gather

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### gather

The header file 'boost/algorithm/gather.hpp' contains two variants of a single algorithm, `gather`.

`gather()` takes a collection of elements defined by a pair of iterators and moves the ones satisfying a predicate to them to a position (called the pivot) within the sequence. The algorithm is stable. The result is a pair of iterators that contains the items that satisfy the predicate.

##### Interface

The function `gather` returns a `std::pair` of iterators that denote the elements that satisfy the predicate.

There are two versions; one takes two iterators, and the other takes a range.

    namespace boost { namespace algorithm {

    template <typename BidirectionalIterator, typename Pred>
    std::pair<BidirectionalIterator,BidirectionalIterator>
    gather ( BidirectionalIterator first, BidirectionalIterator last, BidirectionalIterator pivot, Pred pred );

    template <typename BidirectionalRange, typename Pred>
    std::pair<typename boost::range_iterator<const BidirectionalRange>::type, typename boost::range_iterator<const BidirectionalRange>::type>
    gather ( const BidirectionalRange &range, typename boost::range_iterator<const BidirectionalRange>::type pivot, Pred pred );

    }}

##### Examples

Given an sequence containing:

    0 1 2 3 4 5 6 7 8 9

a call to gather ( arr, arr + 10, arr + 4, IsEven ) will result in:

    1 3 0 2 4 6 8 5 7 9
        |---|-----|
      first |  second
          pivot

where `first` and `second` are the fields of the pair that is returned by the call.

##### Iterator Requirements

`gather` work on bidirectional iterators or better. This requirement comes from the usage of `stable_partition`, which requires bidirectional iterators. Some standard libraries (libstdc++ and libc++, for example) have implementations of `stable_partition` that work with forward iterators. If that is the case, then `gather` will work with forward iterators as well.

##### Storage Requirements

`gather` uses `stable_partition`, which will attempt to allocate temporary memory, but will work in-situ if there is none available.

##### Complexity

If there is sufficient memory available, the run time is linear: `O(N)`

If there is not any memory available, then the run time is `O(N log N)`.

##### Exception Safety

##### Notes