Access Modifiers in C++

2

Access modifiers are used to implement important feature of Object Oriented Programming known as Data Hiding. Consider a real life example: What happens when a driver applies brakes? The car stops. The driver only knows that to stop the car, he needs to apply the brakes. He is unaware of how actually the car stops. That is how the engine stops working or the internal implementation on the engine side. This is what data hiding is.
Access modifiers or Access Specifiers in a class are used to set the accessibility of the class members. That is, it sets some restrictions on the class members not to get directly accessed by the outside functions.

There are 3 types of access modifiers available in C++:

  1. Public
  2. Private
  3. Protected

Note: If we do not specify any access modifiers for the members inside the class then by default the access modifier for the members will be Private.

Let us now look at each one these access modifiers in details:

  • 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.

    // 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 datamember 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:    
            double  compute_area()
            {   // member function can access private 
                // data member radius
                return 3.14*radius*radius;
            }
        
    };
    
    // main function
    int main()
    {   
        // creating object of the class
        Circle obj;
        
        // trying to access private data member
        // directly outside the class
        obj.radius = 1.5;
        
        cout << "Area is:" << obj.compute_area();
        return 0;
    }
    

    The output of above program will be a compile time error because we are not allowed to access the private data members of a class directly outside the class.
    Output:

     In function 'int main()':
    11:16: error: 'double Circle::radius' is private
             double radius;
                    ^
    31:9: error: within this context
         obj.radius = 1.5;
             ^
    

    However we can access the private data members of a class indirectly using the public member functions of the class. Below program explains how to do this:

    // C++ program to demonstrate private
    // access modifier
    
    #include<iostream>
    using namespace std;
    
    class Circle
    {   
        // private data member
        private: 
            double radius;
         
        // public member function    
        public:    
            double  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
    
  • Protected: Protected access modifier is similar to that of private access modifiers, the difference is that the class member declared as Protected are inaccessible outside the class but they can be accessed by any subclass(derived class) of that class.

    // C++ program to demonstrate
    // protected access modifier
    #include <bits/stdc++.h>
    using namespace std;
    
    // base class
    class Parent
    {   
        // protected data members
        protected:
        int id_protected;
       
    };
    
    // sub class or derived class
    class Child : public Parent
    {
       
        
        public:
        void setId(int id)
        {
            
            // Child class is able to access the inherited 
            // protected data members of base class
            
            id_protected = id;
            
        }
        
        void displayId()
        {
            cout << "id_protected is:" << id_protected << endl;
        }
    };
    
    // main function
    int main() {
        
        Child obj1;
        
        // member function of derived class can
        // access the protected data members of base class
        
        obj1.setId(81);
        obj1.displayId();
        return 0;
    }
    

    Output:

    id_protected is:81
    

This article is contributed by Abhirav Kariya and Harsh Agarwal . If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.

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