NAME¶

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

Synopsis¶

Defined in header <ranges>
template< ranges::view V > (1) (since C++20)
class take_view : public ranges::view_interface<take_view<V>>
namespace views {

inline constexpr /*unspecified*/ take = /*unspecified*/; (2) (since C++20)

}
Call signature
template< ranges::viewable_range R, class DifferenceType >

requires /* see below */ (since C++20)

constexpr ranges::view auto take( R&& r, DifferenceType&& count );
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, 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 constructs a take_view
(C++20) (public member function)
base returns a copy of the underlying (adapted) view
(C++20) (public member function)
begin returns an iterator to the beginning
(C++20) (public member function)
end returns an iterator or a sentinel to the end
(C++20) (public member function)
size returns the number of elements. Provided only if the underlying
(C++20) (adapted) range satisfies sized_range.
(public member function)
Inherited from std::ranges::view_interface
empty Returns whether the derived view is empty. Provided if it satisfies
(C++20) sized_range or forward_range.
(public member function of std::ranges::view_interface<D>)
operator bool Returns whether the derived view is not empty. Provided if
(C++20) ranges::empty is applicable to it.
(public member function of std::ranges::view_interface<D>)
data Gets the address of derived view's data. Provided if its iterator type
(C++20) satisfies contiguous_iterator.
(public member function of std::ranges::view_interface<D>)
front Returns the first element in the derived view. Provided if it
(C++20) satisfies forward_range.
(public member function of std::ranges::view_interface<D>)
back Returns the last element in the derived view. Provided if it satisfies
(C++20) bidirectional_range and common_range.
(public member function of std::ranges::view_interface<D>)
operator[] Returns the nth element in the derived view. Provided if it satisfies
(C++20) random_access_range.
(public member function of std::ranges::view_interface<D>)

Deduction guides

Nested classes

sentinel the sentinel type
(C++20) (exposition-only member class template)

Helper templates

template<class T>

inline constexpr bool (since C++20)
enable_borrowed_range<std::ranges::take_view<T>> =

std::ranges::enable_borrowed_range<T>;

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 it is a borrowed_range if its
borrowed_range underlying view is
views::take sometimes fails the result type is adjusted so that
LWG 3407 C++20 to construct construction is always valid
a sized random access range

See also¶

views::counted creates a subrange from an iterator and a count
(C++20) (customization point object)
ranges::take_while_view a view consisting of the initial elements of another view,
views::take_while until the first element on which a predicate returns false
(C++20) (class template) (range adaptor object)
ranges::copy_n copies a number of elements to a new location
(C++20) (niebloid)