Object Oriented Programming using C++: Ch06 Objects and Classes.pptx

Object Oriented Programming using C++: Ch06 Objects and Classes

1. Object Oriented Programming Objects and Classes Engr. Rashid Farid Chishti [email protected] https://youtube.com/rfchishti https://sites.google.com/site/chishti International Islamic University H-10, Islamabad, Pakistan http://www.iiu.edu.pk 1 Class Object Video Lecture

2. Introduction Previously, programmers starting a project would sit down almost immediately and start writing code. As programming projects became large and more complicated, it was found that this approach did not work very well. The problem was complexity. Large programs are probably the most complicated entities ever created by humans. Because of this complexity, programs are prone to error, and software errors can be expensive and even life threatening (in air traffic control, for example ). Three major innovations in programming have been devised to cope with the problem of complexity. 2

3. O bject O riented P rogramming Object Oriented Programming offers a new and powerful way to cope with complexity. Instead of viewing a program as a series of steps to be carried out, it views it as a group of objects that have certain properties and can take certain actions. This may sound obscure until you learn more about it, but it results in programs that are clearer, more reliable, and more easily maintained. A major goal of this course is to teach object-oriented programming using C++ and cover all its major features. 3

4. The U nified M odeling L anguage The Unified Modeling Language ( UML ) is a graphical language consisting of many kinds of diagrams. It helps program analysts figure out what a program should do, and helps programmers design and understand how a program works. The UML is a powerful tool that can make programming easier and more effective. We introduce each UML feature where it will help to clarify the OOP topic being discussed. In this way you learn the UML painlessly at the same time the UML helps you to learn C++. 4

5. Class A class serves as a plan, or template. It specifies what data and what functions will be included in objects of that class . Defining the class doesn’t create any objects , just as the type int doesn’t create any variables. A class is thus a description of a number of similar objects. 5 functions data A class encapsulates attributes and functions

6. Example 1: A Simple Class #include < iostream > #include < stdlib.h > using namespace std ; class student { // define a class (a concept) private : int age; // class data ( attributes ) public : void set_age ( int g) { // member function age = g; // to set age } void show_age () { // member function // to display age cout << "I am " << age << " Years Old." << endl ; } }; 6 int main() { student Ali, Usman; // define two objects // of class student Ali.set_age (21); // call member function Usman.set_age (18); // to set age Ali.show_age (); // call member function Usman.show_age (); // to display age system( "PAUSE" ); return 0; }

7. Classes and Objects An object has the same relationship to a class that a variable has to a data type. An object is said to be an instance of a class, in the same way my Corolla is an instance of a vehicle. In the program the class , whose name is student, is defined in the first part of the program. In main(), we define two objects Ali and Usman that are instances of that class. Each of the two objects is given a value, and each displays its value. 7

8. Defining the Class 8 Syntax for class definition

9. Defining the Class The definition starts with the keyword class , followed by the class name— student in this Example. The body of the class is delimited by braces and terminated by a semicolon. An object has the same relationship to a class that a variable has to a data type. A key feature of object-oriented programming is data hiding. it means that data is concealed within a class so that it cannot be accessed mistakenly by functions outside the class . The primary mechanism for hiding data is to put it in a class and make it private . 9

10. Defining the Class private data or functions can only be accessed from within the class. p ublic data or functions, on the other hand, are accessible from outside the class. The data member age follows the keyword private , so it can be accessed from within the class , but not from outside. Member functions (also called methods or messages ) are functions that are included within a class . There are two member functions in student class : set_age () and show_age () . 10

11. Defining the Class Because set_age () and show_age () follow the keyword public , they can be accessed from outside the class. In a class the functions do not occupy memory until an object of the class is created . 11

12. Defining the Class Remember that the definition of the class student does not create any objects . It only describes how they will look when they are created. Defining objects means creating them. This is also called instantiating them, because an instance of the class is created. An object is an instance of a class. ali.set_age (21 ); This syntax is used to call a member function that is associated with a specific object ali . Member functions of a class can be accessed only by an object of that class . 12

