10.4.1. Insert Iterators

An inserter is an iterator adaptor (§ 9.6, p. 368) that takes a container and yields an iterator that adds elements to the specified container. When we assign a value through an insert iterator, the iterator calls a container operation to add an element at a specified position in the given container. The operations these iterators support are listed in Table 10.2 (overleaf).

Table 10.2. Insert Iterator Operations

There are three kinds of inserters. Each differs from the others as to where elements are inserted:

• back_inserter (§ 10.2.2, p. 382) creates an iterator that uses push_back.

• front_inserter creates an iterator that uses push_front.

• inserter creates an iterator that uses insert. This function takes a second argument, which must be an iterator into the given container. Elements are inserted ahead of the element denoted by the given iterator.

It is important to understand that when we call inserter(c, iter), we get an iterator that, when used successively, inserts elements ahead of the element originally denoted by iter. That is, if it is an iterator generated by inserter, then an assignment such as

* it = va1;

behaves as

it = c.insert(it, val); // it points to the newly added element
++it; // increment it so that it denotes the same element as before

The iterator generated by front_inserter behaves quite differently from the one created by inserter. When we use front_inserter, elements are always inserted ahead of the then first element in the container. Even if the position we pass to inserter initially denotes the first element, as soon as we insert an element in front of that element, that element is no longer the one at the beginning of the container:

list<int> 1st = {1,2,3,4};
list<int> lst2, lst3;     // empty lists
// after copy  completes, 1st2 contains 4 3 2 1
copy(1st.cbegin(), lst.cend(), front_inserter(lst2));
// after copy completes, 1st3 contains 1 2 3 4
copy(1st.cbegin(), lst.cend(), inserter(lst3, lst3.begin()));

When we call front_inserter(c), we get an insert iterator that successively calls push_front. As each element is inserted, it becomes the new first element in c. Therefore, front_inserter yields an iterator that reverses the order of the sequence that it inserts; inserter and back_inserter don’t.