[range.drop]

# 25 Ranges library [[ranges]](./#ranges)

## 25.7 Range adaptors [[range.adaptors]](range.adaptors#range.drop)

### 25.7.12 Drop view [range.drop]

#### [25.7.12.1](#overview) Overview [[range.drop.overview]](range.drop.overview)

[1](#overview-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/ranges.tex#L6345)

drop_view produces a view
excluding the first N elements from another view, or
an empty range if the adapted view contains fewer than N elements[.](#overview-1.sentence-1)

[2](#overview-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/ranges.tex#L6350)

The name views​::​drop denotes
a range adaptor object ([[range.adaptor.object]](range.adaptor.object "25.7.2 Range adaptor objects"))[.](#overview-2.sentence-1)

Let E and F be expressions,
let T be remove_cvref_t<decltype((E))>, and
let D be range_difference_t<decltype((E))>[.](#overview-2.sentence-2)

If decltype((F)) does not model[convertible_to](concept.convertible#concept:convertible_to "18.4.4 Concept convertible_­to [concept.convertible]")<D>,views​::​drop(E, F) is ill-formed[.](#overview-2.sentence-3)

Otherwise, the expression views​::​drop(E, F) is expression-equivalent to:

- [(2.1)](#overview-2.1)

  If T is a specialization ofempty_view ([[range.empty.view]](range.empty.view "25.6.2.2 Class template empty_­view")),
then ((void)F, *decay-copy*(E)),
except that the evaluations of E and F are indeterminately sequenced[.](#overview-2.1.sentence-1)

- [(2.2)](#overview-2.2)

  Otherwise, if T models[random_access_range](range.refinements#concept:random_access_range "25.4.6 Other range refinements [range.refinements]") and [sized_range](range.sized#concept:sized_range "25.4.4 Sized ranges [range.sized]") and is
  * [(2.2.1)](#overview-2.2.1)

a specialization of span ([[views.span]](views.span "23.7.2.2 Class template span")),

  * [(2.2.2)](#overview-2.2.2)

a specialization of basic_string_view ([[string.view]](string.view "27.3 String view classes")),

  * [(2.2.3)](#overview-2.2.3)

a specialization of iota_view ([[range.iota.view]](range.iota.view "25.6.4.2 Class template iota_­view")), or

  * [(2.2.4)](#overview-2.2.4)

a specialization of subrange ([[range.subrange]](range.subrange "25.5.4 Sub-ranges"))
where T​::​*StoreSize* is false,

then U(ranges​::​begin(E) + std​::​min<D>(ranges​::​distance(E), F), ranges​::​end(E)),
except that E is evaluated only once,
where U is span<typename T​::​element_type> if T is a specialization of span and T otherwise[.](#overview-2.2.sentence-1)

- [(2.3)](#overview-2.3)

  Otherwise,
if T is
a specialization of subrange that models [random_access_range](range.refinements#concept:random_access_range "25.4.6 Other range refinements [range.refinements]") and [sized_range](range.sized#concept:sized_range "25.4.4 Sized ranges [range.sized]"),
thenT(ranges​::​begin(E) + std​::​min<D>(ranges​::​distance(E), F), ranges​::​end(E),*to-unsigned-like*(ranges​::​distance(E) - std​::​min<D>(ranges​::​distance(E), F))),
except that E and F are each evaluated only once[.](#overview-2.3.sentence-1)

- [(2.4)](#overview-2.4)

  Otherwise, if T is
a specialization of repeat_view ([[range.repeat.view]](range.repeat.view "25.6.5.2 Class template repeat_­view")):
  * [(2.4.1)](#overview-2.4.1)

if T models [sized_range](range.sized#concept:sized_range "25.4.4 Sized ranges [range.sized]"),
thenviews::repeat(*E.*value_*, ranges::distance(E) - std::min<D>(ranges::distance(E), F)) except that E is evaluated only once;

  * [(2.4.2)](#overview-2.4.2)

otherwise, ((void)F, *decay-copy*(E)),
except that the evaluations of E and F are indeterminately sequenced[.](#overview-2.4.sentence-1)

- [(2.5)](#overview-2.5)

  Otherwise, drop_view(E, F)[.](#overview-2.5.sentence-1)

[3](#overview-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/ranges.tex#L6417)

[*Example [1](#overview-example-1)*: auto ints = views::iota(0) | views::take(10);for (auto i : ints | views::drop(5)) { cout << i << ' '; // prints 5 6 7 8 9} â *end example*]