13. Example 2 : Using Mobile Phone as an Object (1/2) // mobile.cpp demonstrates mobile phone as an object #include < iostream > #include < stdlib.h > using namespace std ; class mobile // class name should be { // the name of a concept private : string company_name ; string model_number ; string emi_number ; float cost; public : void set_info (string cn , string mn , string emi , float price) { // set data company_name = cn ; model_number = mn ; emi_number = emi ; cost = price; } 13

14. Example 2: Using Mobile Phone as an Object (2/2 ) 14 void show_info () { // display data cout << "Company = " << company_name.data () << endl ; cout << "Model = " << model_number.data () << endl ; cout << "EMI = " << emi_number.data () << endl ; cout << "Cost = " << cost << endl ; } }; int main(){ mobile m1; // define object of class part // call member function set_nfo () m1.set_info ( " QMobile " , "Noir A950" , " dont know :)" , 25000.0F); // now call member function show_info () m1.show_info (); system ( "PAUSE " ); return 0; }

15. Using Mobile Phone as an Object If you were designing an inventory program you might actually want to create a class something like mobile . It’s an example of a C++ object representing a physical object in the real world—a mobile phone . Standard C++ includes a new class called string . Using string object, you no longer need to worry about creating an array of the right size to hold string variables. The string class assumes all the responsibility for memory management. 15

16. Example 3 : Using Class to Represent Height (1/2) #include < iostream > #include < stdlib.h > using namespace std ; class Height{ // A Height class private : int feet; float inches; public : void set( int ft , float in){ feet = ft ; inches = in; } void get(){ // get height information from user cout << "\ nEnter Your Height Information " << endl ; cout << "Enter feet: " ; cin >> feet; cout << "Enter inches : " ; cin >> inches ; } void show(){ // display height information cout << "Height = " << feet << " feet and " << inches << " inches." << endl ; } }; 16

17. Example 3 : Using Class to Represent Height (2/2) int main() { Height H1, H2 ; // define two height objects H1.set(4, 9.35F); // set feet = 4 and inches = 9.35 in H1 object H2.get (); // get feet and inches info from user for H2 // display height information from H1 and H2 Objects cout << "For H1, " ; H1.show(); cout << "For H2, " ; H2.show(); system( "PAUSE" ); return 0; } 17

18. Constructors We see that member functions can be used to give values to the data items in an object . Sometimes , however, it’s convenient if an object can initialize itself when it’s first created , without requiring a separate call to a member function. Automatic initialization is carried out using a special member function called a constructor . A constructor is a member function that is executed automatically whenever an object is created . Constructor has the same name of class and it has no return type . 18

19. Example 4 : Constructors A House Example (1/2) #include < iostream > // A Constructor Example #include < stdlib.h > using namespace std ; class House{ private : double area; // area in square feet public : House() : area(1000){ /* a zero (no) argument constructor */ cout << "A House Created \n" ; /* */ } /* */ void set_area ( double a) { area = a; } double return_area () { return area; } void get_area (){ // get area info from user cout << "What is the area of House in Square Feet:" ; cin >> area; } }; 19

20. Example 4: Constructors A House Example (2/2 ) int main() { House H1, H2; /* define two objects and call constructor */ cout << "default area of House H1 is = " << H1.return_area() << endl ; cout << "default area of House H2 is = " << H2.return_area() << endl ; H1.set_area(1500); H2.get_area(); cout << "Now Area of House H1 is = " << H1.return_area() << " Square Feet" << endl ; cout << "Now Area of House H2 is = " << H2.return_area() << " Square Feet" << endl ; system( "PAUSE" ); return 0; } 20

