NAME¶

std::upper_bound - std::upper_bound

Synopsis¶

Defined in header <algorithm>
template< class ForwardIt, class T >
ForwardIt upper_bound( ForwardIt first, (until C++20)
ForwardIt last, const T& value );
template< class ForwardIt, class T >
constexpr ForwardIt upper_bound( ForwardIt (since C++20)
first, ForwardIt last, const T& value );
template< class ForwardIt, class T, class
Compare > (1) (until C++20)
ForwardIt upper_bound( ForwardIt first,
ForwardIt last, const T& value, Compare comp );
template< class ForwardIt, class T, class (2)
Compare >
constexpr ForwardIt upper_bound( ForwardIt (since C++20)
first, ForwardIt last, const T& value, Compare
comp );

Returns an iterator pointing to the first element in the range [first, last) such
that value < element (or comp(value, element)) is true (i.e. strictly greater), or
last if no such element is found.

The range [first, last) must be partitioned with respect to the expression !(value <
element) 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 first version uses operator< to compare the elements, the second version uses
the given comparison function comp.

Parameters¶

first, last - iterators defining the partially-ordered range to examine
value - value to compare the elements to
binary predicate which returns true if the first argument is less
than (i.e. 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);

comp - 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.

Type requirements¶

-
ForwardIt must meet the requirements of LegacyForwardIterator.
-
Compare must meet the requirements of BinaryPredicate. it is not required to satisfy
Compare

Return value¶

Iterator pointing to the first element in the range [first, last) such that value <
element (or comp(value, element)) is true, or last if no such element is found.

Complexity¶

The number of comparisons performed is logarithmic in the distance between first and
last (At most log
2(last - first) + O(1) comparisons). However, for non-LegacyRandomAccessIterators,
the number of iterator increments is linear. Notably, std::set and std::multiset
iterators are not random access, and so their member functions std::set::upper_bound
(resp. std::multiset::upper_bound) should be preferred.

Possible implementation¶

See also the implementations in libstdc++ and libc++.

First version¶

template<class ForwardIt, class T>
ForwardIt upper_bound(ForwardIt first, ForwardIt last, const T& value)
{
ForwardIt it;
typename std::iterator_traits<ForwardIt>::difference_type count, step;
count = std::distance(first, last);

while (count > 0) {
it = first;
step = count / 2;
std::advance(it, step);
if (!(value < *it)) {
first = ++it;
count -= step + 1;
}
else
count = step;
}
return first;
}

Second version¶

template<class ForwardIt, class T, class Compare>
ForwardIt upper_bound(ForwardIt first, ForwardIt last, const T& value, Compare comp)
{
ForwardIt it;
typename std::iterator_traits<ForwardIt>::difference_type count, step;
count = std::distance(first, last);

while (count > 0) {
it = first;
step = count / 2;
std::advance(it, step);
if (!comp(value, *it)) {
first = ++it;
count -= step + 1;
}
else
count = step;
}
return first;
}

Example¶

// Run this code

#include <algorithm>
#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';
}
}

Output:¶

0 < 1 at index 0
1 < 2 at index 1
2 < 4 at index 2
3 < 4 at index 2
4 < 5 at index 3
5 < 6 at index 5
6 < not found
107.3 at index 4
110.2 not found

Defect reports

The following behavior-changing defect reports were applied retroactively to
previously published C++ standards.

DR Applied to Behavior as published Correct behavior
LWG 270 C++98 Compare was required to be a only a partitioning is needed;
strict weak ordering heterogeneous comparisons permitted

See also¶

equal_range returns range of elements matching a specific key
(function template)
returns an iterator to the first element not less than the given
lower_bound value
(function template)
partition divides a range of elements into two groups
(function template)
partition_point locates the partition point of a partitioned range
(C++11) (function template)
ranges::upper_bound returns an iterator to the first element greater than a certain
(C++20) value
(niebloid)
returns an iterator to the first element greater than the given
upper_bound key
(public member function of std::set<Key,Compare,Allocator>)
returns an iterator to the first element greater than the given
upper_bound key
(public member function of std::multiset<Key,Compare,Allocator>)