+
+namespace s4u {
+class Activity;
+/** Smart pointer to a simgrid::s4u::Actor */
+using ActivityPtr = boost::intrusive_ptr<Activity>;
+XBT_PUBLIC void intrusive_ptr_release(const Activity* actor);
+XBT_PUBLIC void intrusive_ptr_add_ref(const Activity* actor);
+
+class Actor;
+/** Smart pointer to a simgrid::s4u::Actor */
+using ActorPtr = boost::intrusive_ptr<Actor>;
+XBT_PUBLIC void intrusive_ptr_release(const Actor* actor);
+XBT_PUBLIC void intrusive_ptr_add_ref(const Actor* actor);
+
+class Barrier;
+/** Smart pointer to a simgrid::s4u::Barrier */
+using BarrierPtr = boost::intrusive_ptr<Barrier>;
+XBT_PUBLIC void intrusive_ptr_release(Barrier* m);
+XBT_PUBLIC void intrusive_ptr_add_ref(Barrier* m);
+
+class Comm;
+/** Smart pointer to a simgrid::s4u::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 ConditionVariable;
+/** @beginrst
+ * Smart pointer to a :cpp:type:`simgrid::s4u::ConditionVariable`
+ * @endrst
+ */
+using ConditionVariablePtr = boost::intrusive_ptr<ConditionVariable>;
+XBT_PUBLIC void intrusive_ptr_release(const ConditionVariable* c);
+XBT_PUBLIC void intrusive_ptr_add_ref(const ConditionVariable* c);
+
+class Engine;
+
+class Exec;
+/** Smart pointer to a simgrid::s4u::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 ExecSeq; // FIXME: hide this class in implementation
+class ExecPar; // FIXME: hide this class in implementation
+
+class Host;
+
+class Io;
+/** Smart pointer to a simgrid::s4u::Io */
+using IoPtr = boost::intrusive_ptr<Io>;
+XBT_PUBLIC void intrusive_ptr_release(Io* i);
+XBT_PUBLIC void intrusive_ptr_add_ref(Io* i);
+
+class Link;
+
+class Mailbox;
+
+class Mutex;
+XBT_PUBLIC void intrusive_ptr_release(const Mutex* m);
+XBT_PUBLIC void intrusive_ptr_add_ref(const Mutex* m);
+/**
+ * @beginrst
+ * Smart pointer to a :cpp:type:`simgrid::s4u::Mutex`
+ * @endrst
+ */
+using MutexPtr = boost::intrusive_ptr<Mutex>;
+
+class NetZone;
+class VirtualMachine;
+class File;
+
+class Semaphore;
+/** Smart pointer to a simgrid::s4u::Semaphore */
+using SemaphorePtr = boost::intrusive_ptr<Semaphore>;
+XBT_PUBLIC void intrusive_ptr_release(Semaphore* m);
+XBT_PUBLIC void intrusive_ptr_add_ref(Semaphore* m);
+
+class Disk;
+class Storage;
+} // namespace s4u
+
+namespace config {
+template <class T> class Flag;
+}
+