NAME¶

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

Synopsis¶

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, (1) (since C++20)
ranges::iterator_t<V>>
class drop_while_view

: public ranges::view_interface<drop_while_view<V, Pred>>
namespace views {

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

}
Call signature
template< ranges::viewable_range R, class Pred >

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

constexpr ranges::view auto drop_while( R&& r, Pred&& pred );
template< class Pred > (since C++20)
constexpr /*range adaptor closure*/ drop_while( Pred&& pred );

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) RangeAdaptorObject. 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.

Member functions¶

constructor constructs a drop_while_view
(C++20) (public member function)
base returns a copy of the underlying (adapted) view
(C++20) (public member function)
pred returns a reference to the stored predicate
(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)
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>)
returns the number of elements in the derived view. Provided if it
size satisfies forward_range and its sentinel and iterator type satisfy
(C++20) sized_sentinel_for.
(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, class Pred >

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

std::ranges::enable_borrowed_range<T>;

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 <algorithm>
#include <iomanip>
#include <iostream>
#include <ranges>
#include <string>
#include <string_view>

using std::operator""sv;

[[nodiscard]]
constexpr bool is_space(char q) noexcept
{
static constexpr auto ws = {' ', '\t', '\n', '\v', '\r', '\f'};
return std::ranges::any_of(ws, [q](auto p) { return p == q; });
};

[[nodiscard]]
constexpr std::string trim_left(std::string_view const in) noexcept
{
auto view = in | std::views::drop_while(is_space);
return {view.begin(), view.end()};
}

[[nodiscard]]
constexpr std::string trim(std::string_view const in)
{
auto view = in
| std::views::drop_while(is_space) | std::views::reverse
| std::views::drop_while(is_space) | std::views::reverse
;
return {view.begin(), view.end()};
}

int main()
{
static_assert(trim_left(" \n C++23") == "C++23"sv);
constexpr std::string_view src = " \f\n\t\r\vHello, C++20!\f\n\t\r\v ";

const auto s = trim(src);
std::cout << "s = " << 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 << ' ';
std::cout << '\n';
}

Output:¶

s = "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 it is a borrowed_range if its
borrowed_range underlying view is

See also¶

ranges::drop_view a view consisting of elements of another view, skipping the first
views::drop N elements
(C++20) (class template) (range adaptor object)