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1 Inheritance in C++ Lesson #6 Note: CIS 601 notes were originally developed by H. Zhu for NJIT DL Program. The notes were subsequently revised by M.
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1 Inheritance in C++ Lesson #6 Note: CIS 601 notes were originally developed by H. Zhu for NJIT DL Program. The notes were subsequently revised by M. Deek.

Dec 21, 2015

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Page 1: 1 Inheritance in C++ Lesson #6 Note: CIS 601 notes were originally developed by H. Zhu for NJIT DL Program. The notes were subsequently revised by M. Deek.

1

Inheritance in C++

Lesson #6

Note: CIS 601 notes were originally developed by H. Zhu for NJIT DL Program. The notes were subsequently revised by M. Deek.

Page 2: 1 Inheritance in C++ Lesson #6 Note: CIS 601 notes were originally developed by H. Zhu for NJIT DL Program. The notes were subsequently revised by M. Deek.

2

Content

Base and Derived Classes Single Inheritance Declaration of derived classes Order of Constructor and Destructor Execution

Inherited member accessibility Multiple Inheritance Virtual Base Classes

Page 3: 1 Inheritance in C++ Lesson #6 Note: CIS 601 notes were originally developed by H. Zhu for NJIT DL Program. The notes were subsequently revised by M. Deek.

3

Base and Derived Classes

A base class is a previously defined class that is used to define new classes

A derived class inherits all the data and function members of a base class (in addition to its explicitly declared members.)

Page 4: 1 Inheritance in C++ Lesson #6 Note: CIS 601 notes were originally developed by H. Zhu for NJIT DL Program. The notes were subsequently revised by M. Deek.

4

Single Inheritance

Implement an “is-a” relationship The derived class only has one base

class.

Page 5: 1 Inheritance in C++ Lesson #6 Note: CIS 601 notes were originally developed by H. Zhu for NJIT DL Program. The notes were subsequently revised by M. Deek.

5

Example 1:Student• name• id• major

Undergraduate• year• minor• etc.

Graduate• advisor• thesis• research• etc...

Page 6: 1 Inheritance in C++ Lesson #6 Note: CIS 601 notes were originally developed by H. Zhu for NJIT DL Program. The notes were subsequently revised by M. Deek.

6

Example 2:Publication:• publisher• date

Magazine:• # of issues per year• circulation

Book:• ISBN• author

Page 7: 1 Inheritance in C++ Lesson #6 Note: CIS 601 notes were originally developed by H. Zhu for NJIT DL Program. The notes were subsequently revised by M. Deek.

7

Example: Publication #include “FString.h"

class Publication {public: void SetPublisher( const FString & p ) {publisher.Assign(p);}; void SetDate( unsigned long dt ) {date = dt;}; FString GetPublisher(){return publisher;}; unsigned long GetDate(){return date;};private: FString publisher; unsigned long date;};

Page 8: 1 Inheritance in C++ Lesson #6 Note: CIS 601 notes were originally developed by H. Zhu for NJIT DL Program. The notes were subsequently revised by M. Deek.

8

Publicationclass Magazine :public Publication {public:

void SetIssuesPerYear( unsigned n ){issuesPerYear=n;};void SetCirculation( unsigned long n ){ circulation=n;};

unsigned GetIssuesPerYear(){return issuesPerYear;};

unsigned long GetCirculation(){return circulation;};private: unsigned issuesPerYear; unsigned long circulation;};

Page 9: 1 Inheritance in C++ Lesson #6 Note: CIS 601 notes were originally developed by H. Zhu for NJIT DL Program. The notes were subsequently revised by M. Deek.

9

Publicationclass Book :public Publication {public: void SetISBN( const FString & s ) {ISBN.Assign(s);}; void SetAuthor( const FString & s ) {author.Assign(s);}; FString GetISBN() {return ISBN;}; FString GetAuthor() {return author;};private: FString ISBN; FString author;};

Page 10: 1 Inheritance in C++ Lesson #6 Note: CIS 601 notes were originally developed by H. Zhu for NJIT DL Program. The notes were subsequently revised by M. Deek.

