C++ Classes and Objects

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In Object Oriented Programming, classes and objects are basic concepts of that are used to represent real-world concepts and entities.

• A class is a template to create objects having similar properties and behavior, or in other words, we can say that a class is a blueprint for objects.
• An object is an instance of a class. For example, the animal type Dog is a class, while a particular dog named Tommy is an object of the Dog class.

C++ Classes

A class is a user-defined data type, which holds its own data members and member functions that can be accessed and used by creating an instance of that class. A C++ class is like a blueprint for an object.

Create a Class : A class must be defined before its use. C++ class is defined using the keyword class keyword as shown:

class className {
access_specifier:

    // data member

    // member method
};

where,

• Data Members: These are the variables that are defined inside the class.
• Member Functions: Functions declared inside a class. Also referred to as a member method.

Example:

#include <iostream>
using namespace std;

// Class definition
class GfG {
public:
    // Data member
    int val;

    // Member function
    void show() {
        cout << "Value: " << val << endl;
    }
};

int main() {
    
   // Object will declareed here 
    return 0;
}

In the above, GfG class is created with a data member val and member function show(). Here, member function is defined inside the class, but they can also be just declared in the class and then defined outside using scope resolution operator ::

The above is called class definition or template.

C++ Objects

When a class is defined, only the specification (attributes and behaviour) for the object is defined. No memory is allocated to the class definition. To use the data and access functions defined in the class, we need to create its objects.

Objects are the actual entities that are created as an instance of a class. There can be as many objects of a class as desired. For example, in the above, we discussed the class of Cars. If we create an actual car based on the properties of the Car class, the car we made is the object of that class.

Create Object : Once the class is defined, we can create its object in the same way we declare the variables of any other inbuilt data type.

className objectName;

This statement creates an object of className class.

Member Access : Members of the class can be accessed inside the class itself simply by using their assigned name.

To access them outside, the (.) dot operatoris used with the object of the class.

obj.member1 // For data members
obj.member2(..) // For functions

There obj is the name of the object of the given class, member1 is data member and member2 is member function.

Local Class

Classes are generally declared in global scope and are accessible to every function or other classes once they are defined. But C++ also provides facility to define a class within a function. It is called local class in C++ and is only accessible in that function.

Nested Class

A nested class is a class defined within another enclosing class. As a member of the enclosing class, it has the same access rights as any other member. The members of the enclosing class do not have special access to the members of the nested class; the standard access rules apply.

This is the most common form:

class Outer {
public:
    class Inner {
    public:
        void display() {
            std::cout << "Inner class\n";
        }
    };
};

Enum Class

Enum classes in C++ are a safer and more organized way of using enums. They help to group related constants together while avoiding naming problems and ensuring better type safety.

Class vs Object

The following table lists the primary differences between the classes and objects in C++: