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std::nullopt_t

STD：：Nullopt[医]T型

Defined in header ​​
struct nullopt_t;​(since C++17)

std::nullopt_t是用于指示optional输入未初始化状态。特别是，std::optional有一个构造函数nullopt_t作为单个参数，创建不包含值的可选参数。

std::nullopt_t必须是非聚合的LiteralType并且不能具有默认构造函数或初始化-列表构造函数。

它一定有一个constexpr构造函数，该构造函数采用某些实现定义的文字类型。

注记

对...的限制nullopt_t%27s构造函数存在以支持两者op = {};和op = nullopt;作为分离可选对象的语法。

这个类的一个可能的实现是。

二次

struct nullopt_t {
    explicit constexpr nullopt_t(int) {}
};

二次

另见

nullopt (C++17)an object of type nullopt_t (constant)

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