10

Publicationint main(){Book B;B.SetPublisher( "Prentice Hall" );B.SetDate( 970101L );B.SetISBN( "0-02-359852-2" );B.SetAuthor( "Irvine, Kip" );cout << B.GetPublisher()<<endl<<B.GetDate()<<endl<<B.GetISBN().CString()<<endl<<B.GetAuthor().CString()<<endl;Magazine M;M.SetIssuesPerYear( 12 );M.SetCirculation( 500000L );cout << M.GetIssuesPerYear()<<endl<<M.GetCirculation()<<endl; return 0;}//ex6pub(Fstring.h, fstring.cpp, ex6pub.cpp)

Page 11: 1 Inheritance in C++ Lesson #6 Note: CIS 601 notes were originally developed by H. Zhu for NJIT DL Program. The notes were subsequently revised by M. Deek.

11

Different Views of an Employee

Full-time or part-time Permanent or Temporary How do you define its base class?

How td you define derived classes based on this base class?

Page 12: 1 Inheritance in C++ Lesson #6 Note: CIS 601 notes were originally developed by H. Zhu for NJIT DL Program. The notes were subsequently revised by M. Deek.

12

Declaring Derived Classes

Class class_name: access_specifieropt or base_class { Member_list

}

access_specifier ::= public|protected|private(default)

Equivalent to :Subclass ::=<Id, SupeId, Ds, Ops, Intfc>

Page 13: 1 Inheritance in C++ Lesson #6 Note: CIS 601 notes were originally developed by H. Zhu for NJIT DL Program. The notes were subsequently revised by M. Deek.

13

3D Point class Point { public: Point(); Point( int xv, int yv ); void SetX( int xv ); void SetY( int yv ); private: int x; int y; };

Page 14: 1 Inheritance in C++ Lesson #6 Note: CIS 601 notes were originally developed by H. Zhu for NJIT DL Program. The notes were subsequently revised by M. Deek.

14

3D Point class Point3D :public Point { public: Point3D(); Point3D( int xv, int yv, int zv ); void SetZ( int zv ); private: int z; };

Page 15: 1 Inheritance in C++ Lesson #6 Note: CIS 601 notes were originally developed by H. Zhu for NJIT DL Program. The notes were subsequently revised by M. Deek.

15

3D Point

int main() { Point3D P; P.SetX( 100 ); P.SetY( 200 ); P.SetZ( 300 ); return 0; }

Point3D::Point3D( int xv, int yv, int zv )

{ SetX( xv );

SetY( yv );

SetZ( zv );

}

Page 16: 1 Inheritance in C++ Lesson #6 Note: CIS 601 notes were originally developed by H. Zhu for NJIT DL Program. The notes were subsequently revised by M. Deek.

16

Order of Constructor and Destructor Execution

Base class constructors are always executed first.

Destructors are executed in exactly the reverse order of constructors

The following example, shows you the ordering of constructors.

Page 17: 1 Inheritance in C++ Lesson #6 Note: CIS 601 notes were originally developed by H. Zhu for NJIT DL Program. The notes were subsequently revised by M. Deek.

17

ExampleClass Employee{Public:

Employee();//…

};Class SalariedEmployee:public Employee{Public:

SalariedEmployee();//…

};Class ManagementEmployee:public SalariedEmployee{Public:

ManagementEmployee();//…

};ManagementEmployee M;

Page 18: 1 Inheritance in C++ Lesson #6 Note: CIS 601 notes were originally developed by H. Zhu for NJIT DL Program. The notes were subsequently revised by M. Deek.

18

Example

CoordinatePoint

Point3D

Shape

Sphere

1

2

1

Page 19: 1 Inheritance in C++ Lesson #6 Note: CIS 601 notes were originally developed by H. Zhu for NJIT DL Program. The notes were subsequently revised by M. Deek.

19

Example #include <iostream.h> class Coordinate { public: Coordinate() { cout << "Coordinate,"; } ~Coordinate() { cout << “~Coordinate,"; } }; class Point { public: Point() { cout << "Point,"; } ~Point() { cout << “~Point,"; } private: Coordinate x; Coordinate y; };

Page 20: 1 Inheritance in C++ Lesson #6 Note: CIS 601 notes were originally developed by H. Zhu for NJIT DL Program. The notes were subsequently revised by M. Deek.

