tmp/tmp2w5it085/{from.md → to.md}
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| 1 |
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### Drop while view <a id="range.drop.while">[[range.drop.while]]</a>
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#### Overview <a id="range.drop.while.overview">[[range.drop.while.overview]]</a>
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Given a unary predicate `pred` and a `view` `r`, `drop_while_view`
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produces a `view` of the range \[`ranges::find_if_not(r, pred)`,
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`ranges::end(r)`).
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The name `views::drop_while` denotes a range adaptor object
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[[range.adaptor.object]]. Given subexpressions `E` and `F`, the
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expression `views::drop_while(E, F)` is expression-equivalent to
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`drop_while_view{E, F}`.
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[*Example 1*:
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``` cpp
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constexpr auto source = " \t \t \t hello there";
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auto is_invisible = [](const auto x) { return x == ' ' || x == '\t'; };
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auto skip_ws = drop_while_view{source, is_invisible};
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for (auto c : skip_ws) {
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cout << c; // prints hello there with no leading space
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}
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```
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— *end example*]
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#### Class template `drop_while_view` <a id="range.drop.while.view">[[range.drop.while.view]]</a>
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``` cpp
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namespace std::ranges {
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template<view V, class Pred>
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requires input_range<V> && is_object_v<Pred> &&
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indirect_unary_predicate<const Pred, iterator_t<V>>
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class drop_while_view : public view_interface<drop_while_view<V, Pred>> {
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public:
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drop_while_view() = default;
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constexpr drop_while_view(V base, Pred pred);
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constexpr V base() const& requires copy_constructible<V> { return base_; }
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constexpr V base() && { return std::move(base_); }
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constexpr const Pred& pred() const;
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constexpr auto begin();
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constexpr auto end()
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{ return ranges::end(base_); }
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private:
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V base_ = V(); // exposition only
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semiregular-box<Pred> pred_; // exposition only
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};
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template<class R, class Pred>
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drop_while_view(R&&, Pred) -> drop_while_view<views::all_t<R>, Pred>;
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}
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```
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``` cpp
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constexpr drop_while_view(V base, Pred pred);
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```
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*Effects:* Initializes *base\_* with `std::move(base)` and *pred\_* with
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`std::move(pred)`.
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``` cpp
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constexpr const Pred& pred() const;
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```
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*Effects:* Equivalent to: `return *`*`pred_`*`;`
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``` cpp
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constexpr auto begin();
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```
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*Returns:* `ranges::find_if_not(base_, cref(*pred_))`.
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*Remarks:* In order to provide the amortized constant-time complexity
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required by the `range` concept when `drop_while_view` models
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`forward_range`, the first call caches the result within the
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`drop_while_view` for use on subsequent calls.
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[*Note 1*: Without this, applying a `reverse_view` over a
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`drop_while_view` would have quadratic iteration
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complexity. — *end note*]
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