Difference between revisions of "cpp/ranges/take view"
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{{dr list begin}} | {{dr list begin}} | ||
{{dr list item|wg=lwg|dr=3494|std=C++20|before={{tt|take_view}} was never a {{tt|borrowed_range}}|after=it is a {{tt|borrowed_range}} if its underlying view is}} | {{dr list item|wg=lwg|dr=3494|std=C++20|before={{tt|take_view}} was never a {{tt|borrowed_range}}|after=it is a {{tt|borrowed_range}} if its underlying view is}} | ||
| − | {{dr list item|wg=lwg|dr=3407|std=C++20|before={{tt|views::take}} sometimes fails to construct a sized random access range|after=the result type is adjusted so that construction is always valid}} | + | {{dr list item|wg=lwg|dr=3407|std=C++20|before={{tt|views::take}} sometimes fails to construct<br>a sized random access range|after=the result type is adjusted so that construction is always valid}} |
{{dr list end}} | {{dr list end}} | ||
Revision as of 13:20, 18 December 2021
| 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) |
| Call signature |
||
| template< ranges::viewable_range R, class DifferenceType > requires /* see below */ |
(since C++20) | |
| template< class DifferenceType > constexpr /*range adaptor closure*/ take( DifferenceType&& count ); |
(since C++20) | |
1) A range adaptor that represents
view of the elements from an underlying sequence, starting at the beginning and ending at a given bound.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; - U(ranges::begin(e), ranges::begin(e) + std::min<D>(ranges::distance(e), f)), if
Tis a specialization of std::span, std::basic_string_view, or ranges::subrange that models bothrandom_access_rangeandsized_range, whereUis
- std::span<typename T::element_type>, if
Tis a specialization of std::span; - T, if
Tis a specialization of std::basic_string_view; - ranges::subrange<ranges::iterator_t<T>>, if
Tis 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))), if
Tis a specialization of ranges::iota_view that models bothrandom_access_rangeandsized_range; - 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 |
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 (exposition-only member class template*) |
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
|
| LWG 3407 | C++20 | views::take sometimes fails to constructa sized random access range |
the result type is adjusted so that construction is always valid |
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) |
