C++ Class Access Modifiers

C++ access modifiers are used for data hiding implementation. Data hiding is one of the important features of object-oriented programming, which allows the functions of a program to access directly the internal representation of a class type.The access restriction to the class members is specified by the labeled public, private, and protected sections within the class body. The keywords public, private, and protected are called access specifiers.

A class can have multiple public, protected, or private labeled sections. Each section remains in effect until either another section label or the closing right brace of the class body is seen. The default access for members and classes is private.

 class Base { public: // public members go here protected: // protected members go here private: // private members go here }; 

Public Access Modifier

The public access modifier defines public data members and member functions that are accessible from anywhere outside the class but within a program. You can set and get the value of public variables without any member function.

Example

The following example demonstrates the use of public access modifier −

 #include <iostream> using namespace std; class Line { public: double length; void setLength( double len ); double getLength( void ); }; // Member functions definitions double Line::getLength(void) { return length ; } void Line::setLength( double len) { length = len; } // Main function for the program int main() { Line line; // set line length line.setLength(6.0); cout << "Length of line : " << line.getLength() <<endl; // set line length without member function line.length = 10.0; // OK: because length is public cout << "Length of line : " << line.length <<endl; return 0; } 

When the above code is compiled and executed, it produces the following result −

 Length of line : 6 Length of line : 10 

Private Access Modifier

The private access modifier defines private data members and member functions that cannot be accessed, or even viewed from outside the class. Only the class and friend functions can access private members.

By default all the members of a class would be private, for example in the following class width is a private member, which means until you label a member, it will be assumed a private member.

Example

The following example demonstrates the use of private access modifier −

 class Box { double width; public: double length; void setWidth( double wid ); double getWidth( void ); }; 

Practically, we define data in private section and related functions in public section so that they can be called from outside of the class as shown in the following program.

 #include <iostream> using namespace std; class Box { public: double length; void setWidth( double wid ); double getWidth( void ); private: double width; }; // Member functions definitions double Box::getWidth(void) { return width ; } void Box::setWidth( double wid ) { width = wid; } // Main function for the program int main() { Box box; // set box length without member function box.length = 10.0; // OK: because length is public cout << "Length of box : " << box.length <<endl; // set box width without member function // box.width = 10.0; // Error: because width is private box.setWidth(10.0); // Use member function to set it. cout << "Width of box : " << box.getWidth() <<endl; return 0; } 

When the above code is compiled and executed, it produces the following result −

 Length of box : 10 Width of box : 10 

Protected Access Modifier

The protected access modifier defines protected data members and member functions that are very similar to a private member, but it provides one additional benefit that they can be accessed in child classes, which are called derived classes.

You will learn derived classes and inheritance in next chapter. For now you can check following example where I have derived one child class SmallBox from a parent class Box.

Example

Following example is similar to above example and here width member will be accessible by any member function of its derived class SmallBox.

 #include <iostream> using namespace std; class Box { protected: double width; }; class SmallBox:Box { // SmallBox is the derived class. public: void setSmallWidth( double wid ); double getSmallWidth( void ); }; // Member functions of child class double SmallBox::getSmallWidth(void) { return width ; } void SmallBox::setSmallWidth( double wid ) { width = wid; } // Main function for the program int main() { SmallBox box; // set box width using member function box.setSmallWidth(5.0); cout << "Width of box : "<< box.getSmallWidth() << endl; return 0; } 

When the above code is compiled and executed, it produces the following result −

 Width of box : 5