Mutable data members

Mutable data members

When we create a constant object, none of its data members can be changed. In a rare situation, however, you may want some data members should be allowed to change despite the object being const.

1.      Mutable data members can be modified inside constant member functions also.

2.      Prefixing the declaration of a data member with the keyword mutable makes it a mutable data member.

The following example makes the use of mutable keyword clear.

EXAMPLE:

class A

{

int x;

mutable int y;

public:

void abc () const

{

x++;  // Error. Since abc () is a constant function and x is not a mutable

             member.

y++;  // No error. Since y is declared as mutable it is allowed to be

             changed even in constant member functions.

}

void def ()

{

x++;   // No error. Since def () is a non constant member function, x can be 

             changed.

y++;   // No error. Since def () is a non constant member function, x can be 

             changed.

}

};

It is important to understand that a mutable data member can never be a constant.