std::ranges::views::drop_while, std::ranges::drop_while_view - cppreference.com (original) (raw)
std::ranges::drop_while_view
| Defined in header | ||
|---|---|---|
| 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) |
- A range adaptor that represents 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
- 1 Data members
- 2 Member functions
- 3 Deduction guides
- 4 Helper templates
- 5 Notes
- 6 Example
- 7 Defect reports
- 8 See also
[edit] Data members
| Member | Description |
|---|---|
| V base_ (private) | the underlying view(exposition-only member object*) |
| copyable-box(until C++23)movable-box(since C++23) pred_ (private) | the underlying function object(exposition-only member object*) |
| non-propagating-cache<ranges::iterator_t<V>> cache_ (private) (present only if V satisfies forward_range) | an object that caches the result of begin()(exposition-only member object*) |
[edit] Member functions
| (constructor) | constructs a drop_while_view (public member function) [edit] |
|---|---|
| base | returns a copy of the underlying (adapted) view (public member function) [edit] |
| pred | returns a reference to the stored predicate (public member function) [edit] |
| begin | returns an iterator to the beginning (public member function) [edit] |
| end | returns an iterator or a sentinel to the end (public member function) [edit] |
| Inherited from std::ranges::view_interface | |
| empty | returns whether the derived view is empty, provided only if it satisfies sized_range or forward_range (public member function of std::ranges::view_interface) [edit] |
| cbegin(C++23) | returns a constant iterator to the beginning of the range (public member function of std::ranges::view_interface) [edit] |
| cend(C++23) | returns a sentinel for the constant iterator of the range (public member function of std::ranges::view_interface) [edit] |
| operator bool | returns whether the derived view is not empty, provided only if ranges::empty is applicable to it (public member function of std::ranges::view_interface) [edit] |
| data | gets the address of derived view's data, provided only if its iterator type satisfies contiguous_iterator (public member function of std::ranges::view_interface) [edit] |
| size | 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) [edit] |
| front | returns the first element in the derived view, provided if it satisfies forward_range (public member function of std::ranges::view_interface) [edit] |
| back | returns the last element in the derived view, provided only if it satisfies bidirectional_range and common_range (public member function of std::ranges::view_interface) [edit] |
| operator[] | returns the nth element in the derived view, provided only if it satisfies random_access_range (public member function of std::ranges::view_interface) [edit] |
[edit] Deduction guides
[edit] Helper templates
| template< class T, class Pred > constexpr bool enable_borrowed_range<std::ranges::drop_while_view<T, Pred>> = 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.
[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 #include #include #include 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:
[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 |