Kapil Sehgal PGT Computer. Science Ankleshwar Gujarat Chapter 6 Inheritance Extending a Class

Size: px

Start display at page:

Download "Kapil Sehgal PGT Computer. Science Ankleshwar Gujarat Chapter 6 Inheritance Extending a Class"

Daniel Wade
5 years ago
Views:

1 Chapter 6 Inheritance Extending a Class Introduction; Need for Inheritance; Different form of Inheritance; Derived and Base Classes; Inheritance and Access control; Multiple Inheritance Revisited; Multilevel Inheritance; Nesting of Class;

2 Introduction of Inheritance The capability of one class to inherit properties from another class; The most important advantage of inheritance is code reusability. Once a base class is written and debugged; it can be used in various situation without having to redefine it. Already tested and saved previous code an be reused. Reusing existing code saves time, money and efforts and increase a program reliability.

3 Need for Inheritance Inheritance is very powerful concept of OOP. There are several reasons for inheritance was introduced in OOP. They are as follows One major reason behind this is the capability to express the inheritance relationship, which ensure the closeness of real world model; Another reasons is the idea of reusability. One reason is transitive nature of inheritance.

4 Advantage of Reusability The advantages of reusability are fastest development time, easier maintenance and easy to extend. Inheritance allows the addition of additional features to an existing class without modifying it. One can derive a new class from existing one and add new features to it.

5 Different form of Inheritance There are many forms of Inheritance; which are as follows. Single Inheritance; Multilevel Inheritance; Multiple Inheritance; Hierarchical Inheritance; Hybrid Inheritance;

6 Single Inheritance When a subclass inherits only from one base class; It is known as Single Inheritance A Base Class B Derived Class

7 Multilevel Inheritance The transitive nature of Inheritance is reflected by this form of inheritance. When a sub class is inherits from a class that itself inherits from another class; It is known as Multilevel Inheritance. A B C Base Class of B Sub Class of A & Base Class of C Sub Class of B

8 Multiple Inheritance When a sub class inherits from multiple base classes, it is known as multiple inheritance. In other words, when more than one classes together generate another class. The such type of inheritance is called multiple Inheritance. Base Class of B Base Class of A B A C Sub Class

9 Hierarchical Inheritance When many sub classes inherit from a single base class, it is known as hierarchical inheritance. A Base Class B C D Sub Class of B Sub Class of C Sub Class D

10 Hybrid Inheritance Hybrid inheritance combines two or more forms of inheritance e.g., When a sub class inherits from ultiple base classes and all of its base classes inherit from a single base class, this form of inheritance is known as hybrid inheritance. A A B C B C D D E F

11 Derived And Base Classes A Derived / Sub / Child class is defined by specifying its relationship with the Base / Super / Parent Class in addition to own details. Syntax to defining the derived class class derived : visibility mode base_class_name The visibility mode controls the visibility and availability of inherited base class members in the derived class.

12 Ex class Sub : public Super // public derivation... // Class Members class Sub : private Super // private derivation... // Class Members class Sub : protected Super // protected derivation... // Class Members class Sub : Super // by default private derivation... // Class Members

13 Extra Notation Actually a derived class inherits all the members of a base class; however the derived class has access privilege only to the non private members of the base class.

14 Declaration of Multiple Inheritance A class inheriting from multiple base classes is known as multiple inheritance. The syntax as class derived : vs_mode base1, vs_mode base2, Ex.. class sub : public base1, private base2.

15 Visibility Mode Visibility modes Inheritable public members becomes in derived class Inheritable protected members becomes in derived class Public Public Protected Protected Protected Protected Private Private Private

The Public Visibility Mode The public derivation means that the derived class can access the protected and public members of base class, but not the

16 The Public Visibility Mode The public derivation means that the derived class can access the protected and public members of base class, but not the private members of the base class. In this type of derivation public members of base class becomes public and protected members becomes protected of derived class.

17 Private Visibility Mode The private derivation means that the derived class can access the public and protected members of the base class. These members becomes private of derived class. Private members can access only the public member functions.

18 Protected Visibility mode The Protected derivation means the public and protected members of the base class becomes the protected members of the derived class.

