std::ranges::binary_search

Defined in header `<algorithm>`
Call signature
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 bool binary_search( I first, S last, const T& value, Comp comp = {}, Proj proj = {} );	(1)	(since C++20)
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 bool binary_search( R&& r, const T& value, Comp comp = {}, Proj proj = {} );	(2)	(since C++20)

1) Checks if a projected element equivalent to value appears within the range [first, last).

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.

For ranges::binary_search to succeed, the range [first, last) must be at least partially ordered with respect to value, i.e. it must satisfy all of the following requirements:

partitioned with respect to std::invoke(comp, std::invoke(proj, element), value) (that is, all projected elements for which the expression is true precedes all elements for which the expression is false).
partitioned with respect to !std::invoke(comp, value, std::invoke(proj, element)).
for all elements, if std::invoke(comp, std::invoke(proj, element), value) is true then !std::invoke(comp, value, std::invoke(proj, element)) is also true.

A fully-sorted range meets these criteria.

The function-like entities described on this page are 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.

In practice, they may be implemented as function objects, or with special compiler extensions.

Parameters

first, last	-	the range of elements to examine
r	-	the range of elements to examine
value	-	value to compare the elements to
comp	-	comparison function to apply to the projected elements
proj	-	projection to apply to the elements

Return value

true if an element equal to value is found, false otherwise.

Complexity

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

Possible implementation

struct binary_search_fn
{
    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 bool
        operator()(I first, S last, const T& value, Comp comp = {}, Proj proj = {}) const
    {
        first = std::lower_bound(first, last, value, comp);
        return (!(first == last) && !(comp(value, *first)));
    }
 
    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 bool 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 binary_search_fn binary_search;

Example

Run this code

#include <algorithm>
#include <iostream>
#include <ranges>
 
int main()
{
    constexpr static auto haystack = {1, 3, 4, 5, 9};
    static_assert(std::ranges::is_sorted(haystack));
 
    for (const int needle : std::views::iota(1)
                          | std::views::take(3))
    {
        std::cout << "Searching for " << needle << ": ";
        std::ranges::binary_search(haystack, needle)
            ? std::cout << "found " << needle << '\n'
            : std::cout << "no dice!\n";
    }
}

Output:

Searching for 1: found 1
Searching for 2: no dice!
Searching for 3: found 3

Compiler support
Freestanding and hosted
Language
Standard library
Standard library headers
Named requirements
Feature test macros (C++20)
Language support library
Concepts library (C++20)
Metaprogramming library (C++11)
Diagnostics library
General utilities library
Strings library
Containers library
Iterators library
Ranges library (C++20)
Algorithms library
Numerics library
Localizations library
Input/output library
Filesystem library (C++17)
Regular expressions library (C++11)
Concurrency support library (C++11)
Technical specifications
Symbols index
External libraries

ranges::equal_range (C++20)	returns range of elements matching a specific key (niebloid)
ranges::lower_bound (C++20)	returns an iterator to the first element not less than the given value (niebloid)
ranges::upper_bound (C++20)	returns an iterator to the first element greater than a certain value (niebloid)
ranges::containsranges::contains_subrange (C++23)(C++23)	checks if the range contains the given element or subrange (niebloid)
binary_search	determines if an element exists in a partially-ordered range (function template)