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

Latest revision as of 13:21, 14 August 2024

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, decay-copy(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]

[edit] Deduction guides

[edit] Nested classes

sentinel

(C++20)

the sentinel type
(exposition-only member class template*)

[edit] Helper templates

template< class T > constexpr bool enable_borrowed_range<std::ranges::take_view<T>> = 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.

[edit] Example

Run this code

#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 to construct 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

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]

@@ Line 1: / Line 1: @@
-{{cpp/ranges/view title | take}}
+{{cpp/ranges/view title|take}}
 {{cpp/ranges/take_view/navbar}}
 {{dcl begin}}
-{{dcl header | ranges}}
+{{dcl header|ranges}}
-{{dcl | num=1 | since=c++20 | 1=
+{{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=
+{{dcl|num=2|since=c++20|1=
 namespace views {
-     inline constexpr /*unspecified*/ take = /*unspecified*/;
+     inline constexpr /* unspecified */ take = /* unspecified */;
 }
 }}
+{{dcl h|Call signature}}
+{{dcl|since=c++20|1=
+template< ranges::viewable_range R >
+    requires /* see below */
+constexpr ranges::view auto
+    take( R&& r, ranges::range_difference_t<R> count );
+}}
+{{dcl|since=c++20|1=
+template< class DifferenceType >
+constexpr /* range adaptor closure */ take( DifferenceType&& count );
+}}
 {{dcl end}}
-@1@ A range adaptor that represents {{lconcept|view}} of the first ''N'' elements from an underlying sequence, or all elements if the underlying sequence contains fewer than ''N''.
+@1@ A range adaptor that represents {{lconcept|view}} of the elements from an underlying sequence, starting at the beginning and ending at a given bound.
-@2@ The expression {{c|views::take(E, F)}} results in a view that represents the first {{tt|F}} elements from {{tt|E}}. The result is not necessarily a {{tt|take_view}}.
+@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}}.
 <!---->
-It 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/core|std::remove_cvref_t<decltype((e))>}} and {{tt|D}} is {{c/core|ranges::range_difference_t<decltype((e))>}}):
 <!---->
-* {{c|((void)F, static_cast<T>(E))}}, if {{tt|T}} is a {{lc|ranges::empty_view}};
+* {{box|{{c/core|((void)f,}}{{nbspt}}{{lti|cpp/standard library/decay-copy}}{{c/core|(e))}}}}, if {{tt|T}} is a {{c/core|ranges::empty_view}}, except that the evaluations of {{c|e}} and {{c|f}} are indeterminately sequenced;<!-- LWG 3524 -->
-* {{c|T{ranges::begin(E), ranges::begin(E) + std::min<D>(ranges::size(E), F)} }}, if {{tt|T}} models both {{lconcept|random_access_range}} and {{lconcept|sized_range}}, and {{tt|T}} is a specialization of
+* {{c|U(ranges::begin(e), ranges::begin(e) + std::min<D>(ranges::distance(e), f))}}, if {{tt|T}} is a specialization of {{c/core|std::span}}, {{lc|std::basic_string_view}}, or {{c/core|ranges::subrange}} that models both {{lconcept|random_access_range}} and {{lconcept|sized_range}}, where {{tt|U}} is
-:* {{lc|std::span}} where {{tt|1=T::extent == std::dynamic_extent}},
+:* {{c/core|std::span<typename T::element_type>}}, if {{tt|T}} is a specialization of {{c/core|std::span}};
-:* {{lc|std::basic_string_view}},
+:* {{tt|T}}, if {{tt|T}} is a specialization of {{lc|std::basic_string_view}};
-:* {{lc|ranges::iota_view}}, or
+:* {{c/core|ranges::subrange<ranges::iterator_t<T>>}}, if {{tt|T}} is a specialization of {{c/core|ranges::subrange}};
-:* {{lc|ranges::subrange}};
+* {{c multi|ranges::iota_view(*ranges::begin(e),|                  *(ranges::begin(e) + std::min<D>(ranges::distance(e), f)))}}, if {{tt|T}} is a specialization of {{c/core|ranges::iota_view}} that models both {{lconcept|random_access_range}} and {{lconcept|sized_range}};
