typedef decltype(nullptr) nullptr_t;
Null pointer type (C++)
Type of the null pointer constant nullptr.

This type can only take one value: nullptr, which when converted to a pointer type takes the proper null pointer value.

Even though nullptr_t it is not a keyword, it identifies a distinct fundamental type: the type of nullptr. As such, it participates in overload resolution as a different type.

This type is only defined for C++ (since C++11).