Categories: iterators, adaptors Component type: type


Front_insert_iterator is an iterator adaptor that functions as an Output Iterator: assignment through a front_insert_iterator inserts an object before the first element of a Front Insertion Sequence. [1] [2]


list<int> L;
front_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 2 1 0 3


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

Template parameters

Parameter Description Default
FrontInsertionSequence The type of Front Insertion Sequence into which values will be inserted.  

Model of

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

Type requirements

The template parameter FrontInsertionSequence must be a Front Insertion Sequence.

Public base classes



Member Where defined Description
front_insert_iterator(FrontInsertionSequence&) front_insert_iterator See below.
front_insert_iterator(const front_insert_iterator&) Trivial Iterator The copy constructor
operator=(const front_insert_iterator&)
Trivial Iterator The assignment operator
front_insert_iterator& operator*() Output Iterator Used to implement the output iterator expression *i = x. [3]
operator=(const FrontInsertionSequence::value_type&)
Output Iterator Used to implement the output iterator expression *i = x. [3]
front_insert_iterator& operator++() Output Iterator Preincrement.
front_insert_iterator& operator++(int) Output Iterator Postincrement.
iterator_category(const front_insert_iterator&)
iterator tags Returns the iterator's category. This is a global function, not a member.
template<class FrontInsertionSequence>
front_inserter(FrontInsertionSequence& S)
front_insert_iterator See below.

New members.

These members are not defined in the Output Iterator requirements, but are specific to front_insert_iterator.
Member Description
front_insert_iterator(FrontInsertionSequence& S) Constructs a front_insert_iterator that inserts objects before the first element of S.
template<class FrontInsertionSequence>
front_inserter(FrontInsertionSequence& S);
Equivalent to front_insert_iterator<FrontInsertionSequence>(S). [4] This is a global function, not a member function.


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

[2] Note the difference between a front_insert_iterator and an insert_iterator. It may seem that a front_insert_iterator is the same as an insert_iterator constructed with an insertion point that is the beginning of a sequence. In fact, though, there is a very important difference: every assignment through a front_insert_iterator corresponds to an insertion before the first element of the sequence. If you are inserting elements at the beginning of a sequence using an insert_iterator, then the elements will appear in the order in which they were inserted. If, however, you are inserting elements at the beginning of a sequence using a front_insert_iterator, then the elements will appear in the reverse of the order in which they were inserted.

[3] Note how assignment through an front_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 front_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.

[4] 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 front_insert_iterator need not be declared explicitly. One easy way to reverse a range and insert it at the beginning of a Front Insertion Sequence S, for example, is copy(first, last, front_inserter(S)).

See also

insert_iterator, back_insert_iterator, Output Iterator, Sequence, Front Insertion Sequence, Iterator overview
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