21. Example 5: Constructors A Dice Example (1/2) #include < iostream > // for cin and cout #include < stdlib.h > // for rand(), srand () and system() #include < time.h > // for time() using namespace std ; class Dice{ private : int number; public : Dice():number(1){ /* A zero (no) argument constructor */ /* Sets the default number rolled by a dice to 1 */ /* seed random number generator with current system time */ srand (time(0)); /* Seed with Current time of PC */ } /* */ int roll() // Function to roll a dice. { // This function uses a random number generator to randomly // generate a number between 1 and 6, and stores the number // in the instance variable number and returns the number. number = rand() % 6 + 1; return number; } 21

22. Example 5: Constructors A Dice Example ( 2 /2 ) int get_number () // Function to return the number on the top face { // of the die. It returns the value of the variable number. return number; } }; int main(){ Dice d1,d2; cout << "d1: " << d1.get_number() << endl ; cout << "d2: " << d2.get_number() << endl ; cout << "After rolling d1: " << d1.roll() << endl ; cout << "After rolling d2: " << d2.roll() << endl ; cout << "The sum of the numbers rolled by the dice is: " << d1. get_number () + d2. get_number () << endl ; cout << "After rolling again, the sum of the numbers rolled is: " << d1.roll() + d2.roll() << endl ; system( "PAUSE" ); return 0; } 22

23. Destructor You might guess that another function is called automatically when an object is destroyed. This is indeed the case. Such a function is called a destructor . A destructor also has the same name as the class name but is preceded by a tilde (~) sign: Like constructors, destructors do not have a return value . They also take no arguments . There cannot be more than one destructor in a class. The most common use of destructors is to de-allocate memory that was allocated for the object by the constructor. 23

24. Example 6 : Destructor A House Example (1/2 ) #include < iostream > using namespace std ; class House{ private : double area; // area in square feet public : House() : area(1000) // a zero argument const. { cout << "House Created \n" ; } House( double a) : area(a) // a 1 argument const. { cout << "House Created 1 \n " ; } ~House() // A Destructor { cout << "House has been destroyed \n" ; } double return_area () { return area; } }; int main(){ House H1 , H2(600.0); cout << "Area of " << "House H1 is = " << H1.return_area() << endl << "House H2 is = " << H2.return_area () << endl ; system ( "PAUSE" ); return ; } 24

25. Example 7: Destructor A House Example (1/2 ) // A Constructor Example #include < iostream > using namespace std ; class House{ private : double area; // Area in square feet public : House() : area(1000) // A zero (no) argument constructor { cout << "House Created \n" ; } ~House() // A Destructor { cout << " House has been destroyed \n "; } }; void Create_House (){ House aHouse ; cout << "End of function" << endl ; } int main(){ Create_House (); Create_House (); system( "PAUSE" ); return 0; } 25

26. Example 8: Destructor A Student Example (1/3) #include < iostream > #include < cstring > // for calling strlen () function using namespace std ; class Student { public : char * name; // A pointer to store address of a string int length; // to store length of a String Student () : length(100) { // Make a zero (no) argument constructor and set length = 100 name = new char [length + 1]; // request 101 bytes memory to OS cout << "No Argument Constructor was called" << endl ; } Student( const char * src ) { // Make a one argument constructor length = strlen ( src ) ; // find length of src string cout << "Requesting " << length+1 << " bytes more Memory to OS" << endl ; name = new char [length+1]; // assign memory to name pointer one extra byte is for NULL strcpy_s (name , length+1, src ); // copy string from src to name } // alternatively you can also use strcpy (name , src ); 26

27. Example 8: Destructor A String Example (2/3) ~ Student() { /* A destructor */ cout << "Object is Destroyed / Deleted " << endl ; cout << "Therefore returning " << length + 1 << " bytes Memory to OS\n" << endl ; delete [] name; /* return occupied memory to OS */ } void Show_Name (){ cout << name << endl ; } void Get_Name (){ cout << "Enter Name:" ; cin.get (name , length + 1); fflush ( stdin ); } }; 27