20

Example class Point3D :public Point { public: Point3D() { cout << "Point3D,"; } ~Point3D() { cout << “~Point3D,"; } private: Coordinate z; }; class Shape { public: Shape() { cout << "Shape,"; } ~Shape() { cout << “~Shape,"; } };

Page 21: 1 Inheritance in C++ Lesson #6 Note: CIS 601 notes were originally developed by H. Zhu for NJIT DL Program. The notes were subsequently revised by M. Deek.

21

Example

class Sphere :public Shape { public: Sphere() { cout << "Sphere"; } ~Sphere() { cout << "Sphere"; } private: Point3D center; unsigned radius; }; int main() { Sphere S; return 0; } //See Ex6-1.cpp

Page 22: 1 Inheritance in C++ Lesson #6 Note: CIS 601 notes were originally developed by H. Zhu for NJIT DL Program. The notes were subsequently revised by M. Deek.

22

Page 23: 1 Inheritance in C++ Lesson #6 Note: CIS 601 notes were originally developed by H. Zhu for NJIT DL Program. The notes were subsequently revised by M. Deek.

23

Overriding

A function in the derived class with the same function name will override the function’s variables in the base class.

You can still retrieve the overridden functions variables by using the scope resolution operator ”::”.

Page 24: 1 Inheritance in C++ Lesson #6 Note: CIS 601 notes were originally developed by H. Zhu for NJIT DL Program. The notes were subsequently revised by M. Deek.

24

Overriding#include <iostream.h>#include <stdlib.h>class A{ int i;public:

A(){i = 5;};int get(){return i;};

};class B: public A{ int i;public:

B(){i = 10;};int get(){return i;};

};

void main()

{ B b;

int x;

cout << b.get()<<endl;

cout << b.A::get()<<endl;

cout << sizeof(x)<<endl;

cout << sizeof(b)<<endl;

}//ex7overriding.cpp

Page 25: 1 Inheritance in C++ Lesson #6 Note: CIS 601 notes were originally developed by H. Zhu for NJIT DL Program. The notes were subsequently revised by M. Deek.

25

Types of Class Members

private protected public

Page 26: 1 Inheritance in C++ Lesson #6 Note: CIS 601 notes were originally developed by H. Zhu for NJIT DL Program. The notes were subsequently revised by M. Deek.

26

notaccessible

Private

Public

Protected

Accessibleto derivedclasses only

Derived Class

Accessible to derivedclasses and the instances

Types of Class Members

?

Page 27: 1 Inheritance in C++ Lesson #6 Note: CIS 601 notes were originally developed by H. Zhu for NJIT DL Program. The notes were subsequently revised by M. Deek.

27

Types of Inheritance

public private protected

Page 28: 1 Inheritance in C++ Lesson #6 Note: CIS 601 notes were originally developed by H. Zhu for NJIT DL Program. The notes were subsequently revised by M. Deek.

28

Public Inheritance

Public and protected members of the base class become respectively public and protected members of the derived class.

Page 29: 1 Inheritance in C++ Lesson #6 Note: CIS 601 notes were originally developed by H. Zhu for NJIT DL Program. The notes were subsequently revised by M. Deek.

29

Private

Public

Protected

Private

Public

Protected

public

Functions(instances)Class

Page 30: 1 Inheritance in C++ Lesson #6 Note: CIS 601 notes were originally developed by H. Zhu for NJIT DL Program. The notes were subsequently revised by M. Deek.

30

Example #include <iostream.h> #include <assert.h> class Item {

Item * ptr; int data; public: Item(){data = 0; ptr = NULL;}; Item(int i){data = i; ptr = NULL; cout <<"Item::Item"<<i <<endl; };

Page 31: 1 Inheritance in C++ Lesson #6 Note: CIS 601 notes were originally developed by H. Zhu for NJIT DL Program. The notes were subsequently revised by M. Deek.

31

Example

void setItem(int i){data = i; cout <<"Item::setItem"<<i <<endl; }; void setPtr(Item * i){ptr = i; cout <<"Item::setPtr"<<endl; }; int getData(){return data;}; Item * getPtr(){return ptr;}; };

Page 32: 1 Inheritance in C++ Lesson #6 Note: CIS 601 notes were originally developed by H. Zhu for NJIT DL Program. The notes were subsequently revised by M. Deek.

