std::ranges::views::drop_while, std::ranges::drop_while_view
<ranges>
requires ranges::input_range <V> &&
std::is_object_v <Pred> &&
std::indirect_unary_predicate <const Pred, ranges::iterator_t <V>>
class drop_while_view
inline constexpr /* unspecified */ drop_while = /* unspecified */;
requires /* see below */
constexpr /*range adaptor closure*/ drop_while( Pred&& pred );
view of elements from an underlying sequence, beginning at the first element for which the predicate returns false.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.
Contents
[edit] Data members
V base_ (private)
the underlying view(exposition-only member object*)
copyable-box <Pred>(until C++23)movable-box <Pred>(since C++23) pred_ (private)
the underlying function object(exposition-only member object*)
cache_ (private) (present only if
V satisfies forward_range)
an object that caches the result of begin() (exposition-only member object*)
[edit] Member functions
Inherited from std::ranges::view_interface
sized_range or forward_range (public member function of
std::ranges::view_interface<D>) [edit]
(public member function of
std::ranges::view_interface<D>) [edit]
(public member function of
std::ranges::view_interface<D>) [edit]
(public member function of
std::ranges::view_interface<D>) [edit]
contiguous_iterator (public member function of
std::ranges::view_interface<D>) [edit]
forward_range and its sentinel and iterator type satisfy sized_sentinel_for. (public member function of
std::ranges::view_interface<D>) [edit]
forward_range (public member function of
std::ranges::view_interface<D>) [edit]
bidirectional_range and common_range (public member function of
std::ranges::view_interface<D>) [edit]
nth element in the derived view, provided only if it satisfies random_access_range (public member function of
std::ranges::view_interface<D>) [edit]
[edit] Deduction guides
[edit] Helper templates
constexpr bool enable_borrowed_range<std::ranges::drop_while_view<T, Pred>> =
This specialization of std::ranges::enable_borrowed_range makes drop_while_view satisfy borrowed_range when the underlying view satisfies it.
[edit] 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.
[edit] Example
#include <iostream> #include <ranges> #include <string> #include <string_view> using std::operator""sv; [[nodiscard]] constexpr bool is_space(char p) noexcept { auto ne = [p](auto q) { return p != q; }; return !!(" \t\n\v\r\f" | std::views::drop_while(ne)); }; [[nodiscard("trims the output")]] constexpr std::string_view trim_left(std::string_view const in) noexcept { auto view = in | std::views::drop_while(is_space); return {view.begin(), view.end()}; } [[nodiscard("trims the output")]] 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 auto src{" \f\n\t\r\vHello, C++20!\f\n\t\r\v "sv}; static_assert(trim(src) == "Hello, C++20!"); 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:
3 4 5
[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_while_view was never a borrowed_range
|
it is a borrowed_range if its underlying view is
|