std::upper_bound

Since C++26

Header: <algorithm>

Searches for the first element in the partitioned range [first,last) which is ordered after value.

# Declarations

template< class ForwardIt, class T >
ForwardIt upper_bound( ForwardIt first, ForwardIt last,
const T& value );

(constexpr since C++20) (until C++26)

template< class ForwardIt, class T = typename std::iterator_traits
<ForwardIt>::value_type >
constexpr ForwardIt upper_bound( ForwardIt first, ForwardIt last,
const T& value );

(since C++26)

template< class ForwardIt, class T, class Compare >
ForwardIt upper_bound( ForwardIt first, ForwardIt last,
const T& value, Compare comp );

(constexpr since C++20) (until C++26)

template< class ForwardIt, class T = typename std::iterator_traits
<ForwardIt>::value_type,
class Compare >
constexpr ForwardIt upper_bound( ForwardIt first, ForwardIt last,
const T& value, Compare comp );

(since C++26)

# Parameters

first, last: the partitioned range of elements to examine
value: value to compare the elements to
comp: binary predicate which returns true if the first argument is ordered before the second. The signature of the predicate function should be equivalent to the following: bool pred(const Type1 &a, const Type2 &b); While the signature does not need to have const &, the function must not modify the objects passed to it and must be able to accept all values of type (possibly const) Type1 and Type2 regardless of value category (thus, Type1 & is not allowed, nor is Type1 unless for Type1 a move is equivalent to a copy(since C++11)). The type Type1 must be such that an object of type T can be implicitly converted to Type1. The type Type2 must be such that an object of type ForwardIt can be dereferenced and then implicitly converted to Type2.

# Return value

Iterator to the first element of the range [first,last) ordered after value, or last if no such element is found.

# Notes

Although std::upper_bound only requires [first,last) to be partitioned, this algorithm is usually used in the case where [first,last) is sorted, so that the binary search is valid for any value.

For any iterator iter in [first,last), std::upper_bound requires value < *iter and comp(value, *iter) to be well-formed, while std::lower_bound requires *iter < value and comp(*iter, value) to be well-formed instead.

# Example

#include <algorithm>
#include <cassert>
#include <complex>
#include <iostream>
#include <vector>
 
struct PriceInfo { double price; };
 
int main()
{
    const std::vector<int> data{1, 2, 4, 5, 5, 6};
 
    for (int i = 0; i < 7; ++i)
    {
        // Search first element that is greater than i
        auto upper = std::upper_bound(data.begin(), data.end(), i);
 
        std::cout << i << " < ";
        upper != data.end()
            ? std::cout << *upper << " at index " << std::distance(data.begin(), upper)
            : std::cout << "not found";
        std::cout << '\n';
    }
 
    std::vector<PriceInfo> prices{{100.0}, {101.5}, {102.5}, {102.5}, {107.3}};
 
    for (double to_find : {102.5, 110.2})
    {
        auto prc_info = std::upper_bound(prices.begin(), prices.end(), to_find,
            [](double value, const PriceInfo& info)
            {
                return value < info.price;
            });
 
        prc_info != prices.end()
            ? std::cout << prc_info->price << " at index " << prc_info - prices.begin()
            : std::cout << to_find << " not found";
        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 = std::upper_bound(nums.cbegin(), nums.cend(), {2, 0}, cmpz);
    #else
        auto it = std::upper_bound(nums.cbegin(), nums.cend(), CD{2, 0}, cmpz);
    #endif
    assert((*it == CD{3, 0}));
}

# Defect reports

DR	Applied to	Behavior as published	Correct behavior
LWG 270	C++98	Compare was required to satisfy Compare and T was requiredto be LessThanComparable (strict weak ordering required)	only a partitioning is required;heterogeneous comparisons permitted
LWG 384	C++98	at most (\scriptsize \log(N)+1)log2(N)+1 comparisons were allowed	corrected to (\scriptsize \log_{2}(N)+O(1))log2(N)+O(1)
LWG 577	C++98	last could not be returned	allowed
LWG 2150	C++98	if any iterator iter exists in [first, last) such thatbool(comp(value, *iter)) is true, std::upper_boundcould return any iterator in [iter, last)	no iterator afteriter can be returned