32

Example class List {

Item * head, *first, *last;

public:

List(){ head = NULL;

first = head;

last = head; }

Item * RemoveLast();

Item * RemoveFirst();

void PutFirst( Item * I );

void PutLast( Item * I );

protected:

int IsEmpty() const

{return (head==NULL);};

};

Page 33: 1 Inheritance in C++ Lesson #6 Note: CIS 601 notes were originally developed by H. Zhu for NJIT DL Program. The notes were subsequently revised by M. Deek.

33

Example Item * List::RemoveFirst() { Item * temp; temp = first; first = first -> getPtr(); cout <<"List:: RemoveFirst()"<<endl; return temp; }; Item * List::RemoveLast() { Item * temp; temp = last; last = last -> getPtr(); cout <<"List:: RemoveLast()"<<endl; return temp; };

Page 34: 1 Inheritance in C++ Lesson #6 Note: CIS 601 notes were originally developed by H. Zhu for NJIT DL Program. The notes were subsequently revised by M. Deek.

34

Example

void List::PutFirst(Item * I) { I->setPtr(first); first = I; cout <<"List::PutFirst"<<I->getData() <<endl; }; void List::PutLast(Item * I) { I->setPtr(last); first = I; };

Page 35: 1 Inheritance in C++ Lesson #6 Note: CIS 601 notes were originally developed by H. Zhu for NJIT DL Program. The notes were subsequently revised by M. Deek.

35

Example class Stack :public List { public: void Push( Item * I ); Item * Pop(); }; void Stack::Push( Item * I ) {PutFirst( I ); cout <<"Stack::Push"<<I->getData() <<endl; } Item * Stack::Pop() {cout <<"Stack::Pop()"<<endl; return RemoveFirst(); }

Page 36: 1 Inheritance in C++ Lesson #6 Note: CIS 601 notes were originally developed by H. Zhu for NJIT DL Program. The notes were subsequently revised by M. Deek.

36

Example int main() {Item anItem(50), *p; Stack aStack; aStack.Push( &anItem ); p = aStack.Pop(); cout <<"aStack.Pop"<< p->getData()<<endl<<endl; anItem.setItem(100); aStack.Push( &anItem ); p = aStack.RemoveFirst(); cout <<"aStack.RemoveFirst"<< p-

>getData()<<endl<<endl; return 0; }//ex6-2.cpp

Page 37: 1 Inheritance in C++ Lesson #6 Note: CIS 601 notes were originally developed by H. Zhu for NJIT DL Program. The notes were subsequently revised by M. Deek.

37

Page 38: 1 Inheritance in C++ Lesson #6 Note: CIS 601 notes were originally developed by H. Zhu for NJIT DL Program. The notes were subsequently revised by M. Deek.

38

Page 39: 1 Inheritance in C++ Lesson #6 Note: CIS 601 notes were originally developed by H. Zhu for NJIT DL Program. The notes were subsequently revised by M. Deek.

39

Private Inheritance

Public and protected members of the base class become private members of the derived class.

Page 40: 1 Inheritance in C++ Lesson #6 Note: CIS 601 notes were originally developed by H. Zhu for NJIT DL Program. The notes were subsequently revised by M. Deek.

40

Private

Public

Protected

Private

Public

Protected

private

Function(instances) Class

Page 41: 1 Inheritance in C++ Lesson #6 Note: CIS 601 notes were originally developed by H. Zhu for NJIT DL Program. The notes were subsequently revised by M. Deek.

41

Example class Queue :private List { public: void Enqueue( Item * I ); Item * Serve(); }; void Queue::Enqueue( Item * I ) { List::PutFirst( I ); cout <<"Queue::Enqueue"<<I->getData() <<endl; } Item * Queue::Serve() {cout <<"Queue::Serve"<<endl; return List::RemoveFirst(); }

Page 42: 1 Inheritance in C++ Lesson #6 Note: CIS 601 notes were originally developed by H. Zhu for NJIT DL Program. The notes were subsequently revised by M. Deek.

