std::ranges::views::drop_while, std::ranges::drop_while_view

 

C++

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Feature test macros (C++20)

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Concepts library (C++20)

Diagnostics library

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Containers library

Iterators library

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 primitives
iterator_tsentinel_t	range_size_trange_difference_t	range_reference_trange_rvalue_reference_t	range_value_t
Dangling iterator handling
dangling	borrowed_iterator_t	borrowed_subrange_t
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
Views
view_interface	subrange
Factories
empty_viewviews::empty	single_viewviews::single	iota_viewviews::iota	basic_istream_viewviews::istream
Adaptors
views::all_tviews::all ref_view owning_view common_viewviews::common reverse_viewviews::reverse views::counted zip_viewviews::zip (C++23)(C++23)	take_viewviews::take take_while_viewviews::take_while drop_viewviews::drop drop_while_viewviews::drop_while zip_transform_viewviews::zip_transform (C++23)(C++23)	elements_viewviews::elements keys_viewviews::keys values_viewviews::values transform_viewviews::transform adjacent_viewviews::adjacent (C++23)(C++23)	filter_viewviews::filter join_viewviews::join split_viewviews::split lazy_split_viewviews::lazy_split adjacent_transform_viewviews::adjacent_transform (C++23)(C++23)
Range adaptor objects
Range adaptor closure objects
Helper items
copyable-box	simple-view

 

std::ranges::drop_while_view

Member functions

drop_while_view::drop_while_view (C++20)

drop_while_view::base (C++20)

drop_while_view::pred (C++20)

drop_while_view::begin (C++20)

drop_while_view::end (C++20)

Deduction guides

 

Defined in header <ranges>
template< ranges::view V, class Pred >     requires ranges::input_range<V> &&              std::is_object_v<Pred> &&              std::indirect_unary_predicate<const Pred, ranges::iterator_t<V>> class drop_while_view : public ranges::view_interface<drop_while_view<V, Pred>>	(1)	(since C++20)
namespace views {     inline constexpr /*unspecified*/ drop_while = /*unspecified*/; }	(2)	(since C++20)

1) A range adaptor that represents view of elements from an underlying sequence, beginning at the first element for which the predicate returns false.

2) Range adaptor objects. The expression views::drop_while(e, f) is expression-equivalent to drop_while_view(e, f) for any suitable subexpressions e and f.

drop_while_view models the concepts contiguous_range, random_access_range, bidirectional_range, forward_range, input_range, and common_range when the underlying view V models respective concepts. It also models sized_range if ranges::forward_range<V> and std::sized_sentinel_for<ranges::sentinel_t<V>, ranges::iterator_t<V>> are modeled.

Expression-equivalent

Expression e is expression-equivalent to expression f, if e and f have the same effects, either are both potentially-throwing or are both not potentially-throwing (i.e. noexcept(e) == noexcept(f)), and either are both constant subexpressions or are both not constant subexpressions.

Member functions

(constructor) (C++20)	constructs a drop_while_view (public member function)
base (C++20)	returns a copy of the underlying (adapted) view (public member function)
pred (C++20)	returns a reference to the stored predicate (public member function)
begin (C++20)	returns an iterator to the beginning (public member function)
end (C++20)	returns an iterator or a sentinel to the end (public member function)
Inherited from std::ranges::view_interface
empty (C++20)	Returns whether the derived view is empty. Provided if it satisfies forward_range. (public member function of std::ranges::view_interface<D>)
operator bool (C++20)	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 (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>)
size (C++20)	Returns the number of elements in the derived view. Provided if it satisfies forward_range and its sentinel and iterator type satisfy sized_sentinel_for. (public member function of std::ranges::view_interface<D>)
front (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>)
back (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>)
operator[] (C++20)	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, class Pred> inline constexpr bool enable_borrowed_range<std::ranges::drop_while_view<T, Pred>> =     std::ranges::enable_borrowed_range<T>;		(since C++20)

This specialization of std::ranges::enable_borrowed_range makes drop_while_view satisfy borrowed_range when the underlying view satisfies it.

Notes

In order to provide the amortized constant time complexity required by the range concept, the result of begin is cached within the drop_while_view object. If the underlying range is modified after the first call to begin(), subsequent uses of the drop_while_view object might have unintuitive behavior.

Example

Run this code

#include <cctype>
#include <iomanip>
#include <iostream>
#include <ranges>
#include <string>
#include <string_view>
 
std::string trim(std::string_view const in)
{
    auto view
        = in
        | std::views::drop_while(isspace)
        | std::views::reverse
        | std::views::drop_while(isspace)
        | std::views::reverse
        ;
    return {view.begin(), view.end()};
}
 
int main()
{
    const auto s = trim(" \f\n\t\r\vHello, C++20!\f\n\t\r\v ");
    std::cout << std::quoted(s) << '\n';
 
    static constexpr auto v = {0, 1, 2, 3, 4, 5};
    for (int n : v | std::views::drop_while([](int i) { return i < 3; })) {
        std::cout << n << ' ';
    }
}

Output:

"Hello, C++20!"
3 4 5

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_while_view was never a borrowed_range	it is a borrowed_range if its underlying view is

See also

ranges::drop_viewviews::drop (C++20)

a view consisting of elements of another view, skipping the first N elements (class template) (range adaptor object)