28. Example 8: Destructor A String Example (3/3) int main() { Student * s1 = new Student( "Rashid Farid " ); s1- > Show_Name (); s1- > Get_Name (); s1- > Show_Name (); delete s1; system ( "PAUSE" ); return 0; } 28

29. Objects as Function Arguments Next program demonstrates some new aspects of classes, which are: Constructor Overloading Defining Member Functions Outside The Class. Objects as Function Arguments . 29

30. Example 9 : Objects As Function Arguments (1/3) // englcon.cpp constructors, adds objects using member function #include < iostream > #include < stdlib.h > using namespace std ; class Distance // English Distance class { private : int feet; float inches; public : Distance() : feet(0), inches(0.0) { cout << "No Arguments Constructor has been Called \n" ; } Distance( int ft , float in) : feet( ft ), inches(in) { cout << "Two Arguments Constructor has been Called \n" ; } void getDist (){ // get value of feet and inches from user cout << "\ nEnter feet: " ; cin >> feet; cout << "Enter inches: " ; cin >> inches; } 30

31. Example 9 : Objects As Function Arguments (2/3) void showDist (){ // display distance cout << feet << "\'-" << inches << '\"' ; } void addDist ( Distance, Distance ); // declaration }; // member function is defined outside the class void Distance :: addDist (Distance d1, Distance d2) { inches = d1.inches + d2.inches; // add the inches feet = 0; // (for possible carry) if (inches >= 12.0){ // if total exceeds 12.0, inches -= 12.0; // then decrease inches by 12.0 feet++; // and increase feet by 1 } feet += d1.feet + d2.feet; // add the feet } 31

32. Example 9 : Objects As Function Arguments (3/3) int main() { Distance dist1, dist3 ; // Calls no args . constructor Distance dist2(11, 6.25 ); // Calls two args . constructor dist1.getDist(); // get dist1 from user dist3.addDist(dist1 , dist2); // dist3 = dist1 + dist2 // display all lengths cout << "\ndist1 = " ; dist1.showDist (); cout << "\ndist2 = " ; dist2.showDist (); cout << "\ndist3 = " ; dist3.showDist (); cout << endl ; system( "PAUSE" ); return 0; } 32

33. Constructors Overloading Since there are now two explicit constructors with the same name, Distance (), we say the constructor is overloaded . Which of the two constructors is executed when an object is created depends on how many arguments are used in the definition: Distance length; // calls first constructor Distance width(11, 6.0); // calls second constructor 33

34. Member Functions Defined Outside the Class void add_dist ( Distance, Distance ); This tells the compiler that this function is a member of the class but that it will be defined outside the class declaration, someplace else in the listing. 34

35. Objects as Arguments Since add_dist () is a member function of the Distance class, it can access the private data in any object of class Distance supplied to it as an argument, using names like dist1.inches and dist2.feet . In the following statement dist3.add_dist(dist1, dist2 ); add_dist () can access dist3 , the object for which it was called, it can also access dist1 and dist2 , because they are supplied as arguments. When the variables feet and inches are referred to within this function, they refer to dist3.feet and dist3.inches . Notice that the result is not returned by the function. The return type of add_dist () is void . 35

36. Objects as Arguments The result is stored automatically in the dist3 object. To summarize, every call to a member function is associated with a particular object (unless it’s a static function; we’ll get to that later). Using the member names alone ( feet and inches ), the function has direct access to all the members, whether private or public , of that object . member functions also have indirect access, using the object name and the member name, connected with the dot operator ( dist1.inches or dist2.feet ) to other object s of the same class that are passed as arguments . 36