42

Example int main() {Item anItem(50), *p; Queue aQueue; anItem.setItem(60); aQueue.Enqueue(&anItem); p = aQueue.Serve(); cout <<"aQueue.Serve"<< p->getData()<<endl<<endl; anItem.setItem(600); aQueue.Enqueue(&anItem); p =aQueue.RemoveFirst(); //Unaccessible //cout <<"aQueue.RemoveFirst"<< p->getData()<<endl; return 0; }//ex6-3.cpp

Page 43: 1 Inheritance in C++ Lesson #6 Note: CIS 601 notes were originally developed by H. Zhu for NJIT DL Program. The notes were subsequently revised by M. Deek.

43

Page 44: 1 Inheritance in C++ Lesson #6 Note: CIS 601 notes were originally developed by H. Zhu for NJIT DL Program. The notes were subsequently revised by M. Deek.

44

Protected Inheritance

Public and protected members of the base class become protected members of the derived class.

Page 45: 1 Inheritance in C++ Lesson #6 Note: CIS 601 notes were originally developed by H. Zhu for NJIT DL Program. The notes were subsequently revised by M. Deek.

45

Private

Public

Protected

Private

Public

Protected

protected

Function(instances) Class

Page 46: 1 Inheritance in C++ Lesson #6 Note: CIS 601 notes were originally developed by H. Zhu for NJIT DL Program. The notes were subsequently revised by M. Deek.

46

Example class Stack1 :protected List { public: void Push( Item * I ); Item * Pop(); }; void Stack1::Push( Item * I ) {PutFirst( I ); cout <<"Stack1::Push"<<I->getData() <<endl; } Item * Stack1::Pop() {cout <<"Stack1::Pop()"<<endl; return RemoveFirst(); }

Page 47: 1 Inheritance in C++ Lesson #6 Note: CIS 601 notes were originally developed by H. Zhu for NJIT DL Program. The notes were subsequently revised by M. Deek.

47

Example int main() {Item anItem(50), *p; Stack1 aStack1; aStack1.Push( &anItem ); p = aStack1.Pop(); cout <<"aStack1.Pop"<< p->getData()<<endl<<endl; anItem.setItem(100); aStack1.Push( &anItem ); p = aStack1.RemoveFirst();//Unaccessible! cout <<"aStack1.RemoveFirst"<< p->getData()

<<endl<<endl; return 0; }

Page 48: 1 Inheritance in C++ Lesson #6 Note: CIS 601 notes were originally developed by H. Zhu for NJIT DL Program. The notes were subsequently revised by M. Deek.

48

Page 49: 1 Inheritance in C++ Lesson #6 Note: CIS 601 notes were originally developed by H. Zhu for NJIT DL Program. The notes were subsequently revised by M. Deek.

49

The accessibility of inherited members in a derived class

derived

member

Public Protected Private

Public X X

Protected X X

Private

Page 50: 1 Inheritance in C++ Lesson #6 Note: CIS 601 notes were originally developed by H. Zhu for NJIT DL Program. The notes were subsequently revised by M. Deek.

50

The accessibility of inherited members for an instance

Private

Protected

XPublic

PrivateProtectedPublicderived

member

Page 51: 1 Inheritance in C++ Lesson #6 Note: CIS 601 notes were originally developed by H. Zhu for NJIT DL Program. The notes were subsequently revised by M. Deek.

51

Constructor- Initializers

Point3D::Point3D(param-list): ctor-initializer

{// function body

} ctor-initializer is actually used to

transfer the parameters to the constructors of the base-class

Page 52: 1 Inheritance in C++ Lesson #6 Note: CIS 601 notes were originally developed by H. Zhu for NJIT DL Program. The notes were subsequently revised by M. Deek.

52

Publication #include <iostream.h> #include <string.h> class Date { public: Date( int mo, int dy, int yr ) { month = mo; day = dy; year = yr; cout << "Date constructor\n"; } // Ex6pub2.cpp

Page 53: 1 Inheritance in C++ Lesson #6 Note: CIS 601 notes were originally developed by H. Zhu for NJIT DL Program. The notes were subsequently revised by M. Deek.

