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std::optional::reset

可选：：重置

void reset(​(since C++17)

如果*this包含一个值，将该值销毁为value().T::~T()否则，就没有效果。

*this在此调用后不包含值。

例外

noexcept规格：

noexcept

另见

operator=assigns contents (public member function)

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