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

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< cpp‎ | algorithm‎ | ranges

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:

- 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 | 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

Run this code

    #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

ranges::find ranges::find_if ranges::find_if_not (C++20)(C++20)(C++20) | finds the first element satisfying specific criteria (niebloid)
ranges::search (C++20) | searches for a range of elements (niebloid)
ranges::binary_search (C++20) | determines if an element exists in a partially-ordered range (niebloid)
ranges::includes (C++20) | returns true if one sequence is a subsequence of another (niebloid)
ranges::all_of ranges::any_of ranges::none_of (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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