It is a type of new handler function and this is a typedef of a void function with no parameters, used as the argument and return type in function set_new_handler.
It is an empty class defined in header <new>. This header also defines the standard constant nothrow, which is a value of this type specifically designed to call the overloaded operator functions.
Following is the declaration for std::new_handler.
typedef void (*new_handler)();