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Difference between revisions of "cpp/ranges/take view"

From cppreference.com
< cpp‎ | ranges
m (→‎See also: + take_while_view)
m (→‎See also: + std::ranges::copy_n)
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{{dsc inc | cpp/ranges/dsc view_counted}}
 
{{dsc inc | cpp/ranges/dsc view_counted}}
 
{{dsc inc | cpp/ranges/dsc take_while_view}}
 
{{dsc inc | cpp/ranges/dsc take_while_view}}
 +
{{dsc inc | cpp/algorithm/ranges/dsc copy_n}}
 
{{dsc end}}
 
{{dsc end}}

Revision as of 23:50, 27 January 2021

 
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)
1) A range adaptor that represents view of the first N elements from an underlying sequence, or all elements if the underlying sequence contains fewer than N.
2) 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))>):

  • std::span where T::extent == std::dynamic_extent,
  • std::basic_string_view,
  • ranges::iota_view, or
  • ranges::subrange;
  • 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.

Contents

Expression-equivalent

Expression e is expression-equivalent to expression f, if

  • e and f have the same effects, and
  • either both are constant subexpressions or else neither is a constant subexpression, and
  • either both are potentially-throwing or else neither is potentially-throwing (i.e. noexcept(e) == noexcept(f)).

Member functions

 constructs a take_view
(public member function) [edit]
returns a copy of the underlying (adapted) view
(public member function) [edit]
returns an iterator to the beginning
(public member function) [edit]
returns an iterator or a sentinel to the end
(public member function) [edit]
returns the number of elements. Provided only if the underlying (adapted) range satisfies sized_range.
(public member function) [edit]

Deduction guides

Nested classes

 the sentinel type
(public member class)

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

Output: 

3.1415
3.141592

See also

(C++20)
 creates a subrange from an iterator and a count
(customization point object)[edit]
(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)[edit]
(C++20)
 copies a number of elements to a new location
(niebloid)[edit]
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