std::partition_point

Header: <algorithm>

Examines the partitioned 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.

# Declarations

template< class ForwardIt, class UnaryPred >
ForwardIt partition_point( ForwardIt first, ForwardIt last, UnaryPred p );

(since C++11) (constexpr since C++20)

# Parameters

first, last: the partitioned range of elements to examine
p: unary predicate which returns true for the elements found in the beginning of the range. 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).

# Return value

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

# 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 [&](const auto& e) { return e < value; });.

# Example

#include <algorithm>
#include <array>
#include <iostream>
#include <iterator>
 
auto print_seq = [](auto rem, auto first, auto last)
{
    for (std::cout << rem; first != last; std::cout << *first++ << ' ') {}
    std::cout << '\n';
};
 
int main()
{
    std::array 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);
    print_seq("After partitioning, v: ", v.cbegin(), v.cend());
 
    const auto pp = std::partition_point(v.cbegin(), v.cend(), is_even);
    const auto i = std::distance(v.cbegin(), pp);
    std::cout << "Partition point is at " << i << "; v[" << i << "] = " << *pp << '\n';
 
    print_seq("First partition (all even elements): ", v.cbegin(), pp);
    print_seq("Second partition (all odd elements): ", pp, v.cend());
}