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

 

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Ranges library

Range access
begin cbegin end cend	rbegin crbegin rend crend	size ssize data cdata	empty
Range conversions
to (C++23)
Range primitives
range_size_trange_difference_t range_value_t	range_reference_trange_const_reference_trange_rvalue_reference_t   (C++23)	iterator_tconst_iterator_tsentinel_t (C++23)
Dangling iterator handling
dangling	borrowed_iterator_t	borrowed_subrange_t
Range concepts
range borrowed_range    sized_range constant_range (C++23)	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 iota_viewviews::iota	single_viewviews::single cartesian_product_viewviews::cartesian_product (C++23)(C++23)	basic_istream_viewviews::istream repeat_viewviews::repeat (C++23)(C++23)
Adaptors
ref_view views::all_tviews::all common_viewviews::common owning_view views::counted reverse_viewviews::reverse as_const_viewviews::as_const (C++23)(C++23) zip_viewviews::zip (C++23)(C++23) zip_transform_viewviews::zip_transform (C++23)(C++23)	take_viewviews::take take_while_viewviews::take_while drop_viewviews::drop drop_while_viewviews::drop_while as_rvalue_viewviews::as_rvalue (C++23)(C++23)   adjacent_viewviews::adjacent (C++23)(C++23) adjacent_transform_viewviews::adjacent_transform (C++23)(C++23)	elements_viewviews::elements keys_viewviews::keys values_viewviews::values transform_viewviews::transform chunk_viewviews::chunk (C++23)(C++23) chunk_by_viewviews::chunk_by (C++23)(C++23) slide_viewviews::slide (C++23)(C++23)	filter_viewviews::filter split_viewviews::split lazy_split_viewviews::lazy_split join_viewviews::join join_with_viewviews::join_with (C++23)(C++23) stride_viewviews::stride (C++23)(C++23)
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

Member functions

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

Non-member functions

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)
Call signature
template< ranges::viewable_range R, class DifferenceType >     requires /* see below */ constexpr ranges::view auto take( R&& r, DifferenceType&& 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 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;
• 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.

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

(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 sized_range or 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
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

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)