53

Publication Date( Date & D ) { month = D.month; day = D.day; year = D.year; cout << "Date copy constructor\n"; } ~Date() { cout << "Date destructor\n"; } private: int year; int month; int day; };

Page 54: 1 Inheritance in C++ Lesson #6 Note: CIS 601 notes were originally developed by H. Zhu for NJIT DL Program. The notes were subsequently revised by M. Deek.

54

Publication class Publication { public: Publication( char * publshr, Date & aDate ) : pubDate( aDate ) { strcpy( publisher, publshr); cout << "Publication constructor\n"; } ~Publication() { cout << "Publication destructor\n"; } private: char publisher[30]; Date pubDate; };

Page 55: 1 Inheritance in C++ Lesson #6 Note: CIS 601 notes were originally developed by H. Zhu for NJIT DL Program. The notes were subsequently revised by M. Deek.

55

Publication class Magazine :public Publication {public: Magazine( char * publshr, Date & aDate, int issues ): Publication( publshr, aDate ) {issPYear = issues; cout << "Magazine constructor\n"; } ~Magazine() { "Magazine destructor\n"; } private: int issPYear; // issues per year }; int main() { Magazine aMag( "Zipp", Date(10, 1, 95), 12 ); return 0; }

Page 56: 1 Inheritance in C++ Lesson #6 Note: CIS 601 notes were originally developed by H. Zhu for NJIT DL Program. The notes were subsequently revised by M. Deek.

56

Page 57: 1 Inheritance in C++ Lesson #6 Note: CIS 601 notes were originally developed by H. Zhu for NJIT DL Program. The notes were subsequently revised by M. Deek.

57

Why use the constructor-initializer?

Without it, the default constructor for the base class would be called, which would then have to be followed by calls to access functions to set specific data members.

A constructor initailizer is therefore more efficient.

Page 58: 1 Inheritance in C++ Lesson #6 Note: CIS 601 notes were originally developed by H. Zhu for NJIT DL Program. The notes were subsequently revised by M. Deek.

58

Constructors in Derived Classes

When an object of a derived class is created, the constructor of the object must first explicitly call the constructor of the base class.

This is the same as constructor- initializer.

Page 59: 1 Inheritance in C++ Lesson #6 Note: CIS 601 notes were originally developed by H. Zhu for NJIT DL Program. The notes were subsequently revised by M. Deek.

59

Destructor Function

Destructors are called implicitly starting with the last derived class and moving in the direction of the base class.

Page 60: 1 Inheritance in C++ Lesson #6 Note: CIS 601 notes were originally developed by H. Zhu for NJIT DL Program. The notes were subsequently revised by M. Deek.

60

Compatibility Between Base and Derived Classes

An object of a derived class can be treated as an object of its base class.

The reverse is not true.

Page 61: 1 Inheritance in C++ Lesson #6 Note: CIS 601 notes were originally developed by H. Zhu for NJIT DL Program. The notes were subsequently revised by M. Deek.

61

Nested Class Scope

A public or protected base class member that is hidden from the derived class can be accessed using the scope resolution operator ” ::”

For example: base-class::member The “that” of base class can not access

the members of its derived classes.

Page 62: 1 Inheritance in C++ Lesson #6 Note: CIS 601 notes were originally developed by H. Zhu for NJIT DL Program. The notes were subsequently revised by M. Deek.

62

ExampleClass Parent {

public:

void Print() const;

//…

}

class Child: public Parent {

public:

void Print() const{

Parent::print();// scope resolution !!!

cout <<age<<‘\n’

<<school<<‘\n’;

}

private;

int age;

Fstring school;

}

Page 63: 1 Inheritance in C++ Lesson #6 Note: CIS 601 notes were originally developed by H. Zhu for NJIT DL Program. The notes were subsequently revised by M. Deek.

63

Error

Void Parent::Print() {cout<<name <<‘\n’ <<Child::age <<‘\n’;//base can not access that of derived }

Page 64: 1 Inheritance in C++ Lesson #6 Note: CIS 601 notes were originally developed by H. Zhu for NJIT DL Program. The notes were subsequently revised by M. Deek.

