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

From cppreference.com
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:
(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

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 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

(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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