How do I change a data member of the const object?

To change the data members of the const object the data members are declared as mutable in the class. 

This is shown in the following example:

#include <iostream.h>

class sample

{

    private :

          mutable int i;

    public :

          sample( int ii = 0 )

          { 

                i = ii; 

          }

          void fun( ) const

          {

                i++;

                cout<< i; 

          } 

};

void main( )

{

     const sample s ( 15 ) ;

     s.fun( ); 

}

Here, the object s is const and hence only const functions can operate upon it. When the const function fun( ) gets called to operate upon object s, the data member i is incremented. Ideally the data member should not be changed, as object is defined const. But we can change the data member i because it is declared as mutable in the class sample