Creating instance or Creating object

Instance(also known as object) is a real world entities. We can access the fields and methods of the class using an object. So far, we created variables using datatypes like int, double, string etc. A class can also be used like a datatype. The variables declared using class is known as object or instance. We can create an instance of any class in C++ using below syntax. To access the class members(fields and methods), we use the dot (.) operator after the object name. The dot operator links the name of an object with the name of a class member.

Syntax
className objectName;

CODE

EG 1 . Creating Rectangle class with fields and variables and creating instance.

#include <iostream>
using namespace std;

class Rectangle {
  public:
    double length, width, height;
    double Area() {
        return length * width;
    }
    double Perimeter() {
        return 2 * (length+width);
    }
    double Volume() {
        return Area() * height;
    }
};

int main() {
    //Here wall is an object which is a real world entity
    Rectangle wall;
    wall.length = 5;
    wall.width = 6;
    wall.height = 7;
    cout << wall.Area() << endl;
    cout << wall.Volume() << endl;
    cout << wall.Perimeter() << endl;
}

OUTPUT
30
210
22

CODE EXPLANATION
In the above code, we created an object called wall, Since it is the instance of class rectangle we can access all the fields and methods present inside rectangle class. We assigned the value of length, width, height value of the wall and calculate its Area, Perimeter and Volume.


EG 2 . Creating Class called car with fields and variables

#include <iostream>
using namespace std;

class Car {
  public :
    string color;
    int weight, numberOfSeats;
    void Drive() {
        cout << "Start Driving" << endl;
    }
    double CarryingLoadCapacity() {
        return (numberOfSeats*65)+30; //65 is the average weight of a person in kgs, 30kgs for extra load for accessories,fuel etc.
    }
};

int main() {
        Car TataNexon;
        TataNexon.color = "Red";
        TataNexon.weight = 1315;
        TataNexon.numberOfSeats = 5;
        //Drive() is void method hence cannot be written inside cout
        TataNexon.Drive();
        cout << "Carrying Load Capacity = " << TataNexon.CarryingLoadCapacity() << "kg" << endl;
}

OUTPUT
Start Driving
Carrying Load Capcity = 355kg

CODE EXPLANATION
In the above code, we created an object called TataNexon, Since it is the instance of class Car we can access all the fields and methods present inside Car class. We assigned the value of color, weight, numberOfSeats and calculated Maximum load capacity and also we invoked Drive() using it.


Where to use instance or object?
Objects and classes help us to manage complexity of a program and also it is very helpful in reusability of a program. Suppose if we want to create a game that has hundreds of enemies and each of them has fields like health, ammo, and methods like shoot() and run(). With OOP we can create a single Enemy class with required fields and methods. Then, we can create multiple enemy objects from it. Each of the enemy objects will have its own version of health and ammo fields. And, they can use the common shoot() and run() methods. Now, instead of thinking of projects in terms of variables and methods, we can think of them in terms of objects.

Exercises
1. Create a class called MusicalInstruments with fields and methods, create atleast 5 instances and access class members.
2. Create a class called Circle with fields and methods, create atleast 5 instances and access class members.


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