64

Implicit Conversion of Derived Pointers to Base Pointers

A base type pointer can point to either a base object or a derived object.Point3D center;// Point3D is derived from Point

Point * p = &center;

Point3D *cp = new Point3D;

Point *p;

p = cp;

Page 65: 1 Inheritance in C++ Lesson #6 Note: CIS 601 notes were originally developed by H. Zhu for NJIT DL Program. The notes were subsequently revised by M. Deek.

65

Example class FString { }; class Student { public: //... private: long id; }; class GraduateStudent :public Student { public: //... private: FString thesisTitle; };

Page 66: 1 Inheritance in C++ Lesson #6 Note: CIS 601 notes were originally developed by H. Zhu for NJIT DL Program. The notes were subsequently revised by M. Deek.

66

Example void CalcTuition( Student & sp ) { /* sp is a Student or a derived object */} void RecordThesis( GraduateStudent * p ) { /*... */ } int main() {Student * sp; GraduateStudent * gp; //... sp = gp; //... RecordThesis( (GraduateStudent *) sp ); return 0; }

Page 67: 1 Inheritance in C++ Lesson #6 Note: CIS 601 notes were originally developed by H. Zhu for NJIT DL Program. The notes were subsequently revised by M. Deek.

67

Casting Base Pointers to Derived Pointers

A base pointer can not be implicitly converted to a derived pointer.

This conversion is risky, because the derived object can contain more than the base object.

Page 68: 1 Inheritance in C++ Lesson #6 Note: CIS 601 notes were originally developed by H. Zhu for NJIT DL Program. The notes were subsequently revised by M. Deek.

68

Example void DoSomething(const GraduateStudent *

GS){ cout << GS->GetThesisTitle();}

Student * sp = new Student;DoSomething(sp);DoSomething(GraduateStudent (sp));

Student * sp = new GraduateStudent;DoSomething(GraduateStudent (sp));

//Error

Page 69: 1 Inheritance in C++ Lesson #6 Note: CIS 601 notes were originally developed by H. Zhu for NJIT DL Program. The notes were subsequently revised by M. Deek.

69

Example class Item {/* …*/} class Student: public Item{/* …*/} class Collection { public:

void Append (const Item * ip); Item * Get() const; } const unsigned Count = 10; Collection studentList; Student * p; for (int i=0; i<Count; i++ ){p = new Student;studentList.Append(p);} p = (student*) studentList.Get();//explicit cast

Page 70: 1 Inheritance in C++ Lesson #6 Note: CIS 601 notes were originally developed by H. Zhu for NJIT DL Program. The notes were subsequently revised by M. Deek.

70

Attention Forcing class users to use explicit casting

often leads poor code. class studentCollection:public Collection{ public: student * Get() const { return (student *) Collection::Get(); }//…} studentCollection studentList; student *p; //… p = studentList.Get(); // no cast required

Page 71: 1 Inheritance in C++ Lesson #6 Note: CIS 601 notes were originally developed by H. Zhu for NJIT DL Program. The notes were subsequently revised by M. Deek.

71

Multiple Inheritance

Student

employee

GraduateAssistant

salaried

Page 72: 1 Inheritance in C++ Lesson #6 Note: CIS 601 notes were originally developed by H. Zhu for NJIT DL Program. The notes were subsequently revised by M. Deek.

72

Example

#include <iostream.h> #include "fstring.h" typedef unsigned long ulong; class Student { public: unsigned GetAge() const; ulong GetId() const; unsigned GetMajor() const; void SetAge( unsigned n ); void SetId( ulong n ); void SetMajor( unsigned n ); private: ulong id; unsigned majorCode; unsigned degreeCode; float gpa; unsigned age; };

Page 73: 1 Inheritance in C++ Lesson #6 Note: CIS 601 notes were originally developed by H. Zhu for NJIT DL Program. The notes were subsequently revised by M. Deek.

73

Example class Employee { public: unsigned GetAge() const; const FString & GetBenefits() const; unsigned GetExemptions() const; void SetAge( unsigned n ); void SetBenefits( const FString & benef ); void SetExemptions( unsigned n ); private: unsigned age; unsigned exemptions; FString benefits; };

Page 74: 1 Inheritance in C++ Lesson #6 Note: CIS 601 notes were originally developed by H. Zhu for NJIT DL Program. The notes were subsequently revised by M. Deek.

