Namespaces
Variants
Views
Actions

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

From cppreference.com
< cpp‎ | ranges
(c -> c/core.)
m (DR list order fix.)
Line 103: Line 103:
 
===Defect reports===
 
===Defect reports===
 
{{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=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=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}}
 
{{dr list end}}
  

Revision as of 18:36, 26 April 2023

 
C++
 
Ranges library
Range adaptors
 
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)
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))>):

  • otherwise, if T is a specialization of ranges::repeat_view:
(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.

Contents

Member functions

 constructs a take_view
(public member function) [edit]
returns a copy of the underlying (adapted) view
(public member function) [edit]
returns an iterator to the beginning
(public member function) [edit]
returns an iterator or a sentinel to the end
(public member function) [edit]
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
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]
(C++23)
 returns a constant iterator to the beginning of the range.
(public member function of std::ranges::view_interface<D>) [edit]
(C++23)
 returns a sentinel for the constant iterator of the range.
(public member function of std::ranges::view_interface<D>) [edit]
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]
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]
returns the first element in the derived view. Provided if it satisfies forward_range.
(public member function of std::ranges::view_interface<D>) [edit]
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]
returns the nth 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

(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';
 
    // safely takes 8 chars only, i.e. min(pi.size(), 42)
    std::ranges::for_each(std::ranges::take_view{pi, 42}, print);
 
    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 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

See also

(C++20)
 creates a subrange from an iterator and a count
(customization point object)[edit]
(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]
(C++20)
 copies a number of elements to a new location
(niebloid)[edit]
Retrieved from "https://en.cppreference.com/mwiki/index.php?title=cpp/ranges/take_view&oldid=151054"