24 Ranges library [ranges]

24.7 Range adaptors [range.adaptors]

24.7.9 Drop while view [range.drop.while]

24.7.9.1 Overview [range.drop.while.overview]

1
#
Given a unary predicate pred and a view r, drop_­while_­view produces a view of the range [ranges​::​find_­if_­not(r, pred), ranges​::​end(r)).
2
#
The name views​::​drop_­while denotes a range adaptor object ([range.adaptor.object]).
Given subexpressions E and F, the expression views​::​drop_­while(E, F) is expression-equivalent to drop_­while_­view{E, F}.
3
#
[Example
: 
constexpr auto source = "  \t   \t   \t   hello there";
auto is_invisible = [](const auto x) { return x == ' ' || x == '\t'; };
auto skip_ws = drop_while_view{source, is_invisible};
for (auto c : skip_ws) {
  cout << c;                                    // prints hello there with no leading space
}
— end example
]

24.7.9.2 Class template drop_­while_­view [range.drop.while.view]

namespace std::ranges {
  template<view V, class Pred>
    requires input_range<V> && is_object_v<Pred> &&
             indirect_unary_predicate<const Pred, iterator_t<V>>
  class drop_while_view : public view_interface<drop_while_view<V, Pred>> {
  public:
    drop_while_view() = default;
    constexpr drop_while_view(V base, Pred pred);

    constexpr V base() const& requires copy_constructible<V> { return base_; }
    constexpr V base() && { return std::move(base_); }

    constexpr const Pred& pred() const;

    constexpr auto begin();

    constexpr auto end()
    { return ranges::end(base_); }

  private:
    V base_ = V();                                      // exposition only
    semiregular-box<Pred> pred_;                        // exposition only
  };

  template<class R, class Pred>
    drop_while_view(R&&, Pred) -> drop_while_view<views::all_t<R>, Pred>;
}
🔗
constexpr drop_while_view(V base, Pred pred);
1
#
Effects: Initializes base_­ with std​::​move(base) and pred_­ with std​::​move(pred).
🔗
constexpr const Pred& pred() const;
2
#
Effects: Equivalent to: return *pred_­;
🔗
constexpr auto begin();
3
#
Returns: ranges​::​find_­if_­not(base_­, cref(*pred_­)).
4
#
Remarks: In order to provide the amortized constant-time complexity required by the range concept when drop_­while_­view models forward_­range, the first call caches the result within the drop_­while_­view for use on subsequent calls.
[Note
:
Without this, applying a reverse_­view over a drop_­while_­view would have quadratic iteration complexity.
— end note
]