# 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)
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))>):

- ((void)f,`` `decay-copy`(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, if `T` is a specialization of `ranges::repeat_view`: views::repeat(*e.value_, std::min<D>(ranges::distance(e), f)), if `T` models `sized_range`; is such case e is evaluated only once; views::repeat(*e.value_, static_cast<D>(e)) otherwise; | (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

(constructor) (C++20) | constructs a `take_view` (public member function)
base (C++20) | returns a copy of the underlying (adapted) view (public member function)
begin (C++20) | returns an iterator to the beginning (public member function)
end (C++20) | returns an iterator or a sentinel to the end (public member function)
size (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
empty (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>`)
cbegin (C++23) | returns a constant iterator to the beginning of the range. (public member function of `std::ranges::view_interface<D>`)
cend (C++23) | returns a sentinel for the constant iterator of the range. (public member function of `std::ranges::view_interface<D>`)
operator bool (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>`)
data (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>`)
front (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>`)
back (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>`)
operator[] (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

sentinel (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

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';

        // 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

views::counted (C++20) | creates a subrange from an iterator and a count (customization point object)
ranges::take_while_view views::take_while (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)
ranges::copy_n (C++20) | copies a number of elements to a new location (niebloid)

Retrieved from "https://en.cppreference.com/mwiki/index.php?title=cpp/ranges/take_view&oldid=151126"