mutable的用法

这个修饰词只用在类的非静态和非const成员变量，如果变量声明为mutable，则如果成员函数是const的，也可以修改这个值。

（This 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.  ）