std::nullopt_t

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std::optional
Defined in header <optional>
struct nullopt_t;
 (since C++17)

std::nullopt_t is an empty class type used to indicate that an std::optional does not contain a value.

std::nullopt_t is a non-aggregate LiteralType that has no default constructor, no initializer-list constructor, but does have a constexpr

Notes

The constraints on nullopt_t's constructors exist to support both op = {}; and op = nullopt;

A possible implementation of this class is 

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

See also

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