Introduction to C++ | Object and Class

C++ Object and Class

In C++, the concept of an Object and a Class forms the basis of Object-Oriented Programming (OOP). A class is a blueprint or template for creating objects, and an object is an instance of a class. Classes encapsulate data for the object and define methods (functions) to manipulate that data.

What is a Class?

A Class is a user-defined data type that represents the properties (data members) and behaviors (functions or methods) of objects. It defines a set of attributes and functions that apply to all objects created from it.

What is an Object?

An Object is an instance of a class. When a class is defined, no memory is allocated, but when an object of the class is created, memory is allocated for that object’s data members.

Syntax of a Class in C++

The basic syntax to define a class is as follows:

        class ClassName {
            public:  // Access modifier
                // Data members
                DataType variable;
        
                // Member functions
                void functionName() {
                    // function body
                }
        };
                    

Creating an Object of a Class

Once a class is defined, objects can be created as follows:

        ClassName objectName;
                    

Example: Defining a Class and Creating an Object

Here is an example of a simple class definition and object creation:

        #include <iostream>
        using namespace std;
        
        // Class definition
        class Car {
        public:
            string brand;
            int year;
        
            // Member function
            void displayDetails() {
                cout << "Brand: " << brand << ", Year: " << year << endl;
            }
        };
        
        int main() {
            // Creating an object of class Car
            Car myCar;  // Object creation
        
            // Setting values for the object
            myCar.brand = "Toyota";
            myCar.year = 2020;
        
            // Calling member function
            myCar.displayDetails();  // Output: Brand: Toyota, Year: 2020
        
            return 0;
        }
                    

Output:

Explanation of the Code:

In this example:

• The class Car has two data members: brand and year.
• The class also has a member function displayDetails() that displays the values of brand and year.
• In the main() function, an object myCar of class Car is created.
• We assign values to brand and year using the object myCar and then call the displayDetails() function to display the information.

Access Modifiers in C++ Classes

In C++, class members (data members and functions) can be declared with different access levels using access modifiers:

• public: Members are accessible from outside the class.
• private: Members are not accessible from outside the class (default access level for data members).
• protected: Members are accessible within the class and by derived class instances (used for inheritance).

Example: Using Access Modifiers

This example demonstrates the use of private and public access modifiers:

        #include <iostream>
        using namespace std;
        
        class Person {
        private:
            string name;  // Private data member
        
        public:
            // Public function to access the private data member
            void setName(string n) {
                name = n;
            }
        
            void displayDetails() {
                cout << "Name: " << name << endl;
            }
        };
        
        int main() {
            Person person;  // Object creation
            person.setName("John Doe");  // Accessing private member through public method
            person.displayDetails();  // Output: Name: John Doe
        
            // The following line would cause a compile-time error because name is private:
            // person.name = "Jane Doe";  // Error: 'string' is private within this context
        
            return 0;
        }
                    

Output:

Pro Tip: