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) |
Call signature | ||
template< ranges::viewable_range R, class Pred > requires /* see below */ constexpr ranges::view auto drop_while( R&& r, Pred&& pred ); | (since C++20) | |
template< class Pred > constexpr /*range adaptor closure*/ drop_while( Pred&& pred ); | (since C++20) |
view
of elements from an underlying sequence, beginning at the first element for which the predicate returns false
.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.
(C++20) | constructs a drop_while_view (public member function) |
(C++20) | returns a copy of the underlying (adapted) view (public member function) |
(C++20) | returns a reference to the stored predicate (public member function) |
(C++20) | returns an iterator to the beginning (public member function) |
(C++20) | returns an iterator or a sentinel to the end (public member function) |
Inherited from |
|
(C++20) | 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> ) |
(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> ) |
(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> ) |
(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> ) |
(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> ) |
(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> ) |
(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> ) |
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.
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.
#include <iomanip> #include <iostream> #include <ranges> #include <string> #include <string_view> using std::operator""sv; [[nodiscard]] constexpr bool is_space(char ch) noexcept { switch (ch) { case ' ': case '\t': case '\n': case '\v': case '\r': case '\f': return true; } return false; }; [[nodiscard]] constexpr std::string_view trim_left(std::string_view const in) noexcept { return in | std::views::drop_while(is_space); } [[nodiscard]] 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); const auto s = trim(" \f\n\t\r\vHello, C++20!\f\n\t\r\v "); 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 << ' '; }
Output:
s = "Hello, C++20!" 3 4 5
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 |
(C++20) | a view consisting of elements of another view , skipping the first N elements (class template) (range adaptor object) |
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