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std::nullopt

Defined in header <optional> 
inline constexpr nullopt_t nullopt{/*unspecified*/};
 (since C++17)

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

See also

(C++17)
indicator of optional type with uninitialized state
(class)

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https://en.cppreference.com/w/cpp/utility/optional/nullopt