std::ranges::views::take, std::ranges::take_view
Defined in header <ranges>
|
||
template< ranges::view V > class take_view : public ranges::view_interface<take_view<V>> |
(1) | (since C++20) |
namespace views { inline constexpr /*unspecified*/ take = /*unspecified*/; |
(2) | (since C++20) |
F
elements from E
. The result is not necessarily a take_view
.
It is expression-equivalent to (where T
is std::remove_cvref_t<decltype((E))> and D
is ranges::range_difference_t<decltype((E))>):
- ((void)F, static_cast<T>(E)), if
T
is a ranges::empty_view; - T{ranges::begin(E), ranges::begin(E) + std::min<D>(ranges::size(E), F)}, if
T
models both random_access_range and sized_range, andT
is a specialization of
- std::span where
T::extent == std::dynamic_extent
, - std::basic_string_view,
- ranges::iota_view, or
- ranges::subrange;
- std::span where
- otherwise, take_view{E, F}.
decltype((F))
must model std::convertible_to<D>
.
Contents |
[edit] Expression-equivalent
Expression e is expression-equivalent to expression f, if e and f have the same effects, either are both potentially-throwing or are both not potentially-throwing (i.e. noexcept(e) == noexcept(f)), and either are both constant subexpressions or are both not constant subexpressions.
[edit] Member functions
constructs a take_view (public member function) | |
returns a copy of the underlying (adapted) view (public member function) | |
returns an iterator to the beginning (public member function) | |
returns an iterator or a sentinel to the end (public member function) | |
returns the number of elements. Provided only if the underlying (adapted) range satisfies sized_range (public member function) |
[edit] Deduction guides
[edit] Nested classes
the sentinel type (public member class) |
[edit] Helper templates
template<class T> inline constexpr bool enable_borrowed_range<std::ranges::take_view<T>> = |
(since C++20) | |
This specialization of std::ranges::enable_borrowed_range makes take_view
satisfy borrowed_range when the underlying view satisfies it.
[edit] Example
#include <algorithm> #include <iostream> #include <ranges> auto print = [](char x) { std::cout << x; }; int main() { constexpr char pi[] { '3', '.', '1', '4', '1', '5', '9', '2' }; std::ranges::for_each(pi | std::ranges::views::take(6), print); std::cout << '\n'; std::ranges::for_each(std::ranges::take_view{pi, 42}, print); // safely takes only 8 chars std::cout << '\n'; }
Output:
3.1415 3.141592
[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 3494 | C++20 | take_view was never a borrowed_range
|
it is a borrowed_range if its underlying view is
|
[edit] See also
(C++20) |
creates a subrange from an iterator and a count (customization point object) |
a view consisting of the initial elements of another view, until the first element on which a predicate returns false (class template) (range adaptor object) | |
(C++20) |
copies a number of elements to a new location (niebloid) |