Frank Mori Hess wrote:
If my listen() method were able to tease apart the object returned from bind(), it could detect the case of a shared_ptr<boost::trackable subclass>.
It could, by using boost::visit_each. boost::bind supports visit_each to let you apply a visitor to all the objects bound inside the returned functor. The signals libraries use visit_each to discover trackable objects that have been bound into slots.
I would spend some time learning how to use visit_each with bind. You could implement a wrapper function that inspects slots for your own Trackable base class objects, and then takes whatever step is needed to extract a shared_ptr and pass it to slot::track before connecting.
Thanks for the suggestion! I'm excited about this approach, but I've run into a snag. As you note above, in addition to the case of my own Trackable base class, I want my visitor to detect a bound boost::shared_ptr<anything> and pass the shared_ptr to my new slot_type object's track() method. In fact, for a boost::shared_ptr<SomeTrackableSubclass>, I want the shared_ptr tracking to take precedence. As you said, this is the safer mechanism. I can in fact detect a bound shared_ptr and pass it to track() as I want. The problem is that binding a shared_ptr captures a copy, so the referenced object will live until the connection is explicitly disconnected! That makes the slot_type::track() mechanism moot. It looks as though I could only achieve what I want if my visit_each() visitor could *modify* the boost::bind object to replace the bound shared_ptr with its wrapped plain pointer. I don't believe this is possible? (Aside: I tried extending my visitor's detection to boost::weak_ptr, changing my test code by explicitly converting my shared_ptr to weak_ptr before passing it to boost::bind(). I couldn't instantiate slot_type(boost::bind(...boost