To make it possible to deal with insertion in the same way as writing into an array, a special kind of iterator
adaptors, called
insert iterators,
are provided in the 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 insert iterator will insert corresponding elements into the container
.An 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 meet the requirements of output iterators
.operator*
returns the insert iterator itself
. The assignment
operator=(const 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_insert_iterator
inserts elements at the end of a container,
front_insert_iterator
inserts elements at the beginning of a container, and
insert_iterator
inserts elements where the iterator points to in a container
. back_inserter,
front_inserter,
and
inserter
are three
functions making the insert iterators out of a container
.
namespace std {
template<class Container>
class insert_iterator {
protected:
Container* container = nullptr;
ranges::iterator_t<Container> iter = ranges::iterator_t<Container>();
public:
using iterator_category = output_iterator_tag;
using value_type = void;
using difference_type = ptrdiff_t;
using pointer = void;
using reference = void;
using container_type = Container;
insert_iterator() = default;
constexpr insert_iterator(Container& x, ranges::iterator_t<Container> i);
constexpr insert_iterator& operator=(const typename Container::value_type& value);
constexpr insert_iterator& operator=(typename Container::value_type&& value);
constexpr insert_iterator& operator*();
constexpr insert_iterator& operator++();
constexpr insert_iterator& operator++(int);
};
}
constexpr insert_iterator(Container& x, ranges::iterator_t<Container> i);
Effects: Initializes
container
with
addressof(x) and
iter
with
i. constexpr insert_iterator& operator=(const typename Container::value_type& value);
Effects: As if by:
iter = container->insert(iter, value);
++iter;
constexpr insert_iterator& operator=(typename Container::value_type&& value);
Effects: As if by:
iter = container->insert(iter, std::move(value));
++iter;
constexpr insert_iterator& operator*();
constexpr insert_iterator& operator++();
constexpr insert_iterator& operator++(int);
template<class Container>
constexpr insert_iterator<Container>
inserter(Container& x, ranges::iterator_t<Container> i);
Returns: insert_iterator<Container>(x, i).