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std::nullopt

STD：：Nullopt

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

std::nullopt是类型的常数。std::nullopt_t用于表示optional输入未初始化状态。

另见

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

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