# std::ranges::views::drop, std::ranges::drop_view

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< cpp‎ | ranges

Defined in header `<ranges>` |  |
template< ranges::view V > class drop_view : public ranges::view_interface <drop_view<V>> | (1) | (since C++20)
namespace views { inline constexpr /* unspecified */ drop = /* unspecified */; } | (2) | (since C++20)
Call signature |  |
template< ranges::viewable_range R > requires /* see below */ constexpr ranges::view auto drop( R&& r, ranges::range_difference_t <R> count ); |  | (since C++20)
template< class DifferenceType > constexpr /* range adaptor closure */ drop( DifferenceType&& count ); |  | (since C++20)

1) A range adaptor consisting of elements of the underlying sequence, skipping the first N elements.
2) RangeAdaptorObject. Given `T` is std::remove_cvref_t <decltype((e))> and `D` is ranges::range_difference_t <decltype((e))>), the expression views::drop(e, f) is expression-equivalent to:

- ((void)f,`` `decay-copy` (e)), if `T` is a `ranges::empty_view`, except that the evaluations of e and f are indeterminately sequenced;
- otherwise, T( ranges::begin (e) + inc, ranges::end (e),
  /*to-unsigned-like*/( ranges::distance (e) - inc)), if `T` is a specialization of ranges::subrange that models both `random_access_range` and `sized_range`, and `T` needs to store the size (see `ranges::subrange::subrange()` for details), where inc is std::min <D>( ranges::distance (e), f);
- otherwise, U( ranges::begin (e) + inc, ranges::end (e)), if `T` is a specialization of std::span, std::basic_string_view, ranges::iota_view, or ranges::subrange that models both `random_access_range` and `sized_range`, where `U` is

  - std::span <typename T::element_type>, if `T` is a specialization of std::span;
  - `T` otherwise;

- otherwise, if `T` is a specialization of `ranges::repeat_view`:

  - views::repeat (*e.value_, ranges::distance (e) - inc), if `T` models `sized_range`; in such case e is evaluated only once;
  - ((void)e, auto(f)) otherwise, except that the evaluations of e and f are indeterminately sequenced;

(since C++23)

- otherwise, drop_view(e, f).

In all cases, decltype((f)) must model std::convertible_to <D>.

`drop_view` models the concepts `contiguous_range`, `random_access_range`, `bidirectional_range`, `forward_range`, `input_range`, `common_range`, and `sized_range` when the underlying view `V` models respective concepts.

### Data members

Member name | Definition
`base_` (private) | the underlying (adapted) `view` of type `V` (exposition-only member object*)
`count_` (private) | the number of elements to skip, of type ranges::range_difference_t <V> (exposition-only member object*)
`cache_` (private) (conditionally present) | TODO: mention L4$ (exposition-only member object*)

### Member functions

(constructor) | constructs a `drop_view` (public member function)
base | returns a copy of the underlying (adapted) 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>`)
data | gets the address of derived view's data. Provided if its iterator type satisfies `contiguous_iterator`. (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

### Helper templates

template< class T > constexpr bool enable_borrowed_range<std::ranges::drop_view<T>> = ranges::enable_borrowed_range <T>; |  | (since C++20)

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

### Example

Run this code

    #include <iostream>
    #include <ranges>

    int main()
    {
        const auto nums = {1, 2, 3, 4, 5, 6, 7};

        std::cout << "drop " << 2 << ": ";
        for (int i : std::ranges::drop_view{nums, 2})
            std::cout << i << ' ';
        std::cout << '\n';

        std::cout << "drop " << 3 << ": ";
        for (int i : nums | std::views::drop(3))
            std::cout << i << ' ';
        std::cout << '\n';

        std::cout << "drop " << 4 << ": ";
        for (int i : std::views::iota(1, 8) | std::views::drop(4))
            std::cout << i << ' ';
        std::cout << '\n';

        // Note that dropping more than the number of elements is OK:
        for (int dp : {5, 6, 7, 890, 100500})
        {
            std::cout << "drop " << dp << ": ";
            for (int i : std::views::iota(1, 8) | std::views::drop(dp))
                std::cout << i << ' ';
            std::cout << '\n';
        }
    }

Output:

    drop 2: 3 4 5 6 7
    drop 3: 4 5 6 7
    drop 4: 5 6 7
    drop 5: 6 7
    drop 6: 7
    drop 7:
    drop 890:
    drop 100500:

### 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 3407 | C++20 | `views::drop` sometimes fails to construct a sized random access range | the construction is adjusted so that it is always valid
LWG 3494 | C++20 | `drop_view` was never a `borrowed_range` | it is a `borrowed_range` if its underlying view is

### See also

ranges::drop_while_viewviews::drop_while (C++20) | a `view` consisting of the elements of another `view`, skipping the initial subsequence of elements until the first element where the predicate returns false (class template) (range adaptor object)

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