In c plus plus what is Mutable keyword?

Answer 1

mutable keyword can only be applied to non-static and non-constant member of a class. It indicates that the corresponding data member can be modified even from the constant function. (Constant function: is a function marked with the keyword const)

For eg.

class x {

private:

mutable int query_count;

int value;

public:

int get_value() const

{

query_count++;

return value;

}

};

In the code above member query_count is marked with keyword mutable. Hence it's value can be modified from constant function get_value().

Answer 2

Most class accessors need to cater for both constant and non-constant access. If the class representation doesn't change when an accessor method is invoked then this isn't a problem. However, when an accessor needs to modify the class representation we cannot make that accessor constant and therefore cannot call that method on constant objects of the class. To get around that problem, members that need to be modified by an accessor need to be declared mutable.

Typically, mutable data members are only used internally be the class (for housekeeping purposes) and play no part in the logical representation of the class (as viewed by users of the class).