std::binary_search

Header: <algorithm>

Checks if an element equivalent to value appears within the partitioned range [first,last).

# Declarations

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

(since C++26)

template< class ForwardIt, class T, class Compare >
bool binary_search( 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 bool binary_search( 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 types Type1 and Type2 must be such that an object of type T can be implicitly converted to both Type1 and Type2, and an object of type ForwardIt can be dereferenced and then implicitly converted to both Type1 and Type2.

# Return value

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

# Notes

Although std::binary_search 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.

std::binary_search only checks whether an equivalent element exists. To obtain an iterator to that element (if exists), std::lower_bound should be used instead.

# Example

#include <algorithm>
#include <cassert>
#include <complex>
#include <iostream>
#include <vector>
 
int main()
{
    const auto haystack = {1, 3, 4, 5, 9};
 
    for (const auto needle : {1, 2, 3})
    {
        std::cout << "Searching for " << needle << '\n';
        if (std::binary_search(haystack.begin(), haystack.end(), needle))
            std::cout << "Found " << needle << '\n';
        else
            std::cout << "Not found!\n";
    }
 
    using CD = std::complex<double>;
    std::vector<CD> nums{{1, 1}, {2, 3}, {4, 2}, {4, 3}};
    auto cmpz = [](CD x, CD y){ return abs(x) < abs(y); };
    #ifdef __cpp_lib_algorithm_default_value_type
        assert(std::binary_search(nums.cbegin(), nums.cend(), {4, 2}, cmpz));
    #else
        assert(std::binary_search(nums.cbegin(), nums.cend(), CD{4, 2}, cmpz));
    #endif
}

# Defect reports

# See also

• equal_range
• lower_bound
• upper_bound
• ranges::binary_search