std::ranges::views::take, std::ranges::take_view

 

C++

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Language support library

Concepts library (C++20)

Diagnostics library

Utilities library

Strings library

Containers library

Iterators library

Ranges library (C++20)

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Numerics library

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Technical Specifications

 

Ranges library

Range access
begin cbegin end cend	rbegin crbegin rend crend	size ssize data cdata	empty
Range primitives
iterator_tsentinel_t	range_size_trange_difference_t	range_reference_trange_rvalue_reference_t	range_value_t
Dangling iterator handling
dangling	borrowed_iterator_t	borrowed_subrange_t
Range concepts
range borrowed_range    sized_range	common_range view viewable_range	input_range output_range forward_range	bidirectional_range random_access_range contiguous_range
Views
view_interface	subrange
Factories
empty_viewviews::empty	single_viewviews::single	iota_viewviews::iota	basic_istream_viewistream_view
Adaptors
views::all_tviews::all common_viewviews::common filter_viewviews::filter reverse_viewviews::reverse	take_viewviews::take take_while_viewviews::take_while drop_viewviews::drop drop_while_viewviews::drop_while	elements_viewviews::elements keys_viewviews::keys values_viewviews::values transform_viewviews::transform	join_viewviews::join split_viewviews::split lazy_split_viewviews::lazy_split ref_view views::counted
Range adaptor objects
Range adaptor closure objects
Helper items
copyable-box	simple-view

 

std::ranges::take_view

Member functions

take_view::take_view (C++20)

take_view::base (C++20)

take_view::begin (C++20)

take_view::end (C++20)

take_view::size (C++20)

Deduction guides

Sentinel

take_view::sentinel::sentinel (C++20)

take_view::sentinel::base (C++20)

operator==(std::counted_iterator,take_view::sentinel) (C++20)

 

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)

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 T is 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 T models both random_access_range and sized_range, and T is 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).

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

Expression-equivalent

Expression e is expression-equivalent to expression f, if e and f have the same effects, either are both potentially-throwing or are both not potentially-throwing (i.e. noexcept(e) == noexcept(f)), and either are both constant subexpressions or are both not constant subexpressions.

Member functions

(constructor) (C++20)	constructs a take_view (public member function)
base (C++20)	returns a copy of the underlying (adapted) view (public member function)
begin (C++20)	returns an iterator to the beginning (public member function)
end (C++20)	returns an iterator or a sentinel to the end (public member function)
size (C++20)	returns the number of elements. Provided only if the underlying (adapted) range satisfies sized_range (public member function)
Inherited from std::ranges::view_interface
empty (C++20)	Returns whether the derived view is empty. Provided if it satisfies forward_range. (public member function of std::ranges::view_interface<D>)
operator bool (C++20)	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>)
data (C++20)	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>)
front (C++20)	Returns the first element in the derived view. Provided if it satisfies forward_range. (public member function of std::ranges::view_interface<D>)
back (C++20)	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>)
operator[] (C++20)	Returns the nth element in the derived view. Provided if it satisfies random_access_range. (public member function of std::ranges::view_interface<D>)

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 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

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