std::experimental::nullopt_t (3) Linux Manual Page

std::experimental::nullopt_t – std::experimental::nullopt_t

Synopsis

Defined in header <experimental/optional>
struct nullopt_t; (library fundamentals TS)
std::experimental::nullopt_t is an empty class type used to indicate optional type with uninitialized state. In particular, std::experimental::optional has a constructor with nullopt_t as a single argument, which creates an optional that does not contain a value.
std::experimental::nullopt_t must be a LiteralType and cannot have a default constructor.
It must have a constexpr constructor that takes some implementation-defined literal type.

Notes

nullopt_t is not DefaultConstructible to support both op = {};
and op = nullopt; as the syntax for disengaging an optional object.

A possible implementation of this class is
struct nullopt_t {
    constexpr nullopt_t(int)
    {
    }
};

See also

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