std::inserter

We used the it and end pair as input iterators in the std::copy call. The third parameter must be an output iterator. For that, we cannot just take s.begin() or s.end(). In an empty set, both are the same, so we are not even allowed to dereference it, regardless if that is for reading from it or assigning to it.

This is where std::inserter comes into play. It is a function which returns an std::insert_iterator that behaves like an iterator but does something else than what usual iterators do. When we increment it, it does nothing. When we dereference it and assign something to it, it will take the container it is attached to, and insert that value as a new item into it!

When instantiating an std::insert_iterator via std::inserter, two parameters are needed:

auto insert_it = inserter(s, s.end());

The s is our set, and s.end() is an iterator that points to where the new item shall be inserted. For an empty set which we start with, this makes as much sense as s.begin(). When used for other data structures as vectors or lists, that second parameter is crucial for defining where the insert iterator shall insert new items.