std::ranges:: upper_bound

From cppreference.com

Defined in header `<algorithm>`
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)
template < ranges:: forward_range R, class T, class Proj = std:: identity , std:: indirect_strict_weak_order < const T * , std :: projected < ranges:: iterator_t < R > , Proj >> Comp = ranges:: less > constexpr ranges:: borrowed_iterator_t < R > upper_bound ( R && r, const T & value, Comp comp = { } , Proj proj = { } ) ;				(since C++20) (until C++26)
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 > upper_bound ( R && r, const T & value, Comp comp = { } , Proj proj = { } ) ;				(since C++26)

1) Returns an iterator pointing to the first element in the range

first

last

that is greater than value , or last if no such element is found. The range

first

last

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.

2) Same as (1) , but uses r as the source range, as if using ranges:: begin ( r ) as first and ranges:: end ( r ) as last .

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

Explicit template argument lists cannot be specified when calling any of them.
None of them are visible to argument-dependent lookup .
When any of them are found by normal unqualified lookup as the name to the left of the function-call operator, argument-dependent lookup is inhibited.

Parameters

first, last	-	iterator-sentinel defining the partially-ordered range 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

Return value

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

Complexity

The number of comparisons and applications of the projection performed are logarithmic in the distance between first and last (at most $log 2 (last - first) + O(1)$ comparisons and applications of the projection). However, for an iterator that does not model random_access_iterator , the number of iterator increments is linear.

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;

Notes

Feature-test macro	Value	Std	Feature
`__cpp_lib_algorithm_default_value_type`	202403	(C++26)	List-initialization for algorithms ( 1,2 )

Example

Run this code

#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:

4 4 4 
3 3 3 3

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Technical specifications
Symbols index
External libraries

ranges::equal_range (C++20)	returns range of elements matching a specific key (algorithm function object)
ranges::lower_bound (C++20)	returns an iterator to the first element not less than the given value (algorithm function object)
ranges::partition (C++20)	divides a range of elements into two groups (algorithm function object)
upper_bound	returns an iterator to the first element greater than a certain value (function template)