Some important concept in oops are 1) Classes 2) Objects 3) Data abstraction & Encapsulation. 4) Inheritance 5) Dynamic binding. 6) Message passing

Transcription

1 Classes and Objects

2 Some important concept in oops are 1) Classes 2) Objects 3) Data abstraction & Encapsulation. 4) Inheritance 5) Dynamic binding. 6) Message passing

3 Classes i)theentiresetofdataandcodeofanobjectcanbemadeauserdefineddatatypewiththehelpofaclass. Objects are actually variable of the type class. ii) Once a class has been defined, we can create any number of objects belonging to that class. Thusaclassiscollectionofobjectsofsimilartype. iii)classesareuserdefineddatatypesandbehaveslikethebuiltin type of a programming language. iv)thesyntaxfordefiningclassis class class-name

4 Object i) Object are the basic run-time entities in an object-oriented system. ii)theymayrepresentaperson,aplaceabankaccount,atable ofdataoranyitemthattheprogramhastohandle. iii) Programming problem is analyzed in terms of objects and the nature of communication between them. iv)objectstakeupspaceinthememory&haveanassociated address like structure in c. v) When a program executes, the object interacts by sending messages to one another.

5 Ex. If there are two objects customer and account then the customer object may send a message to account object requesting for the bank balance. Thus each object contains data, and code to manipulate the data.

6 Representation of class Class name Attributes Operations Human Name Hair color talking walking Circle Radius center Calculate area() Draw()

7 Structure of Object Oriented Program in c++ class #include<iostream.h> Header File Class class-name access-specifiers int i,j; float f; char ch; Variables or fields access-specifiers void function-name() statement 1; statement 2; Function or Method main() class-name.obj-name; Class membe Object of class

8 Data abstraction and Encapsulation i)thewrappingupofdataandfunctionsintoasingleunitcalled class is known as encapsulation. ii)thedataisnotaccessibletotheoutsideworld,andonlythose functions which are wrapped in the class can access it. iii)thisinsulationofthedatafromdirectaccessbytheprogram is called data hiding or information hiding. iv) Abstraction refers to the act of representing essential features without including the background details or explanations. v)classesusetheconceptofabstractionandaredefinedasalist of abstract attributes such as size, weight and cost, and functions to operate on these attributes.

9 Inheritance i)inheritanceistheprocessbywhichobjectofoneclassacquire the properties of objects of another class. ii) It also provides the idea of reusability. This means that we can add additional features to an existing class without modifying it. iii)thisispossiblebyderivinganewclassfromtheexistingone. The new class will have combined features of both the classes.

10 Hierarchy WalkingBird call:? color:? food:? movement:walked Bird call:? color:? food:? movement:? FlyingBird call:? color:? food:? movement:flew Peacock call: color: blue&green food: grass Ostrich call: neek-neek color: brown food: grass Parrot call: color:? food: fruit Owl call:? color:? food:mice TalkingParrot...

11 Polymorphism i) Polymorphism is important oops concept. It means ability to take more than one form. ii) An operations may shows different behavior in different instances. The behavior depends upon the type of data used in the operation. For Ex-Operation of addition for two numbers, will generate a sum. If the operands are strings, then the operation would produce a third string by concatenation. iii) The process of making an operator to show different behavior in different instance is called as operator overloading. C++ support operator overloading.

12 The above figure shows concept of function overloading. Function overloading means using a single function name to perform different types of tasks

13 Dynamic Binding i) Binding refers to the linking of a procedure call to the code to be executed in response to the call. ii) Dynamic binding means that the code associated with a given procedure call is not known until the time of the call at runtime.

14 Message Passing OOPsconsistofasetofobjectsthatcommunicatewitheach other. Messagepassinginvolvesfollowingsteps i) Creating classes that define objects and their behavior ii) Creating objects from class definitions and iii) Amessage for an object is a request for execution of a procedure& therefore will invoke a function in the receiving object that generates the desired result.

