Hi, Currently the wait callback parameter is either a promise or packaged_task template<typename F> void set_wait_callback(F f); prom.set_wait_callback(f); // will call f(&prom) task.set_wait_callback(f); // will call f(&task) I would like to have a wait callback with a user specific parameter. template<typename F,typename U> void set_wait_callback(F f,U* u); // will call f(u) prom.set_wait_callback(f, x); // will call f(x) The use case is a class that contains a promise and wants to provide a set_wait_callback. Which parameter will have the wait calback? A promise. But the user is not aware that my class has promise, and he will surely want to have as parameter a pointer to my class. So the simpler is to provide a user specific parameter that is defaulted to instance providing the wait_callback setting. Of course my class can wrap the user function and the class instance template <typename R> class X { template <typename F> static void wait_callback_wrapper(promise<R>* p, F f, X*x) { f(x); } template<typename F> void set_wait_callback(F f) { prom_.set_wait_callback(bind(wait_callback_wrapper, _1, f, this)); } }; But this implies much more resources, two binds instead of one. Could this be added without to much trouble? Thanks, Vicente