没有O(1)操作来连接两个forward_lists中的元素？

#include<forward_list>

void connect_forward_lists(std::forward_list<int>& fl1, std::forward_list<int>& fl2) {
    if (fl1.empty()) {
        fl1.swap(fl2);
    } else {
        auto it1 = fl1.before_begin();
        auto it2 = fl2.before_begin();
        while (it1 != fl1.end()) {
            it1 = fl1.insert_after(it1, fl2.front());
            fl2.pop_front();
        }
    }
}