15 Messagepassinginvolvesspecifyingthenameofthe object,thenameofthefunctioni.e.messageandthe information to be sent. Ex customer.balance(account no) Object message information

16 Benefits of oops OOP offers several benefits to both the program designer and the user. The principal advantages are. i) Inheritance, eliminate redundant code and extend the use of existing classes ii) Programs are based on the standard working module that communicate with one another, This leads to saving of development time and higher productivity. iii) The principal of data hiding helps the programmer to build secureprogramsthatcannotbeinvadedbycodeinotherpartof the program. iv)itispossibletohavemultipleinstanceofanobjecttoco-exist without any interference

17 Contd v)itiseasytopartitiontheworkinaproject,basedonobjects. vi) Object oriented systems can be easily upgraded from small to large systems. vii) Message passing techniques for communication between objects makes the interface description with external systems much simpler. viii) Software complexity can be easily managed

18 Specifying a class A class is a user defined data type which binds data and its associated functions together. Allows the data and functions to be hidden. Generally, a class specification has two parts. i) Class declaration: describes the type & scope of its members. ii) Class function definitions: describes how the class functions are implemented

19 Class Declaration The general form of a class is class class-name private: variable declaration; variable declaration; public: function declaration; function declaration; ;

20 1. Theclasskeywordspecifiesthatwhatfollowsisanabstractdataof type class name. 2. The body of a class is enclosed within braces & terminated by semicolon. 3. Class body consists of declaration of variables& functions which are called as members& they are grouped under two sections i.e. private &public. 4. Private and public are known as visibility labels, where private can beaccessedonlyfromwithintheclasswherepublicmemberscanbe accessed from outside the class also.

21 1. Bydefault,membersofaclassareprivate. 2. The variable declared inside the class are known as data members & functions are known as member functions. 3. Onlythememberfunctioncanhaveaccesstotheprivatedata members& private functions. 4. However the public members can be accessed from outside the class

22 Simple class example class item int number; float cost; public : void getdata (inta, float b); ; void putdata (void); Variable declaration Function declaration

23 Creating objects Object is an instance or variable of the class. Once a class has been declared, we can create variables of that type by using the class name. Ex. item x; Here class variables are known as object therefore, x is called an object of type item and necessary memory space is allocated to an object

24 Accessing class Members Privatedataofaclasscanbeaccessedbythemember function. Thefollowingistheformatforcallingmemberfunction. Object-name.function-name(actual argument) Ex. x.getdata(10, 20.3); This statementassign value 10 to number & 20.3 to cost ofthe object x. Ifthemembervariableisdeclaredasprivatethenitcannotbe accessible by object. It can only be accessed by a member function. Whereas, if a variable is declared as public it can be accessed by the object directly.

25 Ex. class abc intx, y; public : intz; ; main() abcm; m.x= 100; // error x is private m.z=50; // ok,z is public

26 A C++ PROGRAM WITH CLASS

27 Contd

28 Scope Resolution Operator 1) C++ is a block -structured language. 2) The scope of the variable extends from the point of its declaration till the end of the block containing the declaration Consider following program. intx = 1; === ===== intx = 2; =====

29 The two declaration of x refer to two different memory locations containing different values. Blocks in C++ are often nested. Declaration in a inner block hides a declaration of the same variable in an outer block.

30 # include<iostream.h > intm = 10; intmain ( ) intm = 20; intk = m; intm = 30; cout << k = <<k; cout << m = <<m; cout << : : m = << : : m; cout << m << m; cout<< ::m = <<::m; return 0; The output of program is k = 20 m = 30 :: m = 10 m = 20 :: m =10

31 DEFINING MEMBER FUNCTIONS. Member functions can be defined in two ways. 1) Outside the class definition 2) Inside the class definition

32 Outside the class definition Member function that is declared inside a class has to be defined separately outside the class. These member functions associate a membership identify label in the header. This label tells the compiler which class the function belongs to.

33 Outside the class definition The general format of a member function definition is. Return type class name:: function-name(argument declaration) Function body The membership label class-name:: tells the compiler that the function function-name belongs to the classname. The symbol :: is the scope resolution operator.

34 Ex- the function getdata is coded as void item:: getdata(int a, float b) number = a; cost = b;

35 The member function have some special characteristics :- i) Several different classes can use the same function name the membership label will resolve their scope. ii) Member function can access the private data of the class. A non member function cannot do so. iii) A member function can call another member function directly, without using the dot operator.

