std::partition_point (3) - Linux Manuals

std::partition_point: std::partition_point

NAME

std::partition_point - std::partition_point

Synopsis

Defined in header <algorithm>
template< class ForwardIt, class UnaryPredicate > (since C++11)
ForwardIt partition_point( ForwardIt first, ForwardIt last, UnaryPredicate p ); (1) (until C++20)
template< class ForwardIt, class UnaryPredicate > (since C++20)
constexpr ForwardIt partition_point( ForwardIt first, ForwardIt last, UnaryPredicate p );

Examines the partitioned (as if by std::partition) range [first, last) and locates the end of the first partition, that is, the first element that does not satisfy p or last if all elements satisfy p.

Parameters

first, last - the partitioned range of elements to examine
              unary predicate which returns true for the elements found in the beginning of the range.
p - The expression p(v) must be convertible to bool for every argument v of type (possibly const) VT, where VT is the value type of ForwardIt, regardless of value_category, and must not modify v. Thus, a parameter type of VT&is not allowed
              , nor is VT unless for VT a move is equivalent to a copy
              (since C++11).

Type requirements

-
ForwardIt must meet the requirements of LegacyForwardIterator.
-
UnaryPredicate must meet the requirements of Predicate.

Return value

The iterator past the end of the first partition within [first, last) or last if all elements satisfy p.

Complexity

Given N = std::distance(first, last), performs O(log N) applications of the predicate p.
However, for non-LegacyRandomAccessIterators, the number of iterator increments is O(N).

Notes

This algorithm is a more general form of std::lower_bound, which can be expressed in terms of std::partition_point with the predicate [&](auto const& e) { return e < value; });.

Example

// Run this code

  #include <algorithm>
  #include <array>
  #include <iostream>
  #include <iterator>

  int main()
  {
      std::array<int, 9> v = { 1, 2, 3, 4, 5, 6, 7, 8, 9 };

      auto is_even = [](int i){ return i % 2 == 0; };
      std::partition(v.begin(), v.end(), is_even);

      auto p = std::partition_point(v.begin(), v.end(), is_even);

      std::cout << "Before partition:\n ";
      std::copy(v.begin(), p, std::ostream_iterator<int>(std::cout, " "));
      std::cout << "\nAfter partition:\n ";
      std::copy(p, v.end(), std::ostream_iterator<int>(std::cout, " "));
  }

Output:

  Before partition:
      8 2 6 4
  After partition:
      5 3 7 1 9