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In Object Oriented Programming, objects are packages that contain data and functions (methods) that can be performed on the data. Data could be considered to be attributes and functions are considered to be behaviors of the object. We can say that the attributes and behaviors are encapsulated into an object. The objects interact between each other through their interfaces. As an example a date object may have a set of data consisting of month, day and year, and methods consisting of assign date, display date, yesterday and tomorrow. Data abstraction is another important feature of Object oriented programming. OOP hides the details of data structures from the users. For example, to set a date in the aforementioned example, values of month, day and year are passed to the assign-date method. The actual representation of the date is hidden from the user. There can be various objects made of a particular class (recall that many variables can be made from type). In OOP the general type with which objects can be made is called a class. An object is an instance of a class. Each class contains data (data members) as well as set of functions (member functions) that manipulate the data. Classes have the following features:
Program 5-1 is a program that incorporates a c++ object. The class Grade contains three data members and three member functions (methods) that manipulate the data. The class has a public: section that is accessible to those using the class, and a private: section that only accessible to member functions and not accessible to the class users. Program 5-1
Program Run 5-1
Let us discuss this program in detail. We have declared a class named Grade. We have an object made up of this class, namely n. We could have made other objects of class Grade. An object encapsulates the data and functions that operate on that data. In this case the data used are three integers and three functions (methods) are getGrades, printGrades, and printLetter. The public part of the class is visible and accessible to all users of the class, the private part is not. The public part contains a constructor; a constructor is a function that is automatically called when an instance of a class is created. A constructor is used to initialize any class member variables, and allocate memory that the class will need. The member functions are similar to the functions we have used so far except in the declaration the scope operator :: is used. For example, void Grade::getGrades (int a, int b, intc), clearly indicates that getGrades is a member function of class Grade. Now let us examine the main. Here we have three integer variables a, b, c. and an object n. These values of these variables are read from the keyboard and then the three member functions of the object n are called. Imagine creating an include file of the class Grade. Instead of imagining it, let us do it. /****************************************** Program Grades - class By Dr. John Abraham Written for CSCI 1380 students Objective: introduce object oriented programming *******************************************/ #include <iostream.h> #include <c:\tempc\classgrade.h> int main (void) { int a,b,c; Grade n; cout <<"\nEnter three grades separated by spaces "; cin >> a >> b >> c; n.getGrades(a,b,c); n.printGrades(); n.printLetterGrade(); return (0); } The include file is given below. /****************************************** Program Grades - class By Dr. John Abraham Written for CSCI 1380 students Objective: introduce object oriented programming *******************************************/
#include <iomanip.h> class Grade { public: Grade(); //constructor void getGrades(int, int, int); void printGrades(); void printLetterGrade(); private: int g1; int g2; int g3; }; Grade::Grade() { g1=g2=g3=0; } void Grade:: getGrades (int a, int b,int c) { g1=a; g2=b; g3=c; } void Grade:: printGrades() { cout << "\n\n--------------------------------"; cout << "\nHere are the Grades you entered: " <<g1<<setw(4)<<g2<<setw(4)<<g3; } void Grade::printLetterGrade() { float av; av = (g1+g2+g3)/3.0; cout << "\nYour Average and Letter Grade -> "<<av; if (av >= 90) cout <<" A\n"; else if (av >=80) cout << " B\n"; else if (av >=70) cout <<" C\n"; else if (av >= 60) cout << " D\n"; else cout << " F\n"; } Go to top of this chapter |