74

Example class Salaried :public Employee { public: float GetSalary() const; void SetSalary( float s ); private: float salary; }; class GradAssistant :public Student, public Salarie

d { public: void Display() const; };

Page 75: 1 Inheritance in C++ Lesson #6 Note: CIS 601 notes were originally developed by H. Zhu for NJIT DL Program. The notes were subsequently revised by M. Deek.

75

Example void GradAssistant::Display() const { cout << GetId() << ',' << GetMajor() << ',' << GetSalary() << ',' << GetExemptions() << endl; } int main() { GradAssistant GA; GA.SetId(12345); // GA.SetAge(22); // error: ambiguous GA.Student::SetAge(22); // ok - specific GA.SetMajor(108); GA.SetExemptions(2); GA.SetSalary(10000); GA.Display(); return 0; }//ex6mulin.cpp

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Page 78: 1 Inheritance in C++ Lesson #6 Note: CIS 601 notes were originally developed by H. Zhu for NJIT DL Program. The notes were subsequently revised by M. Deek.

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Page 79: 1 Inheritance in C++ Lesson #6 Note: CIS 601 notes were originally developed by H. Zhu for NJIT DL Program. The notes were subsequently revised by M. Deek.

79

Example

Question: If we want to set a GradAssistant ‘s

age by calling SetAge(), which SetAge() should we use use?

1. Direct solution: Student::SetAge() or Salaried::SetAge().

2. Abstract(Virtual) base classes

Page 80: 1 Inheritance in C++ Lesson #6 Note: CIS 601 notes were originally developed by H. Zhu for NJIT DL Program. The notes were subsequently revised by M. Deek.

80

Virtual base classes

Student

employee

Graduate Assistant

salaried

Person

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81

Example #include <iostream.h> #include "fstring.h" typedef unsigned long ulong; class Person { public: unsigned GetAge() const; const FString & GetSocSecNum() const; void SetAge( unsigned n ); void SetSocSecNum( const FString & ssn ); private: unsigned age; FString socSecNum; };

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82

Example class Student :public virtual Person { public: unsigned GetMajor() const; void SetMajor( unsigned n ); private: unsigned majorCode; unsigned degreeCode; float gpa; };

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83

Example class Employee :public virtual Person { public: const FString & GetBenefits() const; unsigned GetExemptions() const; void SetBenefits( const FString & benef ); void SetExemptions( unsigned n ); private: unsigned exemptions; FString benefits; };

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84

Example class Salaried :public Employee { public: float GetSalary() const; void SetSalary( float s ); private: float salary; }; class GradAssistant :public Student, public Sa

laried { public: void Display() const; };

Page 85: 1 Inheritance in C++ Lesson #6 Note: CIS 601 notes were originally developed by H. Zhu for NJIT DL Program. The notes were subsequently revised by M. Deek.

85

Example

void GradAssistant::Display() const

{ cout << GetSocSecNum() << ','

<< GetAge() << ',‘

// ambiguous if not virtual

}

int main()

{ GradAssistant GA;

return 0;

}

Page 86: 1 Inheritance in C++ Lesson #6 Note: CIS 601 notes were originally developed by H. Zhu for NJIT DL Program. The notes were subsequently revised by M. Deek.

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Page 87: 1 Inheritance in C++ Lesson #6 Note: CIS 601 notes were originally developed by H. Zhu for NJIT DL Program. The notes were subsequently revised by M. Deek.

87

Virtual Base Classes The function calls in GradAssistant::Displ

ay() are ambiguous unless Person is inherited as a virtual base class.

Adding “virtual” lets the compiler decide which function and which variable should be accessed.

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88

Virtual Base Classes

Student(8)

Employee(22)

GraduateAssistant(0)

Salaried(4)

Person(22)

aGA(78)

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89

Virtual Base Classes(end)

Student(8)

Employee(22)

GraduateAssistant(0)

Salaried(4)

Person(22)

aGA(56)

virtual virtual

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90

Readings

Readings Chapter 6 Sections 6.1.3 – 6.5


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