std::ranges::views::drop, std::ranges::drop_view
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Metaprogramming library (C++11)
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Concurrency support library (C++11)
Execution control library (C++26)
Ranges library
(exposition-only member object*)
returns the approximate size of the resulting
(public member function) [edit]
returns a constant iterator to the beginning of the range
(public member function of
returns a sentinel for the constant iterator of the range
(public member function of
(C++23)(C++23)
(C++23)(C++23)
(C++23)(C++23)
(C++26)(C++26)
drop_viewviews::drop
(C++23)(C++23)
(C++26)(C++26)
(C++26)(C++26)
(C++23)(C++23)
(C++23)(C++23)
(C++23)(C++23)
(C++23)(C++23)
(C++23)(C++23)
(C++23)
(C++23)(C++23)
(C++23)
(C++23)(C++23)
(C++23)(C++23)
(C++23)(C++23)
(C++23)(C++23)
(C++23)(C++23)
(C++23)
std::ranges::drop_view
Defined in header
<ranges>
template< ranges::view V >
(1)
(since C++20)
class drop_view
namespace views {
(2)
(since C++20)
inline constexpr /* unspecified */ drop = /* unspecified */;
Call signature
template< ranges::viewable_range R >
(since C++20)
requires /* see below */
constexpr ranges::view auto
template< class DifferenceType >
constexpr /* range adaptor closure */ drop( DifferenceType&& count );
(since C++20)
constexpr /* range adaptor closure */ drop( DifferenceType&& count );
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,
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)), ifT
is a specialization of ranges::subrange that models bothrandom_access_range
andsized_range
, andT
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 bothrandom_access_range
andsized_range
, whereU
is
- otherwise, if
T
is a specialization of ranges::repeat_view:
- views::repeat (*e.value_, ranges::distance (e) - inc), if
T
modelssized_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;
- views::repeat (*e.value_, ranges::distance (e) - inc), if
- otherwise, drop_view(e, f).
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.
Contents
[edit] Data members
Member
Description
V
base_
(private)
the underlying view(exposition-only member object*)
ranges::range_difference_t <V>
(exposition-only member object*)
count_
(private)
the number of elements to skip(exposition-only member object*)
non-propagating-cache <ranges::iterator_t <V>>
(present only if
(exposition-only member object*)
cache_
(private) (present only if
V
satisfies forward_range
but not random_access_range
and sized_range
)
an object that caches the result of calls to begin()
(exposition-only member object*)
[edit] Member functions
returns the number of elements, provided only if the underlying (adapted) range satisfies
(public member function) [edit]
sized_range
(public member function) [edit]
(C++26)
approximately_sized_range
(public member function) [edit]
Inherited from std::ranges::view_interface
returns whether the derived view is empty, provided only if it satisfies
(public member function of
sized_range
or forward_range
(public member function of
std::ranges::view_interface<D>
) [edit]
(C++23)
(public member function of
std::ranges::view_interface<D>
) [edit]
(C++23)
(public member function of
std::ranges::view_interface<D>
) [edit]
returns whether the derived view is not empty, provided only if ranges::empty is applicable to it
(public member function of
(public member function of
std::ranges::view_interface<D>
) [edit]
gets the address of derived view's data, provided only if its iterator type satisfies
(public member function of
contiguous_iterator
(public member function of
std::ranges::view_interface<D>
) [edit]
returns the first element in the derived view, provided if it satisfies
(public member function of
forward_range
(public member function of
std::ranges::view_interface<D>
) [edit]
returns the last element in the derived view, provided only if it satisfies
(public member function of
bidirectional_range
and common_range
(public member function of
std::ranges::view_interface<D>
) [edit]
returns the
(public member function of
n
th element in the derived view, provided only if it satisfies random_access_range
(public member function of
std::ranges::view_interface<D>
) [edit]
[edit] Deduction guides
[edit] Helper templates
template< class T >
(since C++20)
constexpr bool enable_borrowed_range<std::ranges::drop_view<T>> =
This specialization of ranges::enable_borrowed_range makes drop_view
satisfy borrowed_range
when the underlying view satisfies it.
[edit] Example
Run this code
#include <initializer_list> #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:
[edit] 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 toconstruct 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
|