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) {}
};

See also

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