起因是我写了这么一段代码：
template <typename type>
class LIFO_stack : public my_stack<type> {
template <typename ano_type>
friend ostream& operator<<(ostream& os, LIFO_stack<ano_type>& s);
}
template <typename type>
ostream& operator<<(ostream& os, LIFO_stack<type>& s) {
auto it = --s.vec.end();
for (; it != s.vec.begin(); --it) {
os << *it << ' ';
}
os << *it << endl;
return os;
}