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.
V base_ (private) the underlying view
copyable-box<Pred>(until C++23)movable-box<Pred>(since C++23) pred_ (private) the underlying function object
cache_ (private)
V satisfies forward_range) an object that caches the result of begin()
drop_while_view
sized_range or forward_range
std::ranges::view_interface<D>) [edit] returns a constant iterator to the beginning of the range
std::ranges::view_interface<D>) [edit] returns a sentinel for the constant iterator of the range
std::ranges::view_interface<D>) [edit] returns whether the derived view is not empty, provided only if ranges::empty is applicable to it
std::ranges::view_interface<D>) [edit] gets the address of derived view's data, provided only if its iterator type satisfies contiguous_iterator
std::ranges::view_interface<D>) [edit] 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.
std::ranges::view_interface<D>) [edit] returns the first element in the derived view, provided if it satisfies forward_range
std::ranges::view_interface<D>) [edit] returns the last element in the derived view, provided only if it satisfies bidirectional_range and common_range
std::ranges::view_interface<D>) [edit] returns the nth element in the derived view, provided only if it satisfies random_access_range
std::ranges::view_interface<D>) [edit] [edit] Deduction guides [edit] Helper templates (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 <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:
[edit] Defect reportsThe following behavior-changing defect reports were applied retroactively to previously published C++ standards.
DR Applied to Behavior as published Correct behavior LWG 3494 C++20drop_while_view was never a borrowed_range it is a borrowed_range if its underlying view is [edit] See also
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