+
+namespace s4u {
+class Activity;
+class Actor;
+using ActorPtr = boost::intrusive_ptr<Actor>;
+XBT_PUBLIC void intrusive_ptr_release(Actor* actor);
+XBT_PUBLIC void intrusive_ptr_add_ref(Actor* actor);
+class Barrier;
+class Comm;
+using CommPtr = boost::intrusive_ptr<Comm>;
+XBT_PUBLIC void intrusive_ptr_release(Comm* c);
+XBT_PUBLIC void intrusive_ptr_add_ref(Comm* c);
+class Engine;
+class Exec;
+using ExecPtr = boost::intrusive_ptr<Exec>;
+XBT_PUBLIC void intrusive_ptr_release(Exec* e);
+XBT_PUBLIC void intrusive_ptr_add_ref(Exec* e);
+class Host;
+class Link;
+class Mailbox;
+using MailboxPtr = boost::intrusive_ptr<Mailbox>;
+XBT_PUBLIC void intrusive_ptr_release(Mailbox* m);
+XBT_PUBLIC void intrusive_ptr_add_ref(Mailbox* m);
+class Mutex;
+XBT_PUBLIC void intrusive_ptr_release(Mutex* m);
+XBT_PUBLIC void intrusive_ptr_add_ref(Mutex* m);
+class NetZone;
+class VirtualMachine;
+class File;
+class Storage;
+} // namespace s4u
+
+namespace config {
+template <class T> class Flag;
+}
+