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

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< cpp‎ | ranges
m (Add examples where we drop more than the number of elements.)
(Undo revision 170977 by BenFrantzDale (talk). Unbreak page)
Line 72: Line 72:
 
|code=
 
|code=
 
#include <iostream>
 
#include <iostream>
−
#include <format>
 
 
#include <ranges>
 
#include <ranges>
−
+
 
 
int main()
 
int main()
 
{
 
{
 
     const auto nums = {1, 2, 3, 4, 5, 6, 7, 8, 9};
 
     const auto nums = {1, 2, 3, 4, 5, 6, 7, 8, 9};
−
+
   
 
     for (int i : std::ranges::drop_view{nums, 2})
 
     for (int i : std::ranges::drop_view{nums, 2})
 
         std::cout << i << ' ';
 
         std::cout << i << ' ';
 
     std::cout << '\n';
 
     std::cout << '\n';
−
+
   
−
     for (int i : nums | std::views::drop(2))
+
     for (int i : nums {{!}} std::views::drop(2))
 
         std::cout << i << ' ';
 
         std::cout << i << ' ';
 
     std::cout << '\n';
 
     std::cout << '\n';
−
+
   
−
     for (int i : std::views::iota(1, 10) | std::views::drop(2))
+
     for (int i : std::views::iota(1, 10) {{!}} std::views::drop(2))
 
         std::cout << i << ' ';
 
         std::cout << i << ' ';
 
     std::cout << '\n';
 
     std::cout << '\n';
−
 
−
    // Note that dropping more than the number of elements is OK:
 
−
    for (auto i : {8, 9, 10, 100}) {
 
−
        std::cout << std::format("drop {}: ", i);
 
−
        for (int i : std::views::iota(1, 10) | std::views::drop(i))
 
−
            std::cout << i << ' ';
 
−
        std::cout << '\n';
 
−
    }
 
 
}
 
}
 
|output=
 
|output=
−
3 4 5 6 7 8 9  
+
3 4 5 6 7 8 9
−
3 4 5 6 7 8 9  
+
3 4 5 6 7 8 9
−
3 4 5 6 7 8 9  
+
3 4 5 6 7 8 9
−
drop 8: 9
+
−
drop 9:
+
−
drop 10:
+
−
drop 100:
+
 
}}
 
}}
  

Revision as of 12:08, 16 April 2024

 
C++
 
Ranges library
Range adaptors
 
std::ranges::drop_view
 
Defined in header <ranges>
template< ranges::view V >

class drop_view

    : public ranges::view_interface<drop_view<V>>
 (1) (since C++20)
namespace views {

    inline constexpr /* unspecified */ drop = /* unspecified */;

}
 (2) (since C++20)
Call signature
template< ranges::viewable_range R >

    requires /* see below */
constexpr ranges::view auto

    drop( R&& r, ranges::range_difference_t<R> count );
 (since C++20)
template< class DifferenceType >
constexpr /* range adaptor closure */ drop( DifferenceType&& count );
 (since C++20)
1) A range adaptor consisting of elements of the underlying sequence, skipping the first N elements.
2) RangeAdaptorObject. Given T is std::remove_cvref_t<decltype((e))> and D is ranges::range_difference_t<decltype((e))>), the expression views::drop(e, f) is expression-equivalent to:
  • std::span<typename T::element_type>, if T is a specialization of std::span;
  • T otherwise;
  • views::repeat(*e.value_, ranges::distance(e) - inc), if T models sized_range; in such case e is evaluated only once;
  • ((void)e, auto(f)) otherwise, except that the evaluations of e and f are indeterminately sequenced;
(since C++23)
  • otherwise, drop_view(e, f).
In all cases, decltype((f)) must model std::convertible_to<D>.

drop_view models the concepts contiguous_range, random_access_range, bidirectional_range, forward_range, input_range, common_range, and sized_range when the underlying view V models respective concepts.

Contents

Member functions

 constructs a drop_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]
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>) [edit]
(C++23)
 returns a constant iterator to the beginning of the range.
(public member function of std::ranges::view_interface<D>) [edit]
(C++23)
 returns a sentinel for the constant iterator of the 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]
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]
returns the first element in the derived view. Provided if it satisfies forward_range.
(public member function of std::ranges::view_interface<D>) [edit]
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]

Deduction guides

Helper templates

template<class T>

inline constexpr bool enable_borrowed_range<std::ranges::drop_view<T>> =

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

This specialization of std::ranges::enable_borrowed_range makes drop_view satisfy borrowed_range when the underlying view satisfies it.

Example

Run this code
#include <iostream>
#include <ranges>
 
int main()
{
    const auto nums = {1, 2, 3, 4, 5, 6, 7, 8, 9};
 
    for (int i : std::ranges::drop_view{nums, 2})
        std::cout << i << ' ';
    std::cout << '\n';
 
    for (int i : nums | std::views::drop(2))
        std::cout << i << ' ';
    std::cout << '\n';
 
    for (int i : std::views::iota(1, 10) | std::views::drop(2))
        std::cout << i << ' ';
    std::cout << '\n';
}

Output: 

3 4 5 6 7 8 9
3 4 5 6 7 8 9
3 4 5 6 7 8 9

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::drop sometimes fails to
construct a sized random access range 		the construction is adjusted
so that it is always valid
LWG 3494 C++20 drop_view was never a borrowed_range it is a borrowed_range if its underlying view is

See also

(C++20)
 a view consisting of the elements of another view, skipping the initial subsequence of elements until the first element where the predicate returns false
(class template) (range adaptor object)[edit]
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