36 Program to calculate area of rectangle using outside member function #include <iostream.h> #include<conio.h> class Rectangle int width, height; public: void set_values (); void area() cout << Area is : << width*height; ; void Rectangle::set_values () cout << Enter width and height ; cin>>width>> height; void main () Rectangle rect; rect.set_values (); rect.area(); getch();

37 Program to calculate area of rectangle using outside member function #include <iostream.h> #include<conio.h> class Rectangle int width, height; public: void set_values (int,int); void area() cout << Area is : << width*height; ; void Rectangle::set_values (int x, int y) width = x; height = y; void main () Rectangle rect; rect.set_values (3,4); rect.area(); getch();

38 #include<conio.h> #include<iostream.h> class citizen int code; char name[30]; public : void showdata() cout<< code is :\n"<<code cout<< name is : "<<name; void getdata(); ; void citizen::getdata() cout<<"enter Citizen Name : "; cin>>name; cout<<"enter Citizen code "; cin>>code; void main() citizen c; //c A[3]; c.getdata(); //for(int i=0;i<2;i++) c.showdata(); //c A[i].getData(); getch(); //for(int j=0;j<2;j++) //c A[j].showData();

39 Program to calculate area of rectangle using outside member function with two objects #include <iostream.h> #include<conio.h> class Rectangle int width, height; public: void set_values (int,int); void area() cout << Area is : << width*height; ; void main () Rectangle rect, rectb; rect.set_values (3,4); rectb.set_values (5,6); cout << "rect area: " << rect.area<<endl; cout << "rectb area: " << rectb.area() ; getch(); Output: rect area: 12 rectb area: 30

40 Inside the class definition. In this method the function declaration inside the class is replaced by actual function definition. For ex class item int number; float cost; public : void getdata (int a float b); //declaration void putdata (void) cout << number << endl; cout << cost << endl; Remember only small functions can be defined inside the class

41 #include<iostream.h> #include<conio.h> class data int day; int month; int year; public: void date(int dd,int mm,int yy) day=dd; month=mm; year=yy; cout<<"\t"<<day<<"/"; cout<<month<<"/"; cout<<year<<endl; ; main() clrscr(); data f1,f2; f1.date(7,12,2002); f2.date(8,12,2002); f1.date(12,12,2002); getch(); o/p 7/12/2002 8/12/ /12/2002

42 Inline Function Functionssavememoryspace. When the compiler sees a function call, it normally generates a jump to the function. Whilethissequenceofeventsmaysavememoryspace,it takes some extra time. Tosaveexecutiontimeinshortfunctions,youmayelect toputthecodeinthefunctionbody. Directlyinlinewiththecodeinthecallingprogram.That is,eachtimethere safunctioncallinthesourcefile,the actual code from the function is inserted.

43 Contd Long sections of repeated code are generally used as normal functions. It takes more execution time but memory space and complexity of the program is reduced. But making a short section of code into an ordinary function may result in increasing compilation time. Ifafunctionisveryshort,theinstructionsnecessaryto call it may take up as much space as the instructions within the function body, so that space also increases. Insuchcases,itisbettertousecompletecodeoffunction instead of writing its call.

44 Contd Thesolutiontothisproblemistheinlinefunction. This kind of function is written like a normal function in the source file but compiles into inline code instead of into a function. The source file remains well organized and easy to read, since the function is shown as a separate entity. Whentheprogramiscompiled,thefunctionbodyis actually inserted into the program wherever a function call occurs.

45 Contd Difference between normal function and inline function is

46 // demonstrates inline functions #include <iostream.h> // pdstokg() converts pounds to kilograms inline float pdstokg(float pounds) return * pounds; int main() Float pds; cout << \n Enter your weight in pounds: ; cin >> pds; cout << \n Your weight in kilograms is: << pdstokg(pds); return 0;

47 Making an outside function inline. A member function can be defined outside the class definition and still make it inline by just using the qualifier inline in the header line of function definition Ex. class item public : void getdata(int a, float b); inline void item ::getdata(int a, float b) number = a; cost = b;

48 Data Hiding With data hiding accessing the data is restricted to authorized functions main program can t access the data directly this is done by placing the data members in the private section and, placing member functions to access & modify that data in the public section 48

49 Data Hiding The public section includes the data and operations that are visible, accessible, and usable by all of the clients that have objects of this class this means that the information in the public section is transparent, all of the data and operations are accessible outside the scope of this class by default, nothing in a class is public 49

