tmp/tmpri9ens9z/{from.md → to.md}
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#### Iterator support <a id="optional.iterators">[[optional.iterators]]</a>
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``` cpp
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using iterator = implementation-defined;
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using const_iterator = implementation-defined;
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```
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These types model `contiguous_iterator` [[iterator.concept.contiguous]],
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meet the *Cpp17RandomAccessIterator*
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requirements [[random.access.iterators]], and meet the requirements for
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constexpr iterators [[iterator.requirements.general]], with value type
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`remove_cv_t<T>`. The reference type is `T&` for `iterator` and
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`const T&` for `const_iterator`.
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All requirements on container iterators [[container.reqmts]] apply to
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`optional::iterator` and `optional::const_iterator` as well.
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Any operation that initializes or destroys the contained value of an
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optional object invalidates all iterators into that object.
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``` cpp
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constexpr iterator begin() noexcept;
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constexpr const_iterator begin() const noexcept;
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```
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*Returns:* If `has_value()` is `true`, an iterator referring to the
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contained value. Otherwise, a past-the-end iterator value.
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``` cpp
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constexpr iterator end() noexcept;
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constexpr const_iterator end() const noexcept;
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```
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*Returns:* `begin() + has_value()`.
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