-* otherwise, {{c|take_view{E, F} }}.
+{{rrev|since=c++23|
+* otherwise, if {{tt|T}} is a specialization of {{l2tt|cpp/ranges/repeat_view}}:
+:* {{c|views::repeat(*e.value_, std::min<D>(ranges::distance(e), f))}}, if {{tt|T}} models {{lconcept|sized_range}}; is such case {{c|e}} is evaluated only once;
+:* {{c|views::repeat(*e.value_, static_cast<D>(e))}} otherwise;
+}}
+* otherwise, {{c|take_view(e, f)}}.
+<!---->
+In all cases, {{c/core|decltype((f))}} must model {{c|std::convertible_to<D>}}.
-In all cases, {{tt|decltype((F))}} must model {{tt|std::convertible_to<D>}}.
+{{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 {{tt|V}} models respective concepts. It models {{lconcept|common_range}} when the underlying view {{tt|V}} models both {{lconcept|random_access_range}} and {{lconcept|sized_range}}.
-<!--
+===Data members===
-{{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}}.
+{{dsc begin}}
--->
+{{dsc hitem|Member name|Definition}}
+{{dsc expos mem obj|base_|private=yes|the underlying {{lconcept|view}} of type {{tt|V}}}}
-{{cpp/expr-eq}}
+{{dsc expos mem obj|count_|private=yes|the number of elements to take, of type {{c/core|ranges::range_difference_t<V>}}}}
+{{dsc end}}
 ===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}}
 {{dsc end}}
@@ Line 50: / Line 71: @@
 ===Nested classes===
 {{dsc begin}}
-{{dsc mem class| cpp/ranges/take_view/sentinel | title=''sentinel'' | the sentinel type}}
+{{dsc expos mem tclass|cpp/ranges/take_view/sentinel|notes={{mark c++20}}|the sentinel type}}
 {{dsc end}}
 ===Helper templates===
 {{ddcl|since=c++20|1=
-template<class T>
+template< class T >
-inline constexpr bool enable_borrowed_range<std::ranges::take_view<T>> =
+constexpr bool enable_borrowed_range<std::ranges::take_view<T>> =
-     std::ranges::enable_borrowed_range<T>;
+     ranges::enable_borrowed_range<T>;
 }}
 This specialization of {{ltt|cpp/ranges/borrowed_range|std::ranges::enable_borrowed_range}} makes {{tt|take_view}} satisfy {{lconcept|borrowed_range}} when the underlying view satisfies it.
@@ Line 67: / Line 88: @@
 #include <iostream>
 #include <ranges>
-auto print = [](char x) { std::cout << x; };
 int main()
 {
-     constexpr char pi[] { '3', '.', '1', '4', '1', '5', '9', '2' };
+     namespace views = std::views;
+     auto print = [](char x){ std::cout << x; };
-    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
+     for (const char nums[]{'1', '2', '3'};
-    std::cout << '\n';
+         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=
+|output=
-.1415
+take(0):
-.141592
+take(1): 1
+take(2): 12
+take(3): 123
+take(4): 123
 }}
 ===Defect reports===
 {{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=3407|std=C++20|before={{tt|views::take}} sometimes failed to<br>construct a sized random access range|after=the result type is adjusted so<br>that construction is always valid}}
+{{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 end}}
 ===See also===
 {{dsc begin}}
-{{dsc inc | cpp/ranges/dsc view_counted}}
+{{dsc inc|cpp/ranges/dsc view_counted}}
-{{dsc inc | cpp/ranges/dsc take_while_view}}
+{{dsc inc|cpp/ranges/dsc take_while_view}}
-{{dsc inc | cpp/algorithm/ranges/dsc copy_n}}
+{{dsc inc|cpp/algorithm/ranges/dsc copy_n}}
 {{dsc end}}
 {{langlinks|es|ja|ru|zh}}

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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)

Member name	Definition
`base_` (private)	the underlying `view` of type `V` (exposition-only member object*)
`count_` (private)	the number of elements to take, of type ranges::range_difference_t<V> (exposition-only member object*)

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

Latest revision as of 13:21, 14 August 2024

Contents

[edit] Data members

[edit] Member functions

Inherited from std::ranges::view_interface