50 Data Hiding The private section includes the data and operations that are not visible to any other class or client this means that the information in the private section is unclear and therefore is inaccessible outside the scope of this class the client has no direct access to the data and must use the public member functions this is where you should place all data to ensure the memory s integrity 50

51 Data Hiding Benefits : Member functions defined in the public section can use, return, or modify the contents of any of the data members directly Member functions are the only way to work with private data 51

52 Program for data hiding class Square private: int Num; public: void Get() cout<<"enter Number:"; cin>>num; void Display() cout<<"square is:"<<num*num; ; void main() Square Obj; Obj.Get(); Obj.Display(); getch()

53 Static class members Both functions and data inside the class can be made static. 1.Static data members 2. Static function members

54 Static Data Members A static member variable has certain special characteristics these are. i)itisinitializedtozerowhenthefirstobjectofitsclassis created. No other initialization is permitted. ii)onlyonecopyofthatmemberiscreatedfortheentireclassand is shared by all the objects of that class, no matter how many objects are created. iii) It is visible only within the class, but its lifetime is the entire program. Static variables are normally used to maintain values common to the entire class.

55 Program for static data member # include<iostream.h> #include<conio.h> class shared static int a; int b; public: void set(int i,int j) a=i; b=j; void show() cout<< "This is static a:"<<a; cout<<" \n This is non static b:"<<b<<endl; ; int shared::a; // defines static a

56 void main() shared x,y; x.set(1,1); x.show(); y.set(2,2); y.show(); x.show(); Output:- This is static a:1 This is non static b:1 This is static a:2 This is non static b:2 This is static a: 2 This is non static b:1 getch();

57 Program to illustrate static data member # include<iostream.h> #include<conio.h> class item static int count; int number; public : void getdata(int x) number = x; count ++; void getcount(void) cout << count: << count; ; intitem :: count;

58 void main ( ) item a, b, c; a.getcount() ; b.getcount() ; c.getcount() ; a.getdata(100); a.getdata(200); a.getdata(300); b.getdata(50); cout << After reading data ; a.getcount(); b.getcount(); c.getcount(); getch(); The output would be count : 0 count : 0 count : 0 After reading data count : 4 count : 4 count : 4

59 The static variable count is initialized to zero when the objects are created. The count is incremented whenever the data is read into an object. Since the data is read into objects four times, variable count is incremented four times. Static variables are like non inline member functions in that they are declared in a class declaration & defined in the source file.

60 Static Member Function 1) A static member function has following properties. i) A static function can have access to only other static members declared in the same class. ii) A static member function can be called using the class name as follows: class.name:: function.name;

61 #include<iostream.h> #include<conio.h> class test intcode; static int count; public: void setcode(void) code = ++count; void showcode(void) cout <<"object No: " << code<<endl; static void showcount(void) cout<<"count:"<<count<<endl; ; int test:: count; void main() test t1, t2; t1.setcode(); t2.setcode(); test::showcount(); test t3; t3.setcode(); test::showcount(); t1.showcode() ; t2.showcode(); t3.showcode(); getch();

62 Output: count: 2 count:3 Object No:1 Object No:2 Object No:3

63 Constructors and Destructors Classes uses member functions to provide initial values to private member variables. Forex.thestatement, x.getdata(100,299.95); passestheinitialvaluesasargumentstothefunctiongetdata()where these values are assigned to the private variables of object x. Allthesefunctioncallsareusedwithappropriateobjects that have been already created. Providing the initial values as described above does not confirm with the philosophy of C++ language. The aim of C++ is to create user-defined data types such as class, that behave very similar to the built-in types.

64 Contd Thismeansthatweshouldbeabletoinitializeanobjectwhenitis declared, such as initialization of an ordinary variable. Forexample:- intm=20; Floatx=5.75; Similarly, when a variable of built-in-type goes out of scope, the compiler automatically destroys the variable. Classes have some features, that enable us to initialize the object when they are created and destroy them when their presence is no longer necessary. C++providesaspecialmemberfunctioncalledtheconstructor, which enables an object to initialize itself when it is created. Italsoprovidesanothermemberfunctioncalledthedestructor,that destroys the objects when they are no longer required.