#### [25.7.12.2](#view) Class template drop_view [[range.drop.view]](range.drop.view)

[ð](#lib:drop_view)

namespace std::ranges {template<[view](range.view#concept:view "25.4.5 Views [range.view]") V>class drop_view : public view_interface<drop_view<V>> {public: drop_view() requires [default_initializable](concept.default.init#concept:default_initializable "18.4.12 Concept default_­initializable [concept.default.init]")<V> = default; constexpr explicit drop_view(V base, range_difference_t<V> count); constexpr V base() const & requires [copy_constructible](concept.copyconstructible#concept:copy_constructible "18.4.14 Concept copy_­constructible [concept.copyconstructible]")<V> { return *base_*; }constexpr V base() && { return std::move(*base_*); }constexpr auto begin()requires (!([*simple-view*](range.utility.helpers#concept:simple-view "25.5.2 Helper concepts [range.utility.helpers]")<V> &&[random_access_range](range.refinements#concept:random_access_range "25.4.6 Other range refinements [range.refinements]")<const V> && [sized_range](range.sized#concept:sized_range "25.4.4 Sized ranges [range.sized]")<const V>)); constexpr auto begin() constrequires [random_access_range](range.refinements#concept:random_access_range "25.4.6 Other range refinements [range.refinements]")<const V> && [sized_range](range.sized#concept:sized_range "25.4.4 Sized ranges [range.sized]")<const V>; constexpr auto end() requires (![*simple-view*](range.utility.helpers#concept:simple-view "25.5.2 Helper concepts [range.utility.helpers]")<V>){ return ranges::end(*base_*); }constexpr auto end() const requires [range](range.range#concept:range "25.4.2 Ranges [range.range]")<const V>{ return ranges::end(*base_*); }constexpr auto size() requires [sized_range](range.sized#concept:sized_range "25.4.4 Sized ranges [range.sized]")<V> {const auto s = ranges::size(*base_*); const auto c = static_cast<decltype(s)>(*count_*); return s < c ? 0 : s - c; }constexpr auto size() const requires [sized_range](range.sized#concept:sized_range "25.4.4 Sized ranges [range.sized]")<const V> {const auto s = ranges::size(*base_*); const auto c = static_cast<decltype(s)>(*count_*); return s < c ? 0 : s - c; }constexpr auto reserve_hint() requires [approximately_sized_range](range.approximately.sized#concept:approximately_sized_range "25.4.3 Approximately sized ranges [range.approximately.sized]")<V> {const auto s = static_cast<range_difference_t<V>>(ranges::reserve_hint(*base_*)); return *to-unsigned-like*(s < *count_* ? 0 : s - *count_*); }constexpr auto reserve_hint() const requires [approximately_sized_range](range.approximately.sized#concept:approximately_sized_range "25.4.3 Approximately sized ranges [range.approximately.sized]")<const V> {const auto s = static_cast<range_difference_t<const V>>(ranges::reserve_hint(*base_*)); return *to-unsigned-like*(s < *count_* ? 0 : s - *count_*); }private: V *base_* = V(); // *exposition only* range_difference_t<V> *count_* = 0; // *exposition only*}; template<class R> drop_view(R&&, range_difference_t<R>) -> drop_view<views::all_t<R>>;}

[ð](#lib:drop_view,constructor)

`constexpr explicit drop_view(V base, range_difference_t<V> count);
`

[1](#view-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/ranges.tex#L6494)

*Preconditions*: count >= 0 is true[.](#view-1.sentence-1)

[2](#view-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/ranges.tex#L6498)

*Effects*: Initializes *base_* with std​::​move(base) and*count_* with count[.](#view-2.sentence-1)

[ð](#lib:begin,drop_view)

`constexpr auto begin()
  requires (!([simple-view](range.utility.helpers#concept:simple-view "25.5.2 Helper concepts [range.utility.helpers]")<V> &&
              [random_access_range](range.refinements#concept:random_access_range "25.4.6 Other range refinements [range.refinements]")<const V> && [sized_range](range.sized#concept:sized_range "25.4.4 Sized ranges [range.sized]")<const V>));
constexpr auto begin() const
  requires [random_access_range](range.refinements#concept:random_access_range "25.4.6 Other range refinements [range.refinements]")<const V> && [sized_range](range.sized#concept:sized_range "25.4.4 Sized ranges [range.sized]")<const V>;
`

[3](#view-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/ranges.tex#L6514)

*Returns*: ranges​::​next(ranges​::​begin(*base_*), *count_*, ranges​::​end(*base_*))[.](#view-3.sentence-1)

[4](#view-4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/ranges.tex#L6518)

*Remarks*: In order to provide the amortized constant-time complexity required
by the [range](range.range#concept:range "25.4.2 Ranges [range.range]") concept
when drop_view models [forward_range](range.refinements#concept:forward_range "25.4.6 Other range refinements [range.refinements]"),
the first overload caches the result within the drop_view for use on subsequent calls[.](#view-4.sentence-1)

[*Note [1](#view-note-1)*:

Without this,
applying a reverse_view over a drop_view would have quadratic iteration complexity[.](#view-4.sentence-2)

â *end note*]
