std::ranges:: views::drop, std::ranges::

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< cpp‎ | ranges
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;
(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

Data members

Member name Definition
base_ (private) the underlying (adapted) view of type V
(exposition-only member object*
count_ (private) the number of elements to skip, of type ranges::range_difference_t<V>
(exposition-only member object*
cache_ (private)
(conditionally present) 		TODO: mention L4$
(exposition-only member object*

Member functions

 constructs a drop_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

Helper templates

template < class T >

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

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

This specialization of ranges::enable_borrowed_range makes drop_view satisfy borrowed_range

Example

Run this code
#include <iostream>
#include <ranges>
 
int main()
{
    const auto nums = {1, 2, 3, 4, 5, 6, 7};
 
    std::cout << "drop " << 2 << ": ";
    for (int i : std::ranges::drop_view{nums, 2})
        std::cout << i << ' ';
    std::cout << '\n';
 
    std::cout << "drop " << 3 << ": ";
    for (int i : nums | std::views::drop(3))
        std::cout << i << ' ';
    std::cout << '\n';
 
    std::cout << "drop " << 4 << ": ";
    for (int i : std::views::iota(1, 8) | std::views::drop(4))
        std::cout << i << ' ';
    std::cout << '\n';
 
    // Note that dropping more than the number of elements is OK:
    for (int dp : {5, 6, 7, 890, 100500})
    {
        std::cout << "drop " << dp << ": ";
        for (int i : std::views::iota(1, 8) | std::views::drop(dp))
            std::cout << i << ' ';
        std::cout << '\n';
    }
}

Output: 

drop 2: 3 4 5 6 7
drop 3: 4 5 6 7
drop 4: 5 6 7
drop 5: 6 7
drop 6: 7
drop 7:
drop 890: 
drop 100500:

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)
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