create classes that are
  derived from existing classes offer several
  advantages (continued)
• In a derived class, you can extend and revise a
  parent class without corrupting its existing
  class functions
• The more situations in which the code has been
  used, the more likely that logical errors have
  already been found and fixed

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What Is Inheritance?

• In some object-oriented programming languages,
  every new class must inherit from an existing
  class
• C is sometimes called a hybrid object-oriented
  programming language because you may create
  original base classes without deriving them from
  some other class

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Creating a Derived Class

• With C, you cannot inherit regular functions
  and variables, only classes
• The specifier protected
• Allows members to be used by class member
  functions and by derived classes

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class NameAndNumber private int idNum
char lastName20 char firstName15
public NameAndNumber() NameAndNumber()
void inputData(const int num, const char
last, const char first) void
outputData(void)
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class NameAndNumber protected int
idNum char lastName20 char
firstName15 public NameAndNumber()
NameAndNumber() void inputData(const int
num, const char last, const char
first) void outputData(void)
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What is Inheritance?

• To create a derived class, you include the
  following elements in the order listed
• The keyword class
• The derived class name
• A colon
• A class specifier, either public, private, or
  protected

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What is Inheritance?

• To create a derived class, you include the
  following elements in the order listed
  (continued)
• An opening bracket
• The class definition statements
• A closing bracket
• A semicolon

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include "NameAndNumberNew.cpp" class Customer
public NameAndNumber private double
balanceDue public void inputBalanceDue(con
st double bal) void outputBalance(void)
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What is Inheritance?

• Access specifier matters in the following cases
• example

class Customer public NameAndNumber
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What is Inheritance?

• Access specifier matters in the following cases
• If "public" access specifier is used
• based class that public remain public in the
  derived class
• based class that protected remain protected in
  the derived class
• based class that private are inaccessible in the
  derived class

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What is Inheritance?

• Access specifier matters in the following cases
• If "protected" access specifier is used
• based class that public remain protected in the
  derived class
• based class that protected remain protected in
  the derived class
• based class that private are inaccessible in the
  derived class

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What is Inheritance?

• Access specifier matters in the following cases
• If "private" access specifier is used
• based class that public remain private in the
  derived class
• based class that protected remain private in the
  derived class
• based class that private are inaccessible in the
  derived class

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includeltiostream.hgt class Person private
double salary protected int age
public char initial void setPerson(const
char i, const double sal, const int a)
void showPerson(void)
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void PersonshowPerson() coutltlt"The person
initial is "ltltinitialltltendl coutltlt"The person
age is "ltltageltltendl coutltlt"The person salary is
"ltltsalaryltltendl void PersonsetPerson(const
char i, const double sal, const int
a) initiali salarysal agea
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include "Person.cpp" void main() Person
Carmen coutltltCarmen.initial
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include "Person.cpp" class Introvert private
Person private int idNum public
void showIntrovert(void)
Introvert class has four data members salary,
age, initial, and idNum
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showIntrovert() can use idNum. sowIntrovert can
also access initial and age, but can not use
salary, since it is private in Person class. The
way to access salary is througth
showPerson() function and built in side the
showIntrovert(). Other program such as main()
can not access the four data iterm directly.
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include "Person.cpp" class ShomwhatShy
protected Person private int idNum
public void showShy(void)
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It also has four data members. showShy() can
access idNum, and may also reference as
reference age and intial. When a somewhatShy()
object is instantiated in the main(), it may
use the showShy() fucntion to access idNum,
initial, or age. showShy() can not access salary
directly. It has to cal showPerson() to access
salary.
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include "Person.cpp" class Extrovert public
Person private int idNum public
void showIntrovert(void)
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includeltiostream.hgt include"Extrovert.cpp" void
main() Extrovert ex1 ex1.setPerson('J',
30000,40) / can not access id.Num, since it is
private is Extrovert class can not access
age, since it is protected can not access
salary, since it is priavte defined in base
class coutltltex1.idNumltltendl coutltltex1.ageltlten
dl coutltltex1.salaryltltendl /
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ex1.initial'K' //it can access initial,
since it is public in Extrovert
class coutltltex1.initialltltendl ex1.showPerson()

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When you use main() to create an object of
Extrovert class, showExtrovert() can display any
of the four data members. Main() function can
not access salary directly it may call
showPerson() to access salary.
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What is Inheritance?

