is_null_xptr            Check if the external pointer is null.
is_xptr                 Check if an object is an external pointer.
new_xptr                Create an external pointer object.
register_xptr_finalizer
                        Register a finalizer for external pointer.
set_xptr_address        Set the address of the external pointer.
set_xptr_protected      Set a protected R object to the external
                        pointer.
set_xptr_tag            Set a tag to the external pointer.
xptr                    Manipulating External Pointers
xptr_address            External pointer address.
xptr_clear              Clear the pointer address of the external
                        pointer.
xptr_protected          Protected R object of the external pointer.
xptr_tag                Tag of the external pointer.
