std::ranges::contains,std::ranges::contains_subrange(3)

NAME
std::ranges::contains,std::ranges::contains_subrange - std::ranges::contains,std::ranges::contains_subrange

Synopsis
   Defined in header <algorithm>
   Call signature
   template< std::input_iterator I, std::sentinel_for<I> S,
             class T,
             class Proj = std::identity > (since
   requires std::indirect_binary_predicate<ranges::equal_to, C++23)
   std::projected<I, Proj>, (until
                        const T*> C++26)
   constexpr bool contains( I first, S last, const T& value,
   Proj proj = {} );
   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, (since
   std::projected<I, Proj>, C++26)
                        const T*>
   constexpr bool contains( I first, S last, const T& value,
   Proj proj = {} );
   template< ranges::input_range R,
             class T,
             class Proj = std::identity > (since
   requires std::indirect_binary_predicate<ranges::equal_to, C++23)
                        (until
   std::projected<ranges::iterator_t<R>, Proj>, C++26)
                        const T*>
   constexpr bool contains( R&& r, const T& value, Proj proj =
   {} );
   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, (1) (since
                        C++26)
   std::projected<ranges::iterator_t<R>, Proj>,
                        const T*>
   constexpr bool contains( R&& r, const T& value, Proj proj =
   {} );
   template< std::forward_iterator I1, std::sentinel_for<I1>
   S1,
             std::forward_iterator I2, std::sentinel_for<I2> (2)
   S2,
             class Pred = ranges::equal_to,
             class Proj1 = std::identity, class Proj2 =
   std::identity > (3) (since
   requires std::indirectly_comparable<I1, I2, Pred, Proj1, C++23)
   Proj2>
   constexpr bool contains_subrange( I1 first1, S1 last1, I2
   first2, S2 last2,
                        Pred pred = {},
                        Proj1 proj1 = {}, Proj2
   proj2 = {} );
   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>, (4) (since
                        ranges::iterator_t<R2>, C++23)
   Pred, Proj1, Proj2>
   constexpr bool contains_subrange( R1&& r1, R2&& r2, Pred
   pred = {},
                        Proj1 proj1 = {}, Proj2
   proj2 = {} );
   1) Search-based algorithm that checks whether or not a given range contains a value
   with iterator-sentinel pairs.
   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.
   3) Search-based algorithm that checks whether or not a given range is a subrange of
   another range with iterator-sentinel pairs.
   4) Same as (3) but uses r1 as the first source range and r2 as the second source
   range, as if using ranges::begin(r1) as first1, ranges::end(r1) as last1,
   ranges::begin(r2) as first2, and ranges::end(r2) as last2.
   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 the elements to examine
   value - value to compare the elements to
   pred - predicate to apply to the projected elements
   proj - projection to apply to the elements

Return value
   1,2) : ranges::find(std::move(first), last, value, proj) != last
   3,4) : first2 == last2 || !ranges::search(first1, last1, first2, last2, pred, proj1,
   proj2).empty()

Complexity
   At most last - first applications of the predicate and projection.

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 Searchers (such as Boyer-Moore).
             Feature-test macro Value Std Feature
   __cpp_lib_ranges_contains 202207L (C++23) std::ranges::contains and
                        ranges::contains_subrange
   __cpp_lib_algorithm_default_value_type 202403 (C++26) List-initialization for
                        algorithms (1,2)

Possible implementation
                        contains (1,2)
 struct __contains_fn
 {
     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 operator()(I first, S last, const T& value, Proj proj = {}) const
     {
         return ranges::find(std::move(first), last, value, proj) != last;
     }
     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 operator()(R&& r, const T& value, Proj proj = {}) const
     {
         return (*this)(ranges::begin(r), ranges::end(r), std::move(value), proj);
     }
 };
 inline constexpr __contains_fn contains {};
                        contains_subrange (3,4)
 struct __contains_subrange_fn
 {
     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 operator()(I1 first1, S1 last1,
                        I2 first2, S2 last2,
                        Pred pred = {},
                        Proj1 proj1 = {}, Proj2 proj2 = {}) const
     {
         return (first2 == last2) ||
                !ranges::search(first1, last1, first2, last2, pred, proj1, proj2).empty();
     }
     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 operator()(R1&& r1, R2&& r2,
                        Pred pred = {},
                        Proj1 proj1 = {}, Proj2 proj2 = {}) const
     {
         return (*this)(ranges::begin(r1), ranges::end(r1),
                        ranges::begin(r2), ranges::end(r2), std::move(pred),
                        std::move(proj1), std::move(proj2));
     }
 };
 inline constexpr __contains_subrange_fn contains_subrange {};

Example
// Run this code
 #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
   ranges::find
   ranges::find_if
   ranges::find_if_not finds the first element satisfying specific criteria
   (C++20) (niebloid)
   (C++20)
   (C++20)
   ranges::search searches for a range of elements
   (C++20) (niebloid)
   ranges::binary_search determines if an element exists in a partially-ordered range
   (C++20) (niebloid)
   ranges::includes returns true if one sequence is a subsequence of another
   (C++20) (niebloid)
   ranges::all_of
   ranges::any_of checks if a predicate is true for all, any or none of the
   ranges::none_of elements in a range
   (C++20) (niebloid)
   (C++20)
   (C++20)