Namespaces
Variants
Views
Actions

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

From cppreference.com
< cpp‎ | ranges
 
 
Ranges library
Range access
Range conversions
(C++23)
Range primitives
Dangling iterator handling
Range concepts
Views

Factories
Adaptors
Range adaptor objects
Range adaptor closure objects
Helper items
 
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, class DifferenceType >

    requires /* see below */

constexpr ranges::view auto drop( R&& r, DifferenceType&& 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) 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:
  • 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.

Contents

[edit] 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)).

[edit] Member functions

constructs a drop_view
(public member function) [edit]
(C++20)
returns a copy of the underlying (adapted) view
(public member function) [edit]
(C++20)
returns an iterator to the beginning
(public member function) [edit]
(C++20)
returns an iterator or a sentinel to the end
(public member function) [edit]
(C++20)
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
(C++20)
Returns whether the derived view is empty. Provided if it satisfies forward_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]
(C++20)
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]
(C++20)
Returns the first element in the derived view. Provided if it satisfies forward_range.
(public member function of std::ranges::view_interface<D>) [edit]
(C++20)
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]

[edit] Deduction guides

[edit] 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.

[edit] Example

#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

[edit] 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 borrowed_range it is a borrowed_range if its underlying view is
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

[edit] See also

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]