Why is std::nullopt_t part of the C++ standard?

Question

I don\'t understand the reasoning for the inclusion of std::nullopt_t in the standard. Does it exist strictly for convenience, or is it required in some niche c

Answer 1

C++ reference says it all:

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

Answer 2

nullopt_t is the type of nullopt which indicates disengaged optional state. nullopt allows disambiguating overloads such as (example from the optional proposal):

void run(complex<double> v);
void run(optional<string> v);

run(nullopt);              // pick the second overload
run({});                   // ambiguous