std::ranges::views::take, std::ranges::take_view
<ranges>
class take_view
inline constexpr /* unspecified */ take = /* unspecified */;
requires /* see below */
constexpr ranges::view auto
constexpr /* range adaptor closure */ take( DifferenceType&& count );
view
of the elements from an underlying sequence, starting at the beginning and ending at a given bound.views::take
is a RangeAdaptorObject. The expression views::take(e, f) results in a view that represents the first f elements from e. The result is not necessarily a take_view
.
views::take(e, f) is expression-equivalent to (where T
is std::remove_cvref_t <decltype((e))> and D
is ranges::range_difference_t <decltype((e))>):
- ((void)f,
T
is a ranges::empty_view , except that the evaluations of e and f are indeterminately sequenced; - U(ranges::begin (e), ranges::begin (e) + std::min <D>(ranges::distance (e), f)), if
T
is a specialization of std::span , std::basic_string_view , or ranges::subrange that models bothrandom_access_range
andsized_range
, whereU
is
- std::span <typename T::element_type>, if
T
is a specialization of std::span ; -
T
, ifT
is a specialization of std::basic_string_view ; - ranges::subrange <ranges::iterator_t <T>>, if
T
is a specialization of ranges::subrange ;
- std::span <typename T::element_type>, if
- ranges::iota_view (*ranges::begin (e),
*(ranges::begin (e) + std::min <D>(ranges::distance (e), f))), ifT
is a specialization of ranges::iota_view that models bothrandom_access_range
andsized_range
;
- otherwise, if
T
is a specialization of ranges::repeat_view:
- views::repeat (*e.value_, std::min <D>(ranges::distance (e), f)), if
T
modelssized_range
; is such case e is evaluated only once; - views::repeat (*e.value_, static_cast<D>(e)) otherwise;
- views::repeat (*e.value_, std::min <D>(ranges::distance (e), f)), if
- otherwise, take_view(e, f).
take_view
models the concepts contiguous_range
, random_access_range
, bidirectional_range
, forward_range
, input_range
, and sized_range
when the underlying view V
models respective concepts. It models common_range
when the underlying view V
models both random_access_range
and sized_range
.
Contents
[edit] Data members
V
base_
the underlying view(exposition-only member object*)
count_
the number of elements to take(exposition-only member object*)
[edit] Member functions
sized_range
(public member function) [edit]
approximately_sized_range
(public member function) [edit]
Inherited from std::ranges::view_interface
sized_range
or forward_range
(public member function of
std::ranges::view_interface<D>
) [edit]
(public member function of
std::ranges::view_interface<D>
) [edit]
(public member function of
std::ranges::view_interface<D>
) [edit]
(public member function of
std::ranges::view_interface<D>
) [edit]
contiguous_iterator
(public member function of
std::ranges::view_interface<D>
) [edit]
forward_range
(public member function of
std::ranges::view_interface<D>
) [edit]
bidirectional_range
and common_range
(public member function of
std::ranges::view_interface<D>
) [edit]
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] Nested classes
[edit] Helper templates
constexpr bool enable_borrowed_range<std::ranges::take_view<T>> =
This specialization of ranges::enable_borrowed_range makes take_view
satisfy borrowed_range
when the underlying view satisfies it.
[edit] Example
#include <algorithm> #include <iostream> #include <ranges> int main() { namespace views = std::views; auto print = [](char x){ std::cout << x; }; for (const char nums[]{'1', '2', '3'}; int n : views::iota (0, 5)) { std::cout << "take(" << n << "): "; // safely takes only upto min(n, nums.size()) elements: std::ranges::for_each (nums | views::take(n), print); std::cout << '\n'; } }
Output:
take(0): take(1): 1 take(2): 12 take(3): 123 take(4): 123
[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::take sometimes failed toconstruct a sized random access range |
the result type is adjusted so that construction is always valid |
LWG 3494 | C++20 | take_view was never a borrowed_range
|
it is a borrowed_range if its underlying view is
|
[edit] See also
view
consisting of the initial elements of another view
, until the first element on which a predicate returns false(class template) (range adaptor object)[edit]