Have you heard of "mutable" keyword?

Mutable data members can be modified by const member functions.

The mutable keyword can only be applied to non-static and non-const data members of a class. If a data member is declared mutable, then it is legal to assign a value to this data member from a const member function.

SEE FOLLOWING CODE :-

********************************************

class Mutable
{
private :
              int m_iNonMutVar;
 mutable   int m_iMutVar;

public:
  Mutable();
  void TryChange() const;

};

Mutable::Mutable():m_iNonMutVar(10),m_iMutVar(20)  {};

void Mutable::TryChange() const
{
     m_iNonMutVar  = 100; // THis will give ERROR
     m_iMutVar       = 200; // This will WORK coz it is mutable
}
*****************************************

Any query pls write at mohit.gonduley@gmail.com

Answer are correct but not perfect.

The Crux is:
Purpose of 'mutable' is not changing some member's values of a const object at all. (Its a side bug which comes free with functionality.)
You may use mutable for:
1) Reference counter
2) Appling mutexes
3) Storing some heavy evaluation objects.

Please refer:
highprogrammer . com/alan/rants/mutable . html

Rdgs..
Rahul

The mutable storage class specifier is used only on a class data member to make it modifiable even though the member is part of an object declared as const.
You cannot use the mutable specifier with names declared as static or const, or reference members.