27 Algorithms library [algorithms]

27.8 Sorting and related operations [alg.sorting]

27.8.2 Sorting [alg.sort]

27.8.2.3 partial_sort [partial.sort]

template<class RandomAccessIterator>
  constexpr void partial_sort(RandomAccessIterator first,
                              RandomAccessIterator middle,
                              RandomAccessIterator last);
template<class ExecutionPolicy, class RandomAccessIterator>
  void partial_sort(ExecutionPolicy&& exec,
                    RandomAccessIterator first,
                    RandomAccessIterator middle,
                    RandomAccessIterator last);

template<class RandomAccessIterator, class Compare>
  constexpr void partial_sort(RandomAccessIterator first,
                              RandomAccessIterator middle,
                              RandomAccessIterator last,
                              Compare comp);
template<class ExecutionPolicy, class RandomAccessIterator, class Compare>
  void partial_sort(ExecutionPolicy&& exec,
                    RandomAccessIterator first,
                    RandomAccessIterator middle,
                    RandomAccessIterator last,
                    Compare comp);

template<random_access_iterator I, sentinel_for<I> S, class Comp = ranges::less,
         class Proj = identity>
  requires sortable<I, Comp, Proj>
  constexpr I
    ranges::partial_sort(I first, I middle, S last, Comp comp = {}, Proj proj = {});

Let comp be less{} and proj be identity{} for the overloads with no parameters by those names.

Preconditions: [first, middle) and [middle, last) are valid ranges.

For the overloads in namespace std, RandomAccessIterator meets the Cpp17ValueSwappable requirements ([swappable.requirements]) and the type of *first meets the Cpp17MoveConstructible (Table 31) and Cpp17MoveAssignable (Table 33) requirements.

Effects: Places the first middle - first elements from the range [first, last) as sorted with respect to comp and proj into the range [first, middle).

The rest of the elements in the range [middle, last) are placed in an unspecified order.

Returns: last for the overload in namespace ranges.

Complexity: Approximately (last - first) * log(middle - first) comparisons, and twice as many projections.

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template<random_access_range R, class Comp = ranges::less, class Proj = identity>
  requires sortable<iterator_t<R>, Comp, Proj>
  constexpr borrowed_iterator_t<R>
    ranges::partial_sort(R&& r, iterator_t<R> middle, Comp comp = {}, Proj proj = {});

Effects: Equivalent to: return ranges::partial_sort(ranges::begin(r), middle, ranges::end(r), comp, proj);