std::experimental::nullopt - cppreference.com
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    std::experimental::nullopt

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    std::experimental::optional
     
    Defined in header <experimental/optional> 
    constexpr nullopt_t nullopt{/*unspecified*/};
    		(library fundamentals TS)

    std::experimental::nullopt is a constant of type std::experimental::nullopt_t that is used to indicate optional type with uninitialized state.

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