65 Constructor Aconstructorisaspecialmemberfunctionwhosetaskisto initialize the objects of its class. Itisspecialbecauseitsnameissameastheclassname. The constructor is invoked whenever an object of its associated class is created. It is called constructor because it construct the value data members of the class. When a class contains a constructor like the one defined above,itisguaranteedthatanobjectcreatedbytheclasswill be initialized automatically

66 Constructor //class with a constructor class Integer intm, n; public: Integer(void);.... ; Integer :: Integer(void) m = 0; n = 0; Integer int1; //constructor declared // constructor defined // object int1 created

67 Constructor The constructor functions have some special characteristics. They should be declared in the public section. They are invoked automatically when the objects are created. They do not have return types, not even void and therefore, they cannot return values. They cannot be inherited, though a derived class can call the base class constructor. Like other C++ functions, they can have default arguments.

68 Contd Constructors cannot be virtual. We cannot refer to their addresses. An object with a constructor(or destructor) cannot be used asamemberofaunion. They make implicit calls to the operations new and delete when memory allocation is required. remember, when a constructor is declared for a class, initialization of the class objects become mandatory.

69 Default Constructor A constructor that accepts no parameters is called the default constructor. The default constructor for class A is A::A(). If no such constructor is defined the compiler supplies default constructor. Therefore a statement such as A a; invokes the default constructor of the compiler to create the object a. Program to implement default constructor

70 #include<conio.h> #include<string.h> #include<iostream.h> class Employee int code; char *name; public : Employee() // default constructor name=new char[20]; strcpy(name,"default"); code=0; void showdata() cout<<"\n"<<name<<"\t <<code; ; void main() clrscr(); Employee e1; e1.showdata(); getch();

71 Parameterized Constructors Itmaybenecessarytoinitializethevariousdataelementsof different objects with different values when they are created. C++ permits us to achieve this objective by passing arguments to the constructor function when the objects are created. The constructors that can take arguments are called parameterized constructors.

72 Contd When a constructor has been parameterized, the object declaration statement such as integer int1; may not work. Wemustpasstheinitialvaluesasargumentstothe constructor function when an object is declared. Thiscanbedoneintwoways;

73 Contd 1 By calling the constructor explicitly Integer int1 = Integer (0, 100); //explicit call This statement creates in integer object int1 and passes the values 0 and 100 to it. 2 By calling the constructor implicitly Integer int1 (0, 100); //implicit call. This method sometimes called the shorthand method when the constructor is parameterized, arguments for the constructor must be provided.

74 Example of parameterized constructor #include <iostream.h> #include<conio.h> class myclass int a, b; public: myclass(int i, int j) a=i; b=j; void show() cout<< a << " " << b; ; void main() clrscr(); myclass ob(3, 5); ob.show(); getch(); Output: 3 5

75 Constructor as inline function The constructor functions can also be defined as inline functions. Example: class integer intm, n; public integer (int x, int y) //inline constructor m = x; y = n; ;

76 Destructors A destructor, as the name implies, is used to destroy the objects that have been created by a constructor. Likeaconstructor,itisamemberfunctionwhosenameisthe sameastheclassnamebutisprecededbyatilde. Forexample,thedestructorfortheclassintegercanbedefined as shown below: ~integer() Adestructornevertakesanyargumentnor does itreturnany value. Itwillbeinvokedimplicitlybythecompileruponexitfromthe program(orblockorfunctionasthecasemaybe)tocleanup storage that is no longer accessible.

77 Program to display length of a line #include <iostream.h> class Line public: ; void setlength( double len ); double getlength( void ); Line(); private: double length; Line::Line(void) cout << "Object is being created" << endl; void Line::setLength( double len ) length = len; double Line::getLength( void ) return length; int main( ) Line a; a.setlength(6.0); cout << "Length of line : " << a.getlength() <<endl; return 0;

78 Program to find area of rectangle #include<iostream.h> #include<conio.h> class rectangle private: float length,breadth; public: rectangle( ) length = 10.0; breadth=6.0; float area( ) return( length*breadth); ; void main( ) rectangle r; clrscr(); cout<< \n The area of the rectangle is: <<r.area( ) << square units ; getch( ); return 0;

79 Program to find factorial of a number #include<iostream.h> #include<conio.h> class factorial private: long n; public: factorial(); ; factorial::factorial() cout<<"\nenter the number to find factorial: "; cin>>n; long fact =1; while(n>1) fact =fact *n; n=n-1; cout<<"\n\nthe factorial is : "<<fact; void main() factorial f; getch();