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### No-value state indicator <a id="optional.nullopt">[[optional.nullopt]]</a>
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``` cpp
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struct nullopt_t{see below};
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inline constexpr nullopt_t nullopt(unspecified);
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```
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The struct `nullopt_t` is an empty structure type used as a unique type
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to indicate the state of not containing a value for `optional` objects.
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In particular, `optional<T>` has a constructor with `nullopt_t` as a
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single argument; this indicates that an optional object not containing a
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value shall be constructed.
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Type `nullopt_t` shall not have a default constructor or an
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initializer-list constructor, and shall not be an aggregate.
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