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 > (1) (since C++20)
class drop_view : 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, class DifferenceType >

requires /* see below */ (since C++20)

constexpr ranges::view auto drop( R&& r, DifferenceType&& 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) Range adaptor object. 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, static_cast<T>(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) + std::min<D>(ranges::distance(e), f),
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, 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.

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¶

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>)
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 nth 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 <ranges>
#include <iostream>

int main()
{
const auto nums = { 1, 2, 3, 4, 5, 6, 7, 8, 9 };

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';

for (int i : std::ranges::drop_view{nums, 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 3494 C++20 drop_view was never a it is a borrowed_range if its
borrowed_range underlying view is
views::drop sometimes fails to the construction is adjusted so
LWG 3407 C++20 construct that it is always valid
a sized random access range

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)