+S4U classes are designed to be user process interfaces to Maestro resources.
+We provide an uniform interface to them:
+
+- automatic reference count with intrusive smart pointers `simgrid::s4u::FooPtr`
+ (also called `simgrid::s4u::Foo::Ptr`);
+
+- manual reference count with `intrusive_ptr_add_ref(p)`,
+ `intrusive_ptr_release(p)` (which is the interface used by
+ [`boost::intrusive_ptr`](http://www.boost.org/doc/libs/1_61_0/libs/smart_ptr/intrusive_ptr.html));
+
+- delegation of the operations to a opaque `pimpl` (which is the Maestro object);
+
+- the Maestro object and the corresponding S4U object have the same lifetime
+ (and share the same reference count).
+
+The ability to manipulate thge objects thought pointers and have the ability
+to use explicit reference count management is useful for creating C wrappers
+to the S4U and should play nicely with other language bindings (such as
+SWIG-based ones).
+
+Some objects currently live for the whole duration of the simulation and do
+not have refertence counts. We still provide dummy `intrusive_ptr_add_ref(p)`,
+`intrusive_ptr_release(p)` and `FooPtr` for consistency.
+
+In many cases, we try to have a API which is consistent with the API or
+corresponding C++ standard classes. For example, the methods of
+`simgrid::s4u::Mutex` are based on [`std::mutex`](http://en.cppreference.com/w/cpp/thread/mutex).
+This has several benefits:
+
+ - we use a proven interface with a well defined and documented semantic;
+
+ - the interface is easy to understand and remember for people used to the C++
+ standard interface;
+
+ - we can use some standard C++ algorithms and helper classes with our types
+ (`simgrid::s4u::Mutex` can be used with
+ [`std::lock`](http://en.cppreference.com/w/cpp/thread/lock),
+ [`std::unique_lock`](http://en.cppreference.com/w/cpp/thread/unique_lock),
+ etc.).
+
+Example of `simgrid::s4u::Actor`:
+
+~~~
+class Actor {
+ // This is the corresponding maestro object:
+ friend simgrid::simix::Process;
+ simgrid::simix::Process* pimpl_ = nullptr;
+public:
+
+ Actor(simgrid::simix::Process* pimpl) : pimpl_(pimpl) {}
+ Actor(Actor const&) = delete;
+ Actor& operator=(Actor const&) = delete;
+
+ // Reference count is delegated to the S4u object:
+ friend void intrusive_ptr_add_ref(Actor* actor)
+ {
+ xbt_assert(actor != nullptr);
+ SIMIX_process_ref(actor->pimpl_);
+ }
+ friend void intrusive_ptr_release(Actor* actor)
+ {
+ xbt_assert(actor != nullptr);
+ SIMIX_process_unref(actor->pimpl_);
+ }
+ using Ptr = boost::intrusive_ptr<Actor>;
+
+ // Create processes:
+ static Ptr createActor(const char* name, s4u::Host *host, double killTime, std::function<void()> code);
+
+ // [...]
+};
+
+using ActorPtr = Actor::Ptr;
+~~~
+
+It uses the `simgrid::simix::Process` as a opaque pimple:
+
+~~~
+class Process {
+private:
+ std::atomic_int_fast32_t refcount_ { 1 };
+ // The lifetime of the s4u::Actor is bound to the lifetime of the Process:
+ simgrid::s4u::Actor actor_;
+public:
+ Process() : actor_(this) {}
+
+ // Reference count:
+ friend void intrusive_ptr_add_ref(Process* process)
+ {
+ // Atomic operation! Do not split in two instructions!
+ auto previous = (process->refcount_)++;
+ xbt_assert(previous != 0);
+ (void) previous;
+ }
+ friend void intrusive_ptr_release(Process* process)
+ {
+ // Atomic operation! Do not split in two instructions!
+ auto count = --(process->refcount_);
+ if (count == 0)
+ delete process;
+ }
+
+ // [...]
+};
+
+smx_process_t SIMIX_process_ref(smx_process_t process)
+{
+ if (process != nullptr)
+ intrusive_ptr_add_ref(process);
+ return process;
+}
+
+/** Decrease the refcount for this process */
+void SIMIX_process_unref(smx_process_t process)
+{
+ if (process != nullptr)
+ intrusive_ptr_release(process);
+}
+~~~
+
+@section simgrid_uhood_async Asynchronous operations
+
+@subsection simgrid_uhood_futures Futures
