Generally speaking a const member function can't call a member function that is non-const or a member function that modifies a member variable that's not declar

Generally speaking a const member function can't call a member function that is non-const or a member function that modifies a member variable that's not declared mutable. If the present const member function has a const reference to something that's marked mutable, then it's effectively a no-op as far as const value qualifications are concerned.