# back_insert_iterator<BackInsertionSequence>

Categories: iterators, adaptors | Component type: type

### Description

`Back_insert_iterator` is an iterator adaptor that functions as an Output Iterator: assignment through a `back_insert_iterator` inserts an object after the last element of a Back Insertion Sequence. [1]

### Example

    list<int> L;
    L.push_front(3);
    back_insert_iterator<list<int> > ii(L);
    *ii++ = 0;
    *ii++ = 1;
    *ii++ = 2;
    copy(L.begin(), L.end(), ostream_iterator<int>(cout, " "));
    // The values that are printed are 3 0 1 2

### Definition

Defined in the standard header iterator, and in the nonstandard backward-compatibility header iterator.h.

### Template parameters

Parameter | Description | Default
--- | --- | ---
`BackInsertionSequence` | The type of Back Insertion Sequence into which values will be inserted. |

### Model of

Output Iterator. An insert iterator's set of value types (as defined in the Output Iterator requirements) consists of a single type: `BackInsertionSequence::value_type`.

### Type requirements

The template parameter `BackInsertionSequence` must be a Back Insertion Sequence.

### Public base classes

None.

### Members

Member | Where defined | Description
--- | --- | ---
`back_insert_iterator(BackInsertionSequence&)` | `back_insert_iterator` | See below.
`back_insert_iterator(const back_insert_iterator&)` | Trivial Iterator | The copy constructor
    back_insert_iterator&
    operator=(const back_insert_iterator&)
| Trivial Iterator | The assignment operator
`back_insert_iterator& operator*()` | Output Iterator | Used to implement the output iterator expression `*i = x`. [2]
    back_insert_iterator&
    operator=(const BackInsertionSequence::value_type&)
| Output Iterator | Used to implement the output iterator expression `*i = x`. [2]
`back_insert_iterator& operator++()` | Output Iterator | Preincrement.
`back_insert_iterator& operator++(int)` | Output Iterator | Postincrement.
    output_iterator_tag
    iterator_category(const back_insert_iterator&)
| iterator tags | Returns the iterator's category. This is a global function, not a member.
    template<class BackInsertionSequence>
    back_insert_iterator<BackInsertionSequence>
    back_inserter(BackInsertionSequence& S)
| `back_insert_iterator` | See below.

### New members

These members are not defined in the Output Iterator requirements, but are specific to `back_insert_iterator`.

Member function | Description
--- | ---
`back_insert_iterator(BackInsertionSequence& S)` | Constructs a `back_insert_iterator` that inserts objects after the last element of `S`. (That is, it inserts objects just before `S`'s past-the-end iterator.)
    template<class BackInsertionSequence>
    back_insert_iterator<BackInsertionSequence>
    back_inserter(BackInsertionSequence& S);
| Equivalent to `back_insert_iterator<BackInsertionSequence>(S)`. [3] This is a global function, not a member function.

### Notes

[1] Note the difference between assignment through a `BackInsertionSequence::iterator` and assignment through a `back_insert_iterator<BackInsertionSequence>`. If `i` is a valid `BackInsertionSequence::iterator`, then it points to some particular element in the back insertion sequence; the expression `*i = t` replaces that element with `t`, and does not change the total number of elements in the back insertion sequence. If `ii` is a valid `back_insert_iterator<BackInsertionSequence>`, however, then the expression `*ii = t` is equivalent, to the expression `seq.push_back(t)`. That is, it does not overwrite any of `seq`'s elements and it does change `seq`'s size.

[2] Note how assignment through a `back_insert_iterator` is implemented. In general, unary `operator*` must be defined so that it returns a proxy object, where the proxy object defines `operator=` to perform the insert operation. In this case, for the sake of simplicity, the proxy object is the `back_insert_iterator` itself. That is, `*i` simply returns `i`, and `*i = t` is equivalent to `i = t`. You should not, however, rely on this behavior. It is an implementation detail, and it is not guaranteed to remain the same in future versions.

[3] This function exists solely for the sake of convenience: since it is a non-member function, the template parameters may be inferred and the type of the `back_insert_iterator` need not be declared explicitly. One easy way to reverse a range and insert it at the end of a Back Insertion Sequence `S`, for example, is `reverse_copy(first, last, back_inserter(S))`.

### See also

insert_iterator, front_insert_iterator, Output Iterator, Back Insertion Sequence, Sequence, Iterator overview

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