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

 

C++

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Feature test macros (C++20)

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Concepts library (C++20)

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Ranges library (C++20)

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Technical specifications

Symbols index

External libraries

 

Ranges library

Range access begin cbegin   end cend rbegin crbegin   rend crend size ssize   data cdata empty

Range conversions std::from_range_t std::from_range (C++23)(C++23)     to (C++23)

Dangling iterator handling dangling borrowed_iterator_t borrowed_subrange_t

Range primitives
range_size_trange_difference_trange_value_t elements_of (C++23)	iterator_tconst_iterator_tsentinel_tconst_sentinel_t (C++23)(C++23)	range_reference_trange_const_reference_trange_rvalue_reference_trange_common_reference_t (C++23)

Range concepts
range borrowed_range    sized_range	common_range view viewable_range	input_range output_range forward_range	bidirectional_range    random_access_range contiguous_range	constant_range (C++23)

Views
view_interface	subrange

Range factories
empty_viewviews::empty	single_viewviews::single	basic_istream_viewviews::istream	iota_viewviews::iota	repeat_viewviews::repeat (C++23)(C++23)

Range adaptors
views::all_tviews::all ref_view owning_view as_rvalue_viewviews::as_rvalue (C++23)(C++23)   filter_viewviews::filter transform_viewviews::transform take_viewviews::take take_while_viewviews::take_while concat_viewviews::concat (C++26)(C++26)	drop_viewviews::drop drop_while_viewviews::drop_while join_viewviews::join join_with_viewviews::join_with (C++23)(C++23)   lazy_split_viewviews::lazy_split split_viewviews::split views::counted common_viewviews::common	reverse_viewviews::reverse as_const_viewviews::as_const (C++23)(C++23) elements_viewviews::elements keys_viewviews::keys values_viewviews::values enumerate_viewviews::enumerate (C++23)(C++23) zip_viewviews::zip (C++23)(C++23) zip_transform_viewviews::zip_transform (C++23)(C++23)	adjacent_viewviews::adjacent (C++23)(C++23) views::pairwise (C++23) adjacent_transform_viewviews::adjacent_transform (C++23)(C++23) views::pairwise_transform (C++23) chunk_viewviews::chunk (C++23)(C++23) slide_viewviews::slide (C++23)(C++23) chunk_by_viewviews::chunk_by (C++23)(C++23) stride_viewviews::stride (C++23)(C++23) cartesian_product_viewviews::cartesian_product (C++23)(C++23)

Range generators std::generator (C++23)

Range adaptor closure objects range_adaptor_closure (C++23)

Range adaptor objects

Helper items
copyable-box movable-box (until C++23)  (C++23)	simple-view non-propagating-cache

 

std::ranges::drop_view

Member functions

drop_view::drop_view

drop_view::base

drop_view::begin

drop_view::end

drop_view::size

Deduction guides

 

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:

• ((void)f, decay-copy(e)), if T is a ranges::empty_view, except that the evaluations of e and f are indeterminately sequenced;
• otherwise, T(ranges::begin(e) + inc, ranges::end(e),
      /*to-unsigned-like*/(ranges::distance(e) - inc)), if T is a specialization of ranges::subrange that models both random_access_range and sized_range, and T needs to store the size (see ranges::subrange::subrange() for details), where inc is std::min<D>(ranges::distance(e), f);
• otherwise, U(ranges::begin(e) + inc, ranges::end(e)), if T is a specialization of std::span, std::basic_string_view, ranges::iota_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 otherwise;

otherwise, if T is a specialization of ranges::repeat_view: 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.

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

(constructor)	constructs a drop_view (public member function)
base	returns a copy of the underlying (adapted) view (public member function)
begin	returns an iterator to the beginning (public member function)
end	returns an iterator or a sentinel to the end (public member function)
size	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	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	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	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	returns the first element in the derived view. Provided if it satisfies forward_range. (public member function of std::ranges::view_interface<D>)
back	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[]	returns the nth element in the derived view. Provided if it satisfies random_access_range. (public member function of std::ranges::view_interface<D>)

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 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};
 
    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

ranges::drop_while_viewviews::drop_while (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)