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Engineering LibreTexts

1.11: Private Access Specifier

  • Page ID
    34643
  • By default access to members of a C++ class is 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.

    // 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: 
             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

    Adapted from:
    "C++ Classes and Objects" by Abhirav Kariya, Geeks for Geeks is licensed under CC BY 4.0