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

From cppreference.com
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 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 .

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 when the underlying view satisfies it.

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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