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

 

C++

Language

Standard Library Headers

Freestanding and hosted implementations

Named requirements

Language support library

Concepts library (C++20)

Diagnostics library

Utilities library

Strings library

Containers library

Iterators library

Ranges library (C++20)

Algorithms library

Numerics library

Localizations library

Input/output library

Filesystem library (C++17)

Regular expressions library (C++11)

Atomic operations library (C++11)

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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_value_t	range_reference_trange_rvalue_reference_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
Semiregular wrapper
semiregular_box
Factories
empty_viewviews::empty	single_viewviews::single	iota_viewviews::iota	basic_istream_viewistream_view
Adaptors
views::all_tviews::all common_viewviews::common join_viewviews::join split_viewviews::split	take_viewviews::take drop_viewviews::drop take_while_viewviews::take_while drop_while_viewviews::drop_while	transform_viewviews::transform reverse_viewviews::reverse filter_viewviews::filter ref_view	views::counted elements_viewviews::elements keys_viewviews::keys values_viewviews::values

 

std::ranges::take_view

Member functions

take_view::take_view

take_view::base

take_view::begin

take_view::end

take_view::size

Deduction guides

Sentinel

take_view::sentinel::sentinel

take_view::sentinel::base

operator==(std::counted_iterator,take_view::sentinel)

 

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 first N elements from an underlying sequence, or all elements if the underlying sequence contains fewer than N.

2) 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;
• 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)	constructs a take_view (public member function)
base	returns a copy of the underlying (adapted) view (public member function)
begin	returns an iterator to the beginning (public member function)
end	returns an iterator or a sentinel to the end (public member function)
size	returns the number of elements. Provided only if the underlying (adapted) range satisfies sized_range (public member function)

Nested classes

sentinel

the sentinel type (public member class)

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)