Difference between Public and Private in C++ with Example

Public

All the class members declared under public will be available to everyone. The data members and member functions declared public can be accessed by other classes too. The public members of a class can be accessed from anywhere in the program using the direct member access operator (.) with the object of that class.

Example:

// C++ program to demonstrate public
// access modifier

#include <iostream>
using namespace std;

// class definition
class Circle {
public:
    double radius;

    double compute_area()
    {
        return 3.14 * radius * radius;
    }
};

// main function
int main()
{
    Circle obj;

    // accessing public data member outside class
    obj.radius = 5.5;

    cout << "Radius is: " << obj.radius << "\n";
    cout << "Area is: " << obj.compute_area();
    return 0;
}

Output:

Radius is: 5.5
Area is: 94.985

In the above program, the data member radius is public so we are allowed to access it outside the class.

Private

The class members declared as private can be accessed only by the functions inside the class. They are not allowed to be accessed directly by any object or function outside the class. Only the member functions or the friend functions are allowed to access the private data members of a class.
Example:

// C++ program to demonstrate private
// access modifier

#include <iostream>
using namespace std;

class Circle {
    // private data member
private:
    double radius;

    // public member function
public:
    void compute_area(double r)
    {
        // member function can access private
        // data member radius
        radius = r;

        double area = 3.14 * radius * radius;

        cout << "Radius is: " << radius << endl;
        cout << "Area is: " << area;
    }
};

// main function
int main()
{
    // creating object of the class
    Circle obj;

    // trying to access private data member
    // directly outside the class
    obj.compute_area(1.5);

    return 0;
}

Output:

Radius is: 1.5
Area is: 7.065

Difference between Public and Private

Public	Private
All the class members declared under public will be available to everyone.	The class members declared as private can be accessed only by the functions inside the class.
The data members and member functions declared public can be accessed by other classes too.	Only the member functions or the friend functions are allowed to access the private data members of a class.
The public members of a class can be accessed from anywhere in the program using the direct member access operator (.) with the object of that class.	They are not allowed to be accessed directly by any object or function outside the class.