37. Objects as Arguments 37 9 7.5 11 6.25 21 1.75

38. The Default Copy Constructor We’ve seen two ways to initialize objects. A no-argument constructor can initialize data members to constant values A multi-argument constructor can initialize data members to values passed as arguments. You can also initialize one object with another object of the same type. Surprisingly, you don’t need to create a special constructor for this; one is already built into all classes. It’s called the default copy constructor . It’s a one argument constructor whose argument is an object of the same class as the constructor . The next program shows how this constructor is used. 38

39. Example 10: The Default Copy Constructor ( 1/2 ) // ecopycon.cpp initialize objects using default copy constr. #include < iostream > #include < stdlib.h > using namespace std ; class Distance { // English Distance class private : int feet; float inches; public : Distance() : feet(0), inches(0.0) // constructor (no args ) { cout << "No Arguments Constructor has been Called \n" ; } Distance( int ft , float in) : feet( ft ), inches(in) { cout << "Two Arguments Constructor has been Called \n" ; } void getdist (){ // get value of feet and inches from user cout << "\ nEnter feet: " ; cin >> feet; cout << "Enter inches: " ; cin >> inches; } 39

40. Example 10: The Default Copy Constructor (2/2) void showdist (){ // display distance cout << feet << "\'-" << inches << '\"' ; } }; int main(){ Distance dist1; // calls no arguments constructor Distance dist2(11, 6.25); // calls two- arg constructor Distance dist3(dist2); // calls default copy constructor Distance dist4 = dist2; // also calls default copy const. cout << "\ndist1 = " ; dist1.showdist(); // display all lengths cout << "\ndist2 = " ; dist2.showdist(); cout << "\ndist3 = " ; dist3.showdist(); cout << "\ndist4 = " ; dist4.showdist(); cout << endl ; system( "PAUSE" ); return 0; } 40

41. Example 11: Returning Objects from Functions (1/3) // englret.cpp function returns value of type Distance #include < iostream > #include < stdlib.h > using namespace std ; class Distance{ // English Distance class private : int feet; float inches; public : Distance() : feet(0), inches(0.0) // constructor (no args ) { cout << "No Arguments Constructor has been Called \n" ; } // constructor (two args ) Distance( int ft , float in) : feet( ft ), inches(in) { cout << "Two Arguments Constructor has been Called \n" ; } void getdist (){ // get value of feet and inches from user cout << "Enter feet : " ; cin >> feet; cout << "Enter inches: " ; cin >> inches; } 41

42. Example 11: Returning Objects from Functions ( 2 /3) void showdist () // display feet '- inches " { cout << feet << "\'-" << inches << '\"' ; } Distance add_dist (Distance); // function declaration }; // add this distance to d2 and return the sum using temp object Distance Distance :: add_dist (Distance d2){ // function definition Distance temp; // temporary objet temp.inches = inches + d2.inches; // add the inches if ( temp.inches >= 12.0){ // if total exceeds 12.0, temp.inches -= 12.0; // then decrease inches by 12.0 and temp.feet = 1; // increase feet by 1 } temp.feet += feet + d2.feet; // add the feet return temp; // return value of temporary objet } 42

43. Example 11: Returning Objects from Functions (3/3) int main() { Distance dist1, dist3; // define two lengths Distance dist2(11, 6.25); // define, initialize dist2 dist1.getdist(); // get dist1 from user dist3 = dist1.add_dist(dist2); // dist3 = dist1 + dist2 cout << "\ndist1 = " ; dist1.showdist(); // display all lengths cout << "\ndist2 = " ; dist2.showdist(); cout << "\ndist3 = " ; dist3.showdist(); cout << endl ; system( "PAUSE" ); return 0; } 43

44. Returning Objects from Functions To execute the statement dist3 = dist1.add_dist(dist2 ); A temporary object of class Distance is created to store the sum. The sum is calculated by adding two distances. The first is the object dist1 , of which add_dist () is a member. Its member data is accessed in the function as feet and inches . The second is the object passed as an argument, dist2 . Its member data is accessed as d2.feet and d2.inches . The result is stored in temp and accessed as temp.feet and temp.inches . The temp object is then returned by the function using the statement return temp; The statement in main() assigns it to dist3 . 44

45. Returning Objects from Functions Notice that dist1 is not modified; it simply supplies data to add_dist (). Figure on next slide shows how this looks. In the topic, “Operator Overloading”, we’ll see how to use the arithmetic + operator to achieve the even more natural expression like dist3 = dist1 + dist2; 45

46. Returning Objects from Functions 46 10 8 11 6.25 22 2.25

47. Structures and Classes In fact, you can use structures in almost exactly the same way that you use classes. The only formal difference between class and struct is that in a class the members are private by default , while in a structure they are public by default . You can just as well write class foo { int data1; public : void func (); }; //and the data1 will still be private. 47

48. Structures and Classes If you want to use a structure to accomplish the same thing as this class, you can dispense with the keyword public , provided you put the public members before the private ones struct foo{ void func (); private : int data1; }; // since public is the default . However , in most situations programmers don’t use a struct this way. They use structures to group only data , and classes to group both data and functions . 48

49. Classes, Objects and Memory You might have the impression that each object created from a class contains separate copies of that class’s data and member functions. It’s true that each object has its own separate data items But all the objects in a given class use the same member functions . The member functions are created and placed in memory only once —when they are defined in the class definition. Since the functions for each object are identical. The data items , however, will hold different values . 49 data1 data2 Object 3 data1 data2 Object 3 data1 data2 Object 3 Function1() Function2()

50. s tatic Class Data If a data item in a class is declared as static , only one such item is created for the entire class , no matter how many objects there are. A static data item is useful when all objects of the same class must share a common item of information. A member variable defined as static has characteristics similar to a normal static variable: It is visible only within the class , but its lifetime is the entire program . It continues to exist even if there are no objects of the class . A normal static variable is used to retain information between calls to a function, static class member data is used to share information among the objects of a class . 50

51. static Data in a Class Why would you want to use static member data? As an example, suppose an object needed to know how many other objects of its class were in the program. In a road-racing game, for example, a race car might want to know how many other cars are still in the race. In this case a static variable Total_Cars could be included as a member of the class. All the objects would have access to this variable. It would be the same variable for all of them; they would all see the same number of Total_Cars . 51

52. Example 12: s tatic Data in a Class (1/2) // statdata.cpp demonstrates a simple static data member #include < iostream > #include < stdlib.h > using namespace std ; class Car{ private : static int Total_Cars ; // only one data item for all objects // note: "declaration" only! public : Car() // increments count when object created { Total_Cars ++; } int How_Many () // returns count { return Total_Cars ; } ~Car() { Total_Cars --; } // decrement count when an object is destroyed }; int Car:: Total_Cars = 0; // initialization of count 52

53. Example 12: static Data in a Class (2/2 ) int main() { Car Toyota, Honda, Suzuki; // create three objects cout << Toyota.How_Many () << " Cars are in Race" << endl ; cout << Honda.How_Many () << " Cars are in Race" << endl ; // each object sees the same data cout << Suzuki.How_Many () << " Cars are in Race" << endl ; Car * Pajero = new Car; // create a nameless objet and store // its starting address in a pointer p cout << Suzuki.How_Many () << " Cars are in Race" << endl ; cout << Pajero -> How_Many () << " Cars are in Race" << endl ; delete Pajero ; cout << Honda.How_Many () << " Cars are in Race" << endl ; system( "PAUSE" ); return 0; } 53

54. s tatic Members Data Ordinary variables are usually declared (the compiler is told about their name and type) and defined (the compiler sets aside memory to hold the variable) in the same statement . e.g. int a; Static member data , on the other hand, requires two separate statements. The variable’s declaration appears in the class definition , but the variable is actually defined outside the class , in much the same way as a global variable. If static member data were defined inside the class, it would violate the idea that a class definition is only a blueprint and does not set aside any memory. 54

55. Example 13: const Member Function //constfu.cpp demonstrates const member functions class aClass { private : int alpha; public : void nonFunc () // non- const member function { alpha = 99; // OK } void conFunc () const // const member function { alpha = 99; // ERROR: can’t modify a member } }; 55 A const member function guarantees that it will never modify any of its class’s member data.

56. Example 14: Distance Class and Use of const ( 1/3 ) // const member functions & const arguments to member functions #include < iostream > using namespace std ; class Distance // English Distance class { private : int feet; float inches; // attributes public : Distance() : feet(0), inches(0.0) { } // constructor (no args ) Distance( int ft , float in) : feet( ft ), inches(in) { } // constructor (two args ) void getdist (){ // get value of feet and inches from user cout << "\ nEnter feet: " ; cin >> feet; cout << "Enter inches: " ; cin >> inches; } 56

57. Example 14: Distance Class and Use of const (2/3) void showdist () const // display distance { cout << feet << "\'-" << inches << '\"' ; } Distance add_dist ( const Distance&) const ; // add two objects }; //add this distance to d2, return the sum Distance Distance:: add_dist ( const Distance& d2) const { Distance temp; // temporary variable // feet = 0; // ERROR: can’t modify this // d2.feet = 0; // ERROR: can’t modify d2 temp.inches = inches + d2.inches; // add the inches if ( temp.inches >= 12.0) // if total exceeds 12.0, { temp.inches -= 12.0; // then decrease inches by 12.0 temp.feet = 1; // and increase feet by 1 } temp.feet += feet + d2.feet; // add the feet return temp; // return temp object } 57

58. Example 14: Distance Class and Use of const (3/3) int main() { Distance dist1, dist3; // define two lengths Distance dist2(11, 6.25); // define, initialize dist2 dist1.getdist(); // get dist1 from user dist3 = dist1.add_dist(dist2); // dist3 = dist1 + dist2 //display all lengths cout << "\ndist1 = " ; dist1.showdist(); cout << "\ndist2 = " ; dist2.showdist(); cout << "\ndist3 = " ; dist3.showdist(); cout << endl ; system( "PAUSE" ); return 0; } 58

59. const Member Function Arguments If an argument is passed to an ordinary function by reference, and you don’t want the function to modify it, the argument should be made const in the function declaration (and definition). This is true of member functions as well. Distance Distance :: add_dist ( const Distance& d2) const { In above line, argument to add_dist () is passed by reference , and we want to make sure that won’t modify this variable, which is dist2 in main(). Therefore we make the argument d2 to add_dist () const in both declaration and definition. 59

60. const Objects In several example programs, we’ve seen that we can apply const to variables of basic types such as int to keep them from being modified. In a similar way, we can apply const to objects of classes . When an object is declared as const , you can’t modify it . It follows that you can use only const member functions with it , because they’re the only ones that guarantee not to modify it. e.g. A football field (for American-style football) is exactly 300 feet long. If we were to use the length of a football field in a program, it would make sense to make it const , because changing it would represent the end of the world for football fans . 60

61. Example 15: const Objects (1/2) // constObj.cpp constant Distance objects #include < iostream > using namespace std ; class Distance // English Distance class { private : int feet; float inches; public : Distance( int ft , float in) : feet( ft ), inches(in) { } // 2-arg constructor void getdist () // user input; non- const function { cout << "\ nEnter feet:" ; cin >> feet; cout << "Enter inches:" ; cin >> inches; } 61

62. Example 15: const Objects (2/2) void showdist () const // display distance; const function { cout << feet << "\'-" << inches << '\"' ; } }; int main() { const Distance football(300, 0); // football.getdist (); // ERROR: getdist () not const cout << "football = " ; football.showdist (); // OK cout << endl ; system( "PAUSE" ); return 0; } 62

63. Assignment No. 1 63 Assignment No.1.doc Assignment No.1.pdf