NAME¶

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

Synopsis¶

Defined in header <ranges>
template< ranges::view V >

class drop_view (1) (since C++20)

: public ranges::view_interface<drop_view<V>>
namespace views {

inline constexpr /* unspecified */ drop = /* unspecified */; (2) (since C++20)

}
Call signature
template< ranges::viewable_range R >

requires /* see below */ (since C++20)
constexpr ranges::view auto

drop( R&& r, ranges::range_difference_t<R> count );
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 (since C++23)
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;

* 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.

Member functions¶

constructor constructs a drop_view
(C++20) (public member function)
base returns a copy of the underlying (adapted) view
(C++20) (public member function)
begin returns an iterator to the beginning
(C++20) (public member function)
end returns an iterator or a sentinel to the end
(C++20) (public member function)
size returns the number of elements. Provided only if the underlying
(C++20) (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
(C++20) sized_range or forward_range.
(public member function of std::ranges::view_interface<D>)
cbegin returns a constant iterator to the beginning of the range.
(C++23) (public member function of std::ranges::view_interface<D>)
cend returns a sentinel for the constant iterator of the range.
(C++23) (public member function of std::ranges::view_interface<D>)
operator bool returns whether the derived view is not empty. Provided if
(C++20) 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
(C++20) satisfies contiguous_iterator.
(public member function of std::ranges::view_interface<D>)
front returns the first element in the derived view. Provided if it
(C++20) 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
(C++20) bidirectional_range and common_range.
(public member function of std::ranges::view_interface<D>)
operator[] returns the n^th element in the derived view. Provided if it satisfies
(C++20) random_access_range.
(public member function of std::ranges::view_interface<D>)

Deduction guides

Helper templates

template<class T>

inline constexpr bool (since C++20)
enable_borrowed_range<std::ranges::drop_view<T>> =

std::ranges::enable_borrowed_range<T>;

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};

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
views::drop sometimes fails to the construction is adjusted
LWG 3407 C++20 construct a sized random so that it is always valid
access range
LWG 3494 C++20 drop_view was never a it is a borrowed_range if its
borrowed_range underlying view is

See also¶

ranges::drop_while_view a view consisting of the elements of another view, skipping
views::drop_while the initial subsequence of elements until the first element
(C++20) where the predicate returns false
(class template) (range adaptor object)