Futures can have a registered Callback of type std::function<void(const uint32_t&)> which gets called in a new Fiber with the result of the IO Request. Note the first completion will cause the execution of a callback and partial completion support must be implemented manually in the callback.
Callbacks are stored in a heap allocated std::function on registration and are freed by the new Fiber after the callback returned;
The Future with a registered Callback is not referenced in any way in the IO subsystem and therefore can be dropped after being submitted. This also means that a Future with a registered callback will not be signaled by the IO subsystem on completion. If signaling is desired one must implement it manually in the registered callback.