C++ Classes and Objects
• We learned about functions and variables.
• Sometimes it's desirable to put related functions and data in one
place so that it's logical and easier to work with.
• Suppose, we need to store the length, breadth, and height of a
rectangular room and calculate its area and volume.
• To handle this task, we can create three variables, say, length,
breadth, and height along with the functions calculateArea() and
calculateVolume().
• However, in C++, rather than creating separate variables and
functions, we can also wrap these related data and functions in a
single place (by creating objects).
• This programming paradigm is known as object-oriented
programming.
• But before we can create objects and use them in C++, we first
need to learn about classes.
C++ Class
• A class is a blueprint for the object.
• We can think of a class as a sketch (prototype) of a house.
• It contains all the details about the floors, doors, windows, etc.
• Based on these descriptions we build the house.
• House is the object.
Create a Class
A class is defined in C++ using keyword class followed by the
name of the class.
The body of the class is defined inside the curly brackets and
terminated by a semicolon at the end.
class className {
// some data
// some functions
};
//For example
class Room {
public:
double length;
double breadth;
double height;
double calculateArea(){
return length * breadth;
}
double calculateVolume(){
return length * breadth * height;
}
};
• Here, we defined a class named Room.
• The variables length, breadth, and height declared
inside the class are known as data members. And,
the functions calculateArea() and calculateVolume()
are known as member functions of a class.
C++ Objects
• When a class is defined, only the specification for the object is
defined; no memory or storage is allocated.
• To use the data and access functions defined in the class, we
need to create objects.
• Syntax to Define Object in C++
className objectVariableName;
We can create objects of Room class (defined in the above
example) as follows:
// sample function
void sampleFunction() {
// create objects
Room room1, room2;
}
int main(){
// create objects
Room room3, room4;
}
• Here, two objects room1 and room2 of the Room class are created
in sampleFunction(). Similarly, the objects room3 and room4 are
created in main().
• As we can see, we can create objects of a class in any function of
the program. We can also create objects of a class within the class
itself, or in other classes.
• Also, we can create as many objects as we want from a single
class.
C++ Access Data Members and Member Functions
• We can access the data members and member functions of
a class by using a . (dot) operator.
• For example, room2.calculateArea(); This will call the
calculateArea() function inside the Room class for object
room2.
• Similarly, the data members can be accessed as:
• room1.length = 5.5; In this case, it initializes the length
variable of room1 to 5.5.
// objects and class in C++ Programming int main() {
#include <iostream>
using namespace std; // create object of Room class
// create a class Room room1;
class Room { // assign values to data members
public: room1.length = 42.5;
double length; room1.breadth = 30.8;
double breadth; room1.height = 19.2;
double height;
double calculateArea() { // calculate and display the area and volume of the room
return length * breadth; cout << "Area of Room = " <<
} room1.calculateArea() << endl;
cout << "Volume of Room = " <<
double calculateVolume() { room1.calculateVolume() << endl;
return length * breadth * height;
} return 0;
}; }
• Note the use of the keyword public in the program. This
means the members are public and can be accessed
anywhere from the program.
• As per our needs, we can also create private members
using the private keyword.
• The private members of a class can only be accessed from
within the class.
• For example,
class Test {
private:
int a;
void function1() { }
public:
int b;
void function2() { }
}
Here, a and function1() are private. Thus they cannot be accessed from outside the
class.
On the other hand, b and function2() are accessible from everywhere in the program.
// public and private in C++ Class }
#include <iostream> double calculateVolume() {
using namespace std; return length * breadth * height;
class Room { }
private: };
double length; int main() {
double breadth; // create object of Room class
double height; Room room1;
public: // pass the values of private variables as
// function to initialize private variables arguments
void initData(double len, double brth, double hgt) room1.initData(42.5, 30.8, 19.2);
{ cout << "Area of Room = " <<
length = len; room1.calculateArea() << endl;
breadth = brth; cout << "Volume of Room = " <<
height = hgt; room1.calculateVolume() << endl;
} return 0;
double calculateArea() { }
return length * breadth;
C++ Constructors
A constructor is a special type of member function that is called automatically when
an object is created.
In C++, a constructor has the same name as that of the class and it does not have a
return type. For example,
class Wall {
public:
// create a constructor
Wall() {
// code
}
};
Here, the function Wall() is a constructor of the class Wall. Notice that the
constructor has the same name as the class, does not have a return type, and is
public
C++ Default Constructor
A constructor with no parameters is known as a
default constructor. In the example above, Wall() is
a default constructor.
• Example 1: C++ Default Constructor
#include <iostream> }
using namespace std; };
// declare a class
class Wall { int main()
private: {
double length; Wall wall1;
public: return 0;
// default constructor to initialize variable }
Wall() {
length = 5.5;
cout << "Creating a wall." << endl;
cout << "Length = " << length <<
endl;
Output
Creating a Wall
Length = 5.5
• Here, when the wall1 object is created, the Wall() constructor is
called. This sets the length variable of the object to 5.5.
• Note: If we have not defined a constructor in our class, then the
C++ compiler will automatically create a default constructor with
an empty code and no parameters.
C++ Parameterized Constructor
• In C++, a constructor with parameters is known as a
parameterized constructor. This is the preferred method to
initialize member data.
• Example 2: C++ Parameterized Constructor
// C++ program to calculate the area of a wall
#include <iostream> }
using namespace std; };
// declare a class
class Wall { int main()
private: {
double length; // create object and initialize data members
double height; Wall wall1(10.5, 8.6);
public: Wall wall2(8.5, 6.3);
// parameterized constructor to initialize variables cout << "Area of Wall 1: " <<
Wall(double len, double hgt) { wall1.calculateArea() << endl;
length = len; cout << "Area of Wall 2: " <<
height = hgt; wall2.calculateArea();
} return 0;
double calculateArea() { }
return length * height;
Output
Area of Wall 1: 90.3
Area of Wall 2: 53.55
• Here, we have created a parameterized constructor Wall()
that has 2 parameters: double len and double hgt. The
values contained in these parameters are used to initialize
the member variables length and height.
• When we create an object of the Wall class, we pass the values
for the member variables as arguments. The code for this is:
•
• Wall wall1(10.5, 8.6);
• Wall wall2(8.5, 6.3);
• With the member variables thus initialized, we can now calculate
the area of the wall with the calculateArea() function.
C++ Copy Constructor
• The copy constructor in C++ is used to copy data of one
object to another.
•
• Example 3: C++ Copy Constructor
#include <iostream> int main() {
using namespace std; // create an object of Wall class
// declare a class Wall wall1(10.5, 8.6);
class Wall { // copy contents of wall1 to wall2
private: Wall wall2 = wall1;
double length; // print areas of wall1 and wall2
double height; cout << "Area of Wall 1: " << wall1.calculateArea() << endl;
public: cout << "Area of Wall 2: " << wall2.calculateArea();
// initialize variables with parameterized constructor return 0;
Wall(double len, double hgt) { }
length = len;
height = hgt;
}
// copy constructor with a Wall object as parameter
// copies data of the obj parameter
Wall(Wall &obj) {
length = obj.length;
height = obj.height;
}
double calculateArea() {
return length * height;
}
};
