std::ranges...ns_subrange(3)

📦 version 2024.07.05, in nixpkgs unstable, opensuse-leap 16.0

💖 NAME 💖

okay so this is the std::ranges...ns_subrange(3) page from the C++ Standard Libary manual — yes, the header spells it “Libary,” and I’m keeping the typo because it’s iconic 😭.

The original NAME line is literally:

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

So yeah bestie, it’s about std::ranges::contains and std::ranges::contains_subrange. One checks “is this value in the range?” and the other checks “is this subrange inside the range?” ✨

🌸 Synopsis 🌸

Defined in header <algorithm>.

Call signature — the original is a little feral with line breaks, so here’s the synopsis exactly as printed, no cap:

          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 = {} );

Here’s what the numbered tea is:

  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 — here’s the table tea, bestie:

  • __cpp_lib_ranges_contains — value 202207L — std (C++23) — feature: std::ranges::contains and ranges::contains_subrange
  • __cpp_lib_algorithm_default_value_type — value 202403 — std (C++26) — feature: 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. Original annotations: (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, ranges::none_of — checks if a predicate is true for all, any or none of the elements in a range. Original annotations: (C++20) (niebloid); (C++20); (C++20).

that’s the whole page, bestie 💖 source: http://cppreference.com, version/date: 2024.06.10, footer title: std::ranges...ns_subrange(3). now go slay some ranges ✨👑

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