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

From cppreference.com
< cpp‎ | ranges
 
 
 
 
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 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))>):

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.

Contents

[edit] 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.

[edit] Member functions

constructs a take_view
(public member function) [edit]
(C++20)
returns a copy of the underlying (adapted) view
(public member function) [edit]
(C++20)
returns an iterator to the beginning
(public member function) [edit]
(C++20)
returns an iterator or a sentinel to the end
(public member function) [edit]
(C++20)
returns the number of elements. Provided only if the underlying (adapted) range satisfies sized_range
(public member function) [edit]
Inherited from std::ranges::view_interface
(C++20)
Returns whether the derived view is empty. Provided if it satisfies forward_range.
(public member function of std::ranges::view_interface<D>) [edit]
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>) [edit]
(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>) [edit]
(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>) [edit]
(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>) [edit]
Returns the nth element in the derived view. Provided if it satisfies random_access_range.
(public member function of std::ranges::view_interface<D>) [edit]

[edit] Deduction guides

[edit] Nested classes

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

[edit] 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.

[edit] 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';
 
    std::ranges::for_each(std::ranges::take_view{pi, 42}, print); // safely takes only 8 chars
    std::cout << '\n';
}

Output:

3.1415
3.141592

[edit] 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
LWG 3407 C++20 views::take sometimes fails to construct a sized random access range the result type is adjusted so that construction is always valid

[edit] See also

creates a subrange from an iterator and a count
(customization point object) [edit]
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) [edit]
copies a number of elements to a new location
(niebloid) [edit]