std::ranges::contains, std::ranges::contains_subrange

Since C++26

Header: <algorithm>

  1. Search-based algorithm that checks whether or not a given range contains a value with iterator-sentinel pairs.

# Declarations

Call signature

template< std::input_iterator I, std::sentinel_for<I> S,
class T,
class Proj = std::identity >
requires std::indirect_binary_predicate<ranges::equal_to, std::projected<I, Proj>,
const T*>
constexpr bool contains( I first, S last, const T& value, Proj proj = {} );

(since C++23) (until C++26)

template< std::input_iterator I, std::sentinel_for<I> S,
class Proj = std::identity,
class T = std::projected_value_t<I, Proj> >
requires std::indirect_binary_predicate<ranges::equal_to, std::projected<I, Proj>,
const T*>
constexpr bool contains( I first, S last, const T& value, Proj proj = {} );

(since C++26)

template< ranges::input_range R,
class T,
class Proj = std::identity >
requires std::indirect_binary_predicate<ranges::equal_to,
std::projected<ranges::iterator_t<R>, Proj>,
const T*>
constexpr bool contains( R&& r, const T& value, Proj proj = {} );

(since C++23) (until C++26)

template< ranges::input_range R,
class Proj = std::identity,
class T = std::projected_value_t<ranges::iterator_t<R>, Proj> >
requires std::indirect_binary_predicate<ranges::equal_to,
std::projected<ranges::iterator_t<R>, Proj>,
const T*>
constexpr bool contains( R&& r, const T& value, Proj proj = {} );

(since C++26)

template< std::forward_iterator I1, std::sentinel_for<I1> S1,
std::forward_iterator I2, std::sentinel_for<I2> S2,
class Pred = ranges::equal_to,
class Proj1 = std::identity, class Proj2 = std::identity >
requires std::indirectly_comparable<I1, I2, Pred, Proj1, Proj2>
constexpr bool contains_subrange( I1 first1, S1 last1, I2 first2, S2 last2,
Pred pred = {},
Proj1 proj1 = {}, Proj2 proj2 = {} );

(since C++23)

template< ranges::forward_range R1, ranges::forward_range R2,
class Pred = ranges::equal_to,
class Proj1 = std::identity, class Proj2 = std::identity >
requires std::indirectly_comparable<ranges::iterator_t<R1>,
ranges::iterator_t<R2>, Pred, Proj1, Proj2>
constexpr bool contains_subrange( R1&& r1, R2&& r2, Pred pred = {},
Proj1 proj1 = {}, Proj2 proj2 = {} );

(since C++23)

# Parameters

# Notes

Up until C++20, we’ve had to write std::ranges::find(r, value) != std::ranges::end(r) to determine if a single value is inside a range. And to check if a range contains a subrange of interest, we use not std::ranges::search(haystack, needle).empty(). While this is accurate, it isn’t necessarily convenient, and it hardly expresses intent (especially in the latter case). Being able to say std::ranges::contains(r, value) addresses both of these points.

ranges::contains_subrange, same as ranges::search, but as opposed to std::search, provides no access to (such as Boyer-Moore).

# Example

#include <algorithm>
#include <array>
#include <complex>
 
namespace ranges = std::ranges;
 
int main()
{
    constexpr auto haystack = std::array{3, 1, 4, 1, 5};
    constexpr auto needle = std::array{1, 4, 1};
    constexpr auto bodkin = std::array{2, 5, 2};
 
    static_assert(
        ranges::contains(haystack, 4) &&
       !ranges::contains(haystack, 6) &&
        ranges::contains_subrange(haystack, needle) &&
       !ranges::contains_subrange(haystack, bodkin)
    );
 
    constexpr std::array<std::complex<double>, 3> nums{{{1, 2}, {3, 4}, {5, 6}}};
    #ifdef __cpp_lib_algorithm_default_value_type
        static_assert(ranges::contains(nums, {3, 4}));
    #else
        static_assert(ranges::contains(nums, std::complex<double>{3, 4}));
    #endif
}

# See also