使用.begin（）和.end（）对于std :: set的std :: inserter有区别吗？

Question

If there is any difference between it1 and it2?

std::set<sometype> s;

auto it1 = std::inserter(s, s.begin());
auto it2 = std::inserter(s, s.end());

Answer 1

In practice, not much. If you're inserting a large number of already in order elements into an empty set , the second will be somewhat faster, but that's about it. std::insert_iterator calls insert with the iterator; std::set interprets it as a hint, and inserts in constant time (rather than lg n) if the insertion is immediately before the hint. (Actually, if the set is empty, I think both will do exactly the same thing.)

Answer 2

From http://www.sgi.com/tech/stl/insert_iterator.html

In the case of a Sorted Associative Container, however, the iterator in the insert_iterator's constructor is almost irrelevant. The new elements will not necessarily form a contiguous range; they will appear in the appropriate location in the container, in ascending order by key. The order in which they are inserted only affects efficiency: inserting an already-sorted range into a Sorted Associative Container is an O(N) operation.