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

Defined in header <algorithm>
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 = {} );
 (1) (since C++23) 
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 = {} );
 (2) (since C++23) 
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 = {} );
 (3) (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 = {} );
 (4) (since C++23)
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:

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 Comment
__cpp_lib_ranges_contains 202207L (C++23) std::ranges::contains and ranges::contains_subrange

Possible implementation

contains 
struct __contains_fn
{
    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 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 T, class Proj = std::identity>
    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 
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

#include <algorithm>
#include <array>
 
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 };
    auto increment = [](int x) { return ++x; };
    auto decrement = [](int x) { return --x; };
 
    static_assert(
            std::ranges::contains(haystack, 4) and
        not std::ranges::contains(haystack, 6) and
            std::ranges::contains_subrange(haystack, needle) and
        not std::ranges::contains_subrange(haystack, bodkin) and
            std::ranges::contains(haystack, 6, increment) and
        not std::ranges::contains(haystack, 1, increment) and
            std::ranges::contains_subrange(haystack, bodkin, {}, increment) and
        not std::ranges::contains_subrange(haystack, bodkin, {}, decrement) and
            std::ranges::contains_subrange(haystack, bodkin, {}, {}, decrement)
    );
}

See also

(C++20)(C++20)(C++20)
 finds the first element satisfying specific criteria
(niebloid)
(C++20)
 searches for a range of elements
(niebloid)
(C++20)
 determines if an element exists in a partially-ordered range
(niebloid)
(C++20)
 returns true if one sequence is a subsequence of another
(niebloid)
(C++20)(C++20)(C++20)
 checks if a predicate is true for all, any or none of the elements in a range
(niebloid)

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