19 Extra Notations While deriving a base class, following things should be kept in mind. Members should be intended and at the same time intended to be available to every function, even to non member function. Member intended to be inherited but not intended to be public, should be declared as public. Members not intended to be inherited should be declared as private.

20 Accessibility of Base class Members Access Specifier Accessible from own class Accessible from Derived Class (Inheritance ) Public Yes Yes Yes Protected Yes Yes No. Private Yes No. No. Accessible from object Outside the class

21 Constructor & Destructor in Inhritance When an object of derived class is created, the program first call the constructor of the base class and then constructor of the derived class. This is the same way as the first floor of a building is constructed before the constructing of the second floors. While defining the constructor of the derived class it should be kept in mind the work of constructor may not be duplicate. Similarly, when an object expire, the program should be call derived class destructor and then call base class destructor. It is also same way we first destroy the second floors of the building, before destroying the first floors.

22 EX Kapil Sehgal PGT Computer. Science Ankleshwar Gujarat class super Destructor super() cout<< Call First ; } Constructor class sub : public super sub() cout<< Call Next ; } class super ~super() cout<< Call Next ; } class sub : public super ~sub() cout<< Call First ; }

23 EX Kapil Sehgal PGT Computer. Science Ankleshwar Gujarat #include<iostream.h> #include<conio.h> class base base() cout<<endl<<"base \n"; } class derive:public base derive() cout<<endl<<"derive \n"; } void main() clrscr(); derive d; derive k; getch(); } Base OUTPUT Derive Base Derive

24 Derived Class Parameterized Constructor Some times base class constructor require arguments to construct their object. In such case it is the responsibility of the derived class constructor to pass those argument required by corresponding base class. To Pass these base class argument, the constructor of the class uses a special mechanism. derived(type1 x, type2 y) :base (x, y).. } The colon in the derived class constructor function means pass the derived argument to base class constructor.

25 EX Kapil Sehgal PGT Computer. Science Ankleshwar Gujarat #include<iostream.h> #include<conio.h> class base int a; base(int x) a=x; cout<<endl<<"base "<<a; } class derived:public base int b; derived(int x,int y):base(x) b=y; cout<<endl<<"derived "<<b; } void main() clrscr(); derived d(25,30); derived k(40,50); getch(); } OUTPUT Base 25 Derive 30 Base 40 Derive 50

26 Access Control in Publicly Derived class A class is publicly derived when the keyword public precedes the base class name in the class definition as given below. class derived : public base.. By this type of inheritance Public member of base class becomes public in derived class. Protected member of base class becomes protected in derived class. Private Members of base is not accessible in derived class

27 Access Control in Privately Derived class A class is privately derived when the keyword private precedes the base class name in the class definition as given below. class derived : private base.. By this type of inheritance Public member of base class becomes private in derived class. Protected member of base class becomes private in derived class. Private Members of base is not accessible in derived class

28 Access Control in Protected Derived class A class is protected derived when the keyword protected precedes the base class name in the class definition as given below. class derived : protected base.. By this type of inheritance Public member of base class becomes protected in derived class. Protected member of base class becomes protected in derived class. Private Members of base is not accessible in derived class

29 Facts about Inheritance Private members of base class are not directly accessible in the base class, but that does not affect its visibility in the derived class. The private members of base class are still visible in derived class. int test; // global var. int test1;// global var. class base int test; void getit(int x) cout<<endl<<"enter value "; cin>>test; } class derived :public base void check() test++;//not allowed bcoz private member; // global test is hidden also ::test++//global var. test1++;//global var direct access. }

30 Making a Private member Inheritable In Inheritance, only public and protected members of the base class is inheritable into the derived class. But we want to private members inheritable. Any derived class get not privileges to access private members of the base class directly. But this can be possible into two ways. By Making visibility mode of private as public. By Making visibility mode of private as protected.

