...one of the most highly
regarded and expertly designed C++ library projects in the
world.
— Herb Sutter and Andrei
Alexandrescu, C++
Coding Standards
Similar to standard insert iterators, it's possible to deal with move insertion in the same way as writing into an array. A special kind of iterator adaptors, called move insert iterators, are provided with this library. With regular iterator classes,
while (first != last) *result++ = *first++;
causes a range [first,last) to be copied into a range starting with result. The same code with result being an move insert iterator will move insert corresponding elements into the container. This device allows all of the copying algorithms in the library to work in the move insert mode instead of the regular overwrite mode. This library offers 3 move insert iterators and their helper functions:
// Note: C models Container template <typename C> class back_move_insert_iterator; template <typename C> back_move_insert_iterator<C> back_move_inserter(C& x); template <typename C> class front_move_insert_iterator; template <typename C> front_move_insert_iterator<C> front_move_inserter(C& x); template <typename C> class move_insert_iterator; template <typename C> move_insert_iterator<C> move_inserter(C& x, typename C::iterator it);
A move insert iterator is constructed from a container and possibly one of
its iterators pointing to where insertion takes place if it is neither at the
beginning nor at the end of the container. Insert iterators satisfy the requirements
of output iterators. operator*
returns the move insert iterator itself. The
assignment operator=(T& x)
is defined on insert iterators to allow writing
into them, it inserts x right before where the insert iterator is pointing.
In other words, an insert iterator
is like a cursor pointing into the
container where the insertion takes place. back_move_iterator
move inserts elements at the end of a container, front_insert_iterator
move inserts elements at the beginning of a container, and move_insert_iterator
move inserts elements where the iterator points to in a container. back_move_inserter
, front_move_inserter
,
and move_inserter
are three
functions making the insert iterators out of a container. Here's an example
of how to use them:
#include <boost/container/list.hpp> #include "movable.hpp" #include <cassert> #include <algorithm> using namespace ::boost::container; typedef list<movable> list_t; typedef list_t::iterator l_iterator; template<class MoveInsertIterator> void test_move_inserter(list_t &l2, MoveInsertIterator mit) { //Create a list with 10 default constructed objects list<movable> l(10); assert(!l.begin()->moved()); l2.clear(); //Move insert into l2 containers std::copy(l.begin(), l.end(), mit); //Check size and status assert(l2.size() == l.size()); assert(l.begin()->moved()); assert(!l2.begin()->moved()); } int main() { list_t l2; test_move_inserter(l2, boost::back_move_inserter(l2)); test_move_inserter(l2, boost::front_move_inserter(l2)); test_move_inserter(l2, boost::move_inserter(l2, l2.end())); return 0; }