Docs4dev
Docs
cpp / latest / ranges / take_view.html

std::ranges::views::take, 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))>):

(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.

Member functions

(C++20)
constructs a take_view
(public member function)
(C++20)
returns a copy of the underlying (adapted) view
(public member function)
(C++20)
returns an iterator to the beginning
(public member function)
(C++20)
returns an iterator or a sentinel to the end
(public member function)
(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
(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>)
(C++23)
returns a constant iterator to the beginning of the range.
(public member function of std::ranges::view_interface<D>)
(C++23)
returns a sentinel for the constant iterator of the range.
(public member function of std::ranges::view_interface<D>)
(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>)
(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>)
(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>)
(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>)
(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>)

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

#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)
(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)
(C++20)
copies a number of elements to a new location
(niebloid)

© cppreference.com
Licensed under the Creative Commons Attribution-ShareAlike Unported License v3.0.
https://en.cppreference.com/w/cpp/ranges/take_view