31 Not Inheritable Class Der1:Public Base Private Protected Public Private Protected Public Class Base Class Der2:Private Base Private Protected Public Class Z:Public Der1,Private Der2 Private Protected Public

32 Multiple Inheritance Multiple inheritance means deriving a class from more than one base class. With multiple inheritance, the new classes inherits all the public and protected members of the base classes. class x int a; void fx1(); protected: char b; void fx2(); float c; void fx3(); class y int d; void fy1(); protected: char e; void fy2(); float f; void fy3(); class z : public x, public y int g; protected: int h; void display();

33 Constructor in Multiple Inheritance The constructor play an important role in initializing the object. One important is that, no base class constructor takes argument, then derived class is not need to define constructors. But if base class constructor is needs arguments then derived class constructor is needed and passes argument to base class constructors. Constructor can not be inherited. derived (int a,int b):base1(a),base2(b),... }

35 Virtual Base Class An element of ambiguity can be introduced into C++ program when multiple base classes are inherited. class base int a; class D1 : public base int b; class D2 : public base int c; class D3 : public D1, public D2 int total; void main() D3 ob; ob.a=25; ob.b=30; ob.c=40; ob.total=ob.a + ob.b + ob.c; // a is ambiguous; cout<<ob.a<<ob.b<<ob.c<<ob.total; } D1 Base D3 D2

36 Explanation Both D1 and D2 inherit base, they both have copies of base. However D3 inherit from D1 and D2. This means that there are two copies of base present in the object of D3. ob.a = 25; // this is ambiguous; Here a is of which class? Either D1 or D2 There are two way to overcome this situation. The first is apply is the scope resolution operator The second way is - virtual base class.

37 Solution by :: operator void main() D3 ob; ob.d1::a=25; ob.b=30; ob.c=40; ob.total=ob.d1::a + ob.b + ob.c; // a is ambiguous; cout<<ob.a<<ob.b<<ob.c< <ob.total; }

38 Solution by Virtual Base classes Base When two or more virtual D1 vittual D2 object are derived from a common base class, you can prevent multiple copies of the base class from being present in an D3 object derived from those objects by declaring the base class as virtual, when it is inherited. This is accomplished by putting the keyword virtual before the base class name, when it is inherited.

39 Ex class base int a; class D1 : virtual public base int b; class D2 : virtual public base int c; class D3 : public D1, public D2 int total; void main() D3 ob; ob.a=25; ob.b=30; ob.c=40; ob.total=ob.a + ob.b + ob.c; // a is ambiguous; cout<<ob.a<<ob.b<<ob.c<<ob.total; }

40 Multilevel Inheritance Some times a derived class itself, acts as base class for another class. Such a situation is called multilevel inheritance. The chain of classes forming multilevel inheritance is known as inheritance hierarchy or Inheritance path. The multilevel inheritance implements the transitive nature of real life objects. There can any number of classes in the inheritance hierarchy

41 Nesting of Class When a class contains objects of other class types as its members, it is referred as Containership or Containment or Aggregation. class X... class Y class Z X obj1 ; Y obj2;. All the object of Class Z will be containing object Ob1 and Ob2 of X and Y types respectively. This type of relationship is called containership or nesting. It is also referred as Containment, Composition or Aggregation This can also say that Class Z has HAS A relationship with Class X and Class Y.

42 Ex Kapil Sehgal PGT Computer. Science Ankleshwar Gujarat class subject Initialize the int num; float max-marks; subject (int n, float m) object and call num = n; max-marks=m; } the constructor class student int rollno; subject sub; Student (int r, int n, int m) : sub (n, m) rollno=r; }..

43 Extra Notations The Constructor for a member object of a class is invoked implicitly (through the name of constructor) And the constructor for the base class is invoked explicitly (through the class name)

44 Relationship Between Classes, HAS-A When a class is inherit from another class, is known as IS- A relationship. And when a class contains the object of other class type as member is known as HAS-A relationship. The class having HAS-A relationship with other class has Ownership of the container object. Ownership defines the creation and destruction of an object. It means if Container class object creates then other class also create with it, similarly Container object is destroy, then inner object simultaneously destroy

45 It is same as HAS- A relationship, but ownership is missing in HOLD- A relationship. A class indirectly contains the another object. It means contains a pointer or reference is said to have a HOLD-A relationship. Hold A Relationship. class fax.. } ; class modem fax *connect; long bps; char type; Modem (long s, fax * c) : connect (c) Bps = s; }

Base class or Super class. Subclass or Derived class

Base class or Super class. Subclass or Derived class INHERITANCE is the capability of one class to inherit the properties from another class. generates a model that is closer to the real world. NEED FOR INHERITANCE 1 Closeness with the real world model 3