Objects and classes are used to wrap related functions and data in one place in C++.
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 calculate_area() and calculate_volume().
However, in C++, rather than creating separate variables and functions, we can also wrap the 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 - we build the house based on these descriptions.
The house is the object.
Create a Class
A class is defined in C++ using the keyword class followed by the name of the class.
The body of the class is defined inside 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 calculate_area(){
return length * breadth;
}
double calculate_volume(){
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 calculate_area() and calculate_volume () 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 object_name;
We can create objects of Room class (defined in the above example) as follows:
// sample function
void sample_function() {
// 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 sample_function().
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.calculate_area();
This will call the calculate_area() 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.
Example: Object and Class in C++ Programming
// Program to illustrate the working of
// objects and class in C++ Programming
#include <iostream>
using namespace std;
// create a class
class Room {
public:
double length;
double breadth;
double height;
double calculate_area() {
return length * breadth;
}
double calculate_volume() {
return length * breadth * height;
}
};
int main() {
// create object of Room class
Room room1;
// assign values to data members
room1.length = 42.5;
room1.breadth = 30.8;
room1.height = 19.2;
// calculate and display the area and volume of the room
cout << "Area of Room = " << room1.calculate_area() << endl;
cout << "Volume of Room = " << room1.calculate_volume() << endl;
return 0;
}
Output
Area of Room = 1309 Volume of Room = 25132.8
In this program, we have used the Room class and its object room1 to calculate the area and volume of a room.
In main(), we assigned the values of length, breadth, and height with the code:
room1.length = 42.5;
room1.breadth = 30.8;
room1.height = 19.2;
We then called the functions calculate_area() and calculate_volume() to perform the necessary calculations.
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.
To learn more about public and private keywords, please visit our C++ Class Access Modifiers tutorial.
Also Read