Classes and Objects

Tags: oop java cpp

In object-oriented programming (OOP), real world entities are represented as objects. Each entity will have certain properties and behaviour

Class

A Class is a blueprint design for creating objects of a kind
It defines the properties (data members denoting state) and behaviour (methods) applicable to all instances of that class. Thus, each object will hold some data and code to manipulate that data

Object

Objects are runtime entities representing some real-world item
Each object is an instance of it’s class.It can also be seen as a variable of type as it’s class.

We define the class once and then create as many object of it as we need. Object occupy memory as they are entities that phsically exist, while classes don’t.

Java example

// Class definition of a person
class Person {
    // Data member
    private String name;

    // Constructor to initialize data member
    Person(String _name) {
        name = _name;
    };

    // Method
    void greet() {
        System.out.println("Hello " + name);
    }
}

class Demo {
    public static void main(String[] args) {
        // Creating object of "Person" class and storing reference to it in "p"
        Person p = new Person("Sam");
        // Calling "greet" method of "Person" class on the object that "p" refers to
        p.greet();
        // Output: Hello Sam
    }
}

C++ example

In C++, a class and struct are technically almost the same with the only difference being that the members are private by default inside a class and public by default inside a struct. But semantically, structs are used just as a combined block of data and classes are used for OOP features

class Person {
  private:
    string name;  // Data member

  public:
    // Constructor to initialize data member
    Person (string _name) : name(_name) {}

    // Method
    void greet () { cout << "Hello " << name << endl; }
};

int main () {
    // Creating object in Heap memory and storing it's reference in pointer
    Person *p1 = new Person("Sam");
    p1->greet();
    // Output: Hello Sam

    // Memory allocated on Heap has to be explicitly deallocated
    delete p1;

    // Creating object in Stack memory
    Person p2("Elon");
    p2.greet();
    // Output: Hello Elon

    return 0;
}