Difference between revisions of "cpp/ranges/take view"
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* otherwise, {{c|take_view(e, f)}}. | * otherwise, {{c|take_view(e, f)}}. | ||
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| − | In all cases, {{c|decltype((f))}} must model {{ | + | In all cases, {{c|decltype((f))}} must model {{c|std::convertible_to<D>}}. |
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Revision as of 02:12, 22 June 2021
| Defined in header <ranges>
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| 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) |
1) A range adaptor that represents
view of the first n elements from an underlying sequence, or all elements if the underlying sequence contains fewer than n.2)
views::take is a range adaptor object. 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.
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
Tis a ranges::empty_view, except that the evaluations of e and f are indeterminately sequenced; - T(ranges::begin(e), ranges::begin(e) + std::min<D>(ranges::size(e), f)), if
Tmodels bothrandom_access_rangeandsized_range, andTis a specialization of
- std::span where T::extent == std::dynamic_extent,
- std::basic_string_view,
- ranges::iota_view, or
- ranges::subrange;
- otherwise, take_view(e, f).
Contents |
Expression-equivalent
Expression e is expression-equivalent to expression f, if
- e and f have the same effects, and
- either both are constant subexpressions or else neither is a constant subexpression, and
- either both are potentially-throwing or else neither is potentially-throwing (i.e. noexcept(e) == noexcept(f)).
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) | |
Inherited from std::ranges::view_interface | |
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>)
| |
| (C++23) |
returns a constant iterator to the beginning of the range. (public member function of std::ranges::view_interface<D>)
|
| (C++23) |
returns a sentinel for the constant iterator of the range. (public member function of std::ranges::view_interface<D>)
|
| 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>)
| |
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>)
| |
returns the first element in the derived view. Provided if it satisfies forward_range. (public member function of std::ranges::view_interface<D>)
| |
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>)
| |
returns the nth element in the derived view. Provided if it satisfies random_access_range. (public member function of std::ranges::view_interface<D>)
| |
Deduction guides
Nested classes
| (C++20) |
the sentinel type (public member class) |
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.
Example
Run this code
#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
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
|
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) |
