当不想从外部调用成员函数修改成员变量时，可以将其被调成员函数加上const每个成员函数都有个隐式 this 指针，指向调用对象

-When do not want to call from outside a member function to modify a member variable, it can be coupled with const member functions be transferred functions of each member has one implicit this pointer, point to call the object