std::nullopt_t(3)

NAME
std::nullopt_t - std::nullopt_t

Synopsis
   Defined in header <optional>
   struct nullopt_t; (since C++17)
   std::nullopt_t is an empty class type used to indicate optional type with
   uninitialized state. In particular, std::optional has a constructor with nullopt_t
   as a single argument, which creates an optional that does not contain a value.
   std::nullopt_t must be a non-aggregate LiteralType and cannot have a default
   constructor or an initializer-list constructor.
   It must have a constexpr constructor that takes some implementation-defined literal
   type.

Notes
   The constraints on nullopt_t's constructors exist 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 explicit nullopt_t(int) {}
 };

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