Difference between revisions of "cpp/ranges/take view"

Revision as of 13:17, 26 April 2023

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 > requires /* see below */ constexpr ranges::view auto take( R&& r, ranges::range_difference_t<R> count );		(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 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, static_cast<T>(e)), if 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 both random_access_range and sized_range, where U is

std::span<typename T::element_type>, if T is a specialization of std::span;
T, if T is a specialization of std::basic_string_view;
ranges::subrange<ranges::iterator_t<T>>, if T is a specialization of ranges::subrange;

ranges::iota_view(*ranges::begin(e), *(ranges::begin(e) + std::min<D>(ranges::distance(e), f))), if T is a specialization of ranges::iota_view that models both random_access_range and sized_range;

otherwise, if T is a specialization of ranges::repeat_view:

views::repeat(*e.value_, std::min<D>(ranges::distance(e), f)), if T models sized_range; is such case e is evaluated only once;
views::repeat(*e.value_, static_cast<D>(e)) otherwise;

(since C++23)

otherwise, take_view(e, f).

In all cases, decltype((f)) must model std::convertible_to<D>.

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.

(constructor)	constructs a `take_view` (public member function) [edit]
base	returns a copy of the underlying (adapted) view (public member function) [edit]
begin	returns an iterator to the beginning (public member function) [edit]
end	returns an iterator or a sentinel to the end (public member function) [edit]
size	returns the number of elements. Provided only if the underlying (adapted) range satisfies `sized_range`. (public member function) [edit]
Inherited from std::ranges::view_interface
empty	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>`) [edit]
cbegin (C++23)	returns a constant iterator to the beginning of the range. (public member function of `std::ranges::view_interface<D>`) [edit]
cend (C++23)	returns a sentinel for the constant iterator of the range. (public member function of `std::ranges::view_interface<D>`) [edit]
operator bool	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>`) [edit]
data	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>`) [edit]
front	returns the first element in the derived view. Provided if it satisfies `forward_range`. (public member function of `std::ranges::view_interface<D>`) [edit]
back	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>`) [edit]
operator[]	returns the `n`^th element in the derived view. Provided if it satisfies `random_access_range`. (public member function of `std::ranges::view_interface<D>`) [edit]

Deduction guides

Nested classes

sentinel

(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>> = std::ranges::enable_borrowed_range<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 8 chars only, i.e. min(pi.size(), 42)
 
    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 construct a sized random access range	the result type is adjusted so that construction is always valid

@@ Line 6: / Line 6: @@
 {{dcl|num=1|since=c++20|1=
 template< ranges::view V >
-class take_view : public ranges::view_interface<take_view<V>>
+class take_view :
+    public ranges::view_interface<take_view<V>>
 }}
 {{dcl|num=2|since=c++20|1=
@@ Line 28: / Line 29: @@
 @2@ {{tt|views::take}} is a {{named req|RangeAdaptorObject}}. The expression {{c|views::take(e, f)}} results in a view that represents the first {{c|f}} elements from {{c|e}}. The result is not necessarily a {{tt|take_view}}.
 <!---->
-{{c|views::take(e, f)}} is ''expression-equivalent'' to (where {{tt|T}} is {{c|std::remove_cvref_t<decltype((e))>}} and {{tt|D}} is {{c|ranges::range_difference_t<decltype((e))>}}):
+{{c|views::take(e, f)}} is [[cpp/language/expressions#Expression-equivalence|expression-equivalent]] to (where {{tt|T}} is {{c|std::remove_cvref_t<decltype((e))>}} and {{tt|D}} is {{c|ranges::range_difference_t<decltype((e))>}}):
 <!---->
 * {{c|((void)f, static_cast<T>(e))}}, if {{tt|T}} is a {{lc|ranges::empty_view}}, except that the evaluations of {{c|e}} and {{c|f}} are indeterminately sequenced;<!-- LWG 3524 -->
@@ Line 45: / Line 46: @@
 In all cases, {{c|decltype((f))}} must model {{c|std::convertible_to<D>}}.
-{{c|take_view}} models the concepts {{lconcept|contiguous_range}}, {{lconcept|random_access_range}}, {{lconcept|bidirectional_range}}, {{lconcept|forward_range}},  {{lconcept|input_range}}, and {{lconcept|sized_range}} when the underlying view {{c|V}} models respective concepts. It models {{lconcept|common_range}} when the underlying view {{c|V}} models both {{lconcept|random_access_range}} and {{lconcept|sized_range}}.
+{{tt|take_view}} models the concepts {{lconcept|contiguous_range}}, {{lconcept|random_access_range}}, {{lconcept|bidirectional_range}}, {{lconcept|forward_range}},  {{lconcept|input_range}}, and {{lconcept|sized_range}} when the underlying view {{c|V}} models respective concepts. It models {{lconcept|common_range}} when the underlying view {{c|V}} models both {{lconcept|random_access_range}} and {{lconcept|sized_range}}.
-{{cpp/expr-eq}}
 ===Member functions===
 {{dsc begin}}
-{{dsc inc|cpp/ranges/adaptor/dsc constructor|take_view }}
+{{dsc inc|cpp/ranges/adaptor/dsc constructor|take_view}}
-{{dsc inc|cpp/ranges/adaptor/dsc base|take_view }}
+{{dsc inc|cpp/ranges/adaptor/dsc base|take_view}}
-{{dsc inc|cpp/ranges/adaptor/dsc begin|take_view }}
+{{dsc inc|cpp/ranges/adaptor/dsc begin|take_view}}
-{{dsc inc|cpp/ranges/adaptor/dsc end|take_view }}
+{{dsc inc|cpp/ranges/adaptor/dsc end|take_view}}
-{{dsc inc|cpp/ranges/adaptor/dsc size|take_view }}
+{{dsc inc|cpp/ranges/adaptor/dsc size|take_view}}
 {{cpp/ranges/view_interface/inherit|embedded=yes|size=yes}}

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Feature test macros (C++20)
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External libraries

Member functions
take_view::take_view
take_view::base
take_view::begin
take_view::end
take_view::size
Deduction guides
Sentinel
Member functions
take_view::sentinel::sentinel
Non-member functions
take_view::sentinel::base
operator==(std::counted_iterator,take_view::sentinel)

views::counted (C++20)	creates a subrange from an iterator and a count (customization point object)[edit]
ranges::take_while_viewviews::take_while (C++20)	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)[edit]
ranges::copy_n (C++20)	copies a number of elements to a new location (niebloid)[edit]

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Difference between revisions of "cpp/ranges/take view"

Revision as of 13:17, 26 April 2023

Contents

Member functions

Inherited from std::ranges::view_interface

Deduction guides

Nested classes

Helper templates

Example

Defect reports

See also

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