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

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< cpp‎ | ranges
C++
Ranges library
Range adaptors
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
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

Data members

Member name Definition
base_ (private) the underlying view of type V
(exposition-only member object*
count_ (private) the number of elements to take, of type ranges::range_difference_t<V>
(exposition-only member object*

Member functions

 constructs a take_view
(public member function)
returns a copy of the underlying (adapted) view
(public member function)
returns an iterator to the beginning
(public member function)
returns an iterator or a sentinel to the end
(public member function)
returns the number of elements. Provided only if the underlying (adapted) range satisfies sized_range.
(public member function)
Inherited from std::ranges::view_interface
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>)
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>)
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>)
returns the first element in the derived view. Provided if it satisfies forward_range.
(public member function of std::ranges::view_interface<D>)
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>)
returns the n th 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 >

constexpr bool enable_borrowed_range<std:: ranges :: take_view <T>> =

ranges::enable_borrowed_range <T> ;
(since C++20)

This specialization of std::ranges::enable_borrowed_range makes take_view satisfy borrowed_range

Example

Run this code
#include <algorithm>
#include <iostream>
#include <ranges>
 
int main()
{
    namespace views = std::views;
    auto print = [](char x){ std::cout << x; };
 
    for (const char nums[]{'1', '2', '3'};
         int n : views::iota(0, 5))
    {
        std::cout << "take(" << n << "): ";
        // safely takes only upto min(n, nums.size()) elements:
        std::ranges::for_each(nums | views::take(n), print);
        std::cout << '\n';
    }
}

Output: 

take(0): 
take(1): 1
take(2): 12
take(3): 123
take(4): 123

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
(algorithm function object)
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