# std::ranges::views::adjacent, std::ranges::adjacent_view, std::ranges::views::pairwise

From cppreference.com
< cpp‎ | ranges

Defined in header `<ranges>` |  |
template< ranges::forward_range V, std::size_t N > requires ranges::view <V> && (N > 0) class adjacent_view : public ranges::view_interface <adjacent_view<V, N>> | (1) | (since C++23)
namespace views { template< std::size_t N > constexpr /* unspecified */ adjacent = /* unspecified */ ; } | (2) | (since C++23)
namespace views { inline constexpr auto pairwise = adjacent<2>; } | (3) | (since C++23)
Call signature |  |
template< ranges::viewable_range R > requires /* see below */ constexpr ranges::view auto adjacent<N>( R&& r ); |  | (since C++23)

1) `adjacent_view` is a range adaptor that takes a `view`, and produces a `view` whose `i`th element (a "window") is a std::tuple that holds `N` references to the elements of the original view, from `i`th up to i + N - 1th inclusively.
Let `S` be the size of the original view. Then the size of produced view is:

- S - N + 1, if `S >= N`,
- ​0​ otherwise, and the resulting view is empty.

2) The name views::adjacent<N> denotes a RangeAdaptorObject. Given a subexpression e and a constant expression N, the expression views::adjacent<N>(e) is expression-equivalent to

- ((void)e, auto( views::empty <tuple<>>)) if N is equal to ​0​ and decltype((e)) models `forward_range`,
- adjacent_view< views::all_t <decltype((e))>, N>(e) otherwise.

3) The name views::pairwise denotes a RangeAdaptorObject that behaves exactly as views::adjacent<2>.

`adjacent_view` always models `forward_range`, and models `bidirectional_range`, `random_access_range`, or `sized_range` if adapted `view` type models the corresponding concept.

### Data members

Member name | Definition
`base_` (private) | the underlying `view` of type `V`. (exposition-only member object*)

### Member functions

(constructor) | constructs a `adjacent_view` (public member function)
begin | returns an iterator to the beginning (public member function)
end | returns an iterator or a sentinel to the end (public member function)
size | returns the number of elements. Provided only if the underlying (adapted) range satisfies `sized_range`. (public member function)

##### Inherited from std::ranges::view_interface

empty | returns whether the derived view is empty. Provided if it satisfies `sized_range` or `forward_range`. (public member function of `std::ranges::view_interface<D>`)
cbegin (C++23) | returns a constant iterator to the beginning of the range. (public member function of `std::ranges::view_interface<D>`)
cend (C++23) | returns a sentinel for the constant iterator of the range. (public member function of `std::ranges::view_interface<D>`)
operator bool | returns whether the derived view is not empty. Provided if ranges::empty is applicable to it. (public member function of `std::ranges::view_interface<D>`)
front | returns the first element in the derived view. Provided if it satisfies `forward_range`. (public member function of `std::ranges::view_interface<D>`)
back | returns the last element in the derived view. Provided if it satisfies `bidirectional_range` and `common_range`. (public member function of `std::ranges::view_interface<D>`)
operator[] | returns the `n`th element in the derived view. Provided if it satisfies `random_access_range`. (public member function of `std::ranges::view_interface<D>`)

### Deduction guides

(none)

### Nested classes

iterator | the iterator type (exposition-only member class template*)
sentinel | the sentinel type used when `adjacent_view` is not a `common_range` (exposition-only member class template*)

### Helper templates

template< class V, size_t N > constexpr bool ranges::enable_borrowed_range <adjacent_view<V, N>> = ranges::enable_borrowed_range <V>; |  | (since C++23)

This specialization of `ranges::enable_borrowed_range` makes `adjacent_view` satisfy `borrowed_range` when the underlying view satisfies it.

### Notes

views::adjacent only accepts forward ranges even when `N` is `0`.

There are similarities between ranges::adjacent_view and ranges::slide_view:

- Both create a "sliding window" of size `N`.
- Both have the same size `S - N + 1`, where `S` is the size of an adapted `view` such that `S >= N > 0`.

The following table shows the differences between these adaptors:

View adaptor | `value_type` | The window size `N`
--- | --- | ---
ranges::adjacent_view | std::tuple | A template parameter
ranges::slide_view | ranges::range | A runtime argument

Feature-test macro | Value | Std | Feature
--- | --- | --- | ---
`__cpp_lib_ranges_zip` | `202110L` | (C++23) | ranges::zip_view, ranges::zip_transform_view, `ranges::adjacent_view`, ranges::adjacent_transform_view

### Example

Run this code

    #include <array>
    #include <format>
    #include <iostream>
    #include <ranges>
    #include <tuple>

    int main()
    {
        constexpr std::array v{1, 2, 3, 4, 5, 6};
        std::cout << "v = [1 2 3 4 5 6]\n";

        for (int i{}; std::tuple t : v | std::views::adjacent<3>)
        {
            auto [t0, t1, t2] = t;
            std::cout << std::format("e = {:<{}}[{} {} {}]\n", "", 2 * i++, t0, t1, t2);
        }
    }

Output:

    v = [1 2 3 4 5 6]
    e = [1 2 3]
    e =   [2 3 4]
    e =     [3 4 5]
    e =       [4 5 6]

### Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
LWG 4098 | C++23 | views::adjacent<0> used to accept input-only ranges | made rejected

### References

- C++23 standard (ISO/IEC 14882:2024):

  - 26.7.25 Adjacent view [range.adjacent]

### See also

ranges::adjacent_transform_viewviews::adjacent_transform (C++23) | a `view` consisting of results of application of a transformation function to adjacent elements of the adapted view (class template) (range adaptor object)
ranges::slide_viewviews::slide (C++23) | a `view` whose Mth element is a `view` over the Mth through (M + N - 1)th elements of another `view` (class template) (range adaptor object)
ranges::chunk_viewviews::chunk (C++23) | a range of `view`s that are `N`-sized non-overlapping successive chunks of the elements of another `view` (class template) (range adaptor object)
ranges::stride_viewviews::stride (C++23) | a `view` consisting of elements of another `view`, advancing over N elements at a time (class template) (range adaptor object)

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