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Showing content from https://en.cppreference.com/w/cpp/language/../../cpp/iterator/../algorithm/ranges/upper_bound.html below:

std::ranges::upper_bound - cppreference.com

Call signature

(1) template< std::forward_iterator I, std::sentinel_for<I> S,

          class T, class Proj = std::identity,
          std::indirect_strict_weak_order
              <const T*, std::projected<I, Proj>> Comp = ranges::less >
constexpr I upper_bound( I first, S last, const T& value,

                         Comp comp = {}, Proj proj = {} );
(since C++20)
(until C++26) template< std::forward_iterator I, std::sentinel_for<I> S,

          class Proj = std::identity,
          class T = std::projected_value_t<I, Proj>,
          std::indirect_strict_weak_order
              <const T*, std::projected<I, Proj>> Comp = ranges::less >
constexpr I upper_bound( I first, S last, const T& value,

                         Comp comp = {}, Proj proj = {} );
(since C++26) (2) (since C++20)
(until C++26) (since C++26)

1) Returns an iterator pointing to the first element in the range [firstlast) that is greater than value, or last if no such element is found. The range [firstlast) must be partitioned with respect to the expression or !comp(value, element), i.e., all elements for which the expression is true must precede all elements for which the expression is false. A fully-sorted range meets this criterion.

The function-like entities described on this page are algorithm function objects (informally known as niebloids), that is:

[edit] Parameters first, last - the iterator-sentinel pair defining the partially-ordered range of elements to examine r - the partially-ordered range to examine value - value to compare the elements to pred - predicate to apply to the projected elements proj - projection to apply to the elements [edit] Return value

Iterator pointing to the first element that is greater than value, or last if no such element is found.

[edit] Complexity

The number of comparisons and applications of the projection performed are logarithmic in the distance between first and last (at most log2(last - first) + O(1) comparisons and applications of the projection). However, for an iterator that does not model random_access_iterator, the number of iterator increments is linear.

[edit] Possible implementation
struct upper_bound_fn
{
    template<std::forward_iterator I, std::sentinel_for<I> S,
             class Proj = std::identity, class T = std::projected_value_t<I, Proj>,
             std::indirect_strict_weak_order
                 <const T*, std::projected<I, Proj>> Comp = ranges::less>
    constexpr I operator()(I first, S last, const T& value,
                           Comp comp = {}, Proj proj = {}) const
    {
        I it;
        std::iter_difference_t<I> count, step;
        count = ranges::distance(first, last);
 
        while (count > 0)
        {
            it = first; 
            step = count / 2;
            ranges::advance(it, step, last);
            if (!comp(value, std::invoke(proj, *it)))
            {
                first = ++it;
                count -= step + 1;
            }
            else
                count = step;
        }
        return first;
    }
 
    template<ranges::forward_range R, class Proj = std::identity,
             class T = std::projected_value_t<ranges::iterator_t<R>, Proj>,
             std::indirect_strict_weak_order
                 <const T*, std::projected<ranges::iterator_t<R>,
                                           Proj>> Comp = ranges::less>
    constexpr ranges::borrowed_iterator_t<R>
        operator()(R&& r, const T& value, Comp comp = {}, Proj proj = {}) const
    {
        return (*this)(ranges::begin(r), ranges::end(r), value,
                       std::ref(comp), std::ref(proj));
    }
};
 
inline constexpr upper_bound_fn upper_bound;
[edit] Notes [edit] Example
#include <algorithm>
#include <cassert>
#include <complex>
#include <iostream>
#include <iterator>
#include <vector>
 
int main()
{
    namespace ranges = std::ranges;
 
    std::vector<int> data{1, 1, 2, 3, 3, 3, 3, 4, 4, 4, 5, 5, 6};
 
    {
        auto lower = ranges::lower_bound(data.begin(), data.end(), 4);
        auto upper = ranges::upper_bound(data.begin(), data.end(), 4);
 
        ranges::copy(lower, upper, std::ostream_iterator<int>(std::cout, " "));
        std::cout << '\n';
    }
    {
        auto lower = ranges::lower_bound(data, 3);
        auto upper = ranges::upper_bound(data, 3);
 
        ranges::copy(lower, upper, std::ostream_iterator<int>(std::cout, " "));
        std::cout << '\n';
    }
 
    using CD = std::complex<double>;
    std::vector<CD> nums{{1, 0}, {2, 2}, {2, 1}, {3, 0}, {3, 1}};
    auto cmpz = [](CD x, CD y) { return x.real() < y.real(); };
    #ifdef __cpp_lib_algorithm_default_value_type
        auto it = ranges::upper_bound(nums, {2, 0}, cmpz);
    #else
        auto it = ranges::upper_bound(nums, CD{2, 0}, cmpz);
    #endif
    assert((*it == CD{3, 0}));
}

Output:

[edit] See also returns range of elements matching a specific key
(algorithm function object)[edit] returns an iterator to the first element not less than the given value
(algorithm function object)[edit] divides a range of elements into two groups
(algorithm function object)[edit] returns an iterator to the first element greater than a certain value
(function template) [edit]

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