You need to retrieve OS
singleton first:
uint32_t time = OS::get_singleton()->get_ticks_msec();
Don't forget to include this header file before using it:
#include "core/os/os.h"
Most other stuff could be retrieved similarly, like VisualServer
or PhysicsServer
:
VisualServer *vs = VisualServer::get_singleton();
PhysicsServer *ps = PhysicsServer::get_singleton();
Though appropriate headers need to be included as well.