std::nullopt_t (3) - Linux Manuals

std::nullopt_t: std::nullopt_t

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 {
      explicit constexpr nullopt_t(int) {}
  };