• The following are never inherited
• Constructor functions
• Destructor functions
• friend function
• static data members

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What is Inheritance?

• The following are never inherited (continued)
• static member functions
• Overloaded new operators
• Overloaded operators
• If a derived class requires any of these items,
  they must be explicitly defined within the
  derived class definitions

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Choosing the Class Access Specifier

• When defining a derived class, insert one of
  three class access specifiers (public, private,
  or protected) just prior to the base class name
• No matter which access specifier you use when
  creating a child class, access to parent members
  never becomes more lenient than originally coded

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Choosing the Class Access Specifier

• If a derived class uses the public access
  specifier, the following statements are true
• Base class members that are public remain public
  in the derived class
• Base class members that are protected remain
  protected in the derived class
• Base class members that are private are
  inaccessible in the derived class

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Choosing the Class Access Specifier

• If a derived class uses the protected access
  specifier, the following statements are true
• Base class members that are public become
  protected in the derived class
• Base class members that are protected remain
  protected in the derived class
• Base class members that are private are
  inaccessible in the derived class

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Choosing the Class Access Specifier

• If a derived class uses the private access
  specifier, the following statements are true
• Base class members that are public become private
  in the derived class
• Base class members that are protected become
  private in the derived class
• Base class members that are private are
  inaccessible in the derived class

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Choosing the Class Access Specifier

• If a class has private data members, they can be
  used only by member functions of that class
• If a class has protected data members, they can
  be used by member functions of that class and by
  member functions of described classes
• If a class had public data members, they can be
  used by member functions of that class, by member
  function of derived classes, and by the main()
  program

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class GrandParant public int
x class Parent class Child public
Parent
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Overriding Parent Class Functions

• When a new class is derived from an existing
  class, the derived class has access to nonprivate
  member functions in the base class
• The new class may also have its own member
  functions
• Those functions may have names that are identical
  to the function name in the base class

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Overriding Parent Class Functions

• When any class member function is called, the
  following steps take place
• Compiler looks for a matching function name in
  the class of the object using the function name
• If no match is found, compiler looks for a
  matching function name in the parent class
• If no match is found, compiler continues up the
  inheritance hierarchy until the base class is
  reached
• If no match is found in any class, an error
  message is issued

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includeltiostream.hgt includeltstring.hgt class
NameAndNumber protected int idNum
char lastName20 char firstName15
public NameAndNumber() NameAndNumber()
void inputData(const int num, const char
last, const char first)
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void outputData(void) NameAndNumberNameA
ndNumber() NameAndNumberidNum123 strcpy(Na
meAndNumberlastName," ") strcpy(NameAndNumber
firstName," ") NameAndNumberNameAndNumber(
)
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void NameAndNumberinputData(const int
num, const char last, const char
first) NameAndNumberidNumnum strcpy(NameA
ndNumberlastName,last) strcpy(NameAndNumberf
irstName,first) void NameAndNumberoutputData(
void) coutltlt"ID "ltltNameAndNumberidNum cout
ltlt" "ltltNameAndNumberfirstNameltlt"
" ltltNameAndNumberlastNameltltendl
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include "NameAndNumberB.cpp" class
Employeepublic NameAndNumber private
int dept double hourlySalary public
void inputData(const int id, const char last,
const char first, const int dept, const double
sal) void outputData(void)
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void EmployeeinputData(const int id, const char
last, const char first, const int dep,
const double sal) NameAndNumberinputData(id,
last, first) Employeedeptdep Employeehour
lySalarysal void EmployeeoutputData(void)
coutltlt"Employee ID "ltltEmployeeidNumltltendl co
utltlt" Employee is "ltltEmployeelastNameltlt" ,
" coutltltEmployeefirstName0ltlt"."ltltendl cout
ltlt" Salary "ltltEmployeehourlySalary
ltlt" Department "ltltEmployeedeptltltendl
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include "Employee.cpp" includeltiostream.hgt void
main() NameAndNumber person person.inputData
(123,"Jun","Ni") person.outputData() coutltltendllt
ltendl Employee worker worker.inputData(987,
"Dean", "Roger", 101, 35433.45) worker.outputData
()
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Lab Exercises and Homework

• Do Exercises 1 on page 282
• Do Exercises 2 on page 282
• Do Exercises 3 on page 282
• Due date

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Lesson B
Inheritance Techniques
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Lesson B Objectives

• To learn
• How to use the constructor initialization lists
• How to inherit from base classes with
  constructors
• How to override inherited access
• How to use multiple inheritance

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Constructor Initialization Lists

• Many constructor functions consist of a series of
  assignment statements
• Constructor initialization list
• Inserted after the argument list for the
  constructor function, preceded by a single colon

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Constructor Initialization Lists

• Understanding use of constructor initialization
  lists
• Many C programmers prefer this method, so it is
  used in many programs
• Technically, constructors should initialize
  values
• Reference variable and constant class members
  cannot be assigned values
• When creating a derived class and instantiating
  an object, a parent class object must be
  constructed first

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class Inventory protected int itemNum
double itemPrice public Inventory(int n,
double p) itemNumn, itemPricep
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class Inventory protected int itemNum
double itemPrice public Inventory(int n,
double p)itemNum(n), itemPrice(p)
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Base Class Construction

• When instantiating a derived class object
• A constructor for its base class is called first,
  followed by the derived class constructor
• If a base class constructor requires arguments,
  you must create a constructor for any derived
  class

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Base Class Construction

• If you fail to call a needed base class
  constructor in the initialization list for a
  derived class, you will receive an error message
• Example Cannot find default constructor to
  initialize base class

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Base Class Construction

• Class object
• May be initialized in a constructor
  initialization list
• When constructing a derived class object, the
  base class constructor is called first
• When a derived class object is destroyed, the
  child class destructor is called first and the
  base class destructor is called last

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Base Class Construction

• If a default base class constructor exists, no
  compiler error will arise if you omit the call to
  the base class constructor when deriving a class

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Overriding Inherited Access

• Nine inheritance access specifier combinations
  are possible
• In addition, you may override the class access
  specifier for any specific class members
• Note that you do not place parentheses after the
  function name in the derived class

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class PetInven protected int stockNum
double price public PetInven(const int
stk, const double pr) stockNum(stk), price(pr)

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class Animalpublic PetInven protected
int petAge public Animal(const int stk,
const double price, const int
age) AnimalAnimal(const int stk, const
double price, const int age)
PetInven(stk,price) AnimalpetAgeage
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class PetInven //alternatively protected
int stockNum double price public
PetInven(const int stk, const double
pr) stockNum(stk), price(pr) class
Animalpublic PetInven protected int
petAge public Animal(const int stk, const
double price, const int age)
PetInven(stk,price), petAge(age)
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Multiple Inheritance

• A derived class may derive from more than one
  base class
• You already use multiple inheritance each time
  you include iostream.h in a program
• cin and cout objects are each derived from other
  classes

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Multiple Inheritance

• Some programmers are vehemently opposed to
  multiple inheritance
• As proof that multiple inheritance is never
  required, consider that the object-oriented
  languages SmallTalk and Java allow only single
  inheritance

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Virtual Base Classes

• A base class may have many descendants through
  single inheritance
• A class may inherit from two other classes
  through multiple inheritance
• The keyword virtual indicates that the base class
  should be used only once

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includeltiostream.hgt class Inventory
protected int itemNumber double itemPrice
public Inventory(void) Inventory(void)
void showData(void)
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class Employee protected int empNumber
double empSalary public Employee(void)
Employee(void) void showData(void)
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class Patentpublic Inventory, public
Employee private int patentNum public void
showAll(void) void PatentshowAll(void) c
outltltPatentpatentNumltltendl InventoryshowData
() EmployeeshowData()
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Patent newInvention newInvention.showAll()
//OK to use showAll() in Patent
class newInvention.showData() //It
confuses to select which one is accessed // So
you can use the following way newInvention.Invento
ryshowData() newInvention.EmplyeeshowData()
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Lab Exercises and Homework

• Do Exercises 1 on page 300
• Do Exercises 2 on page 300
• Do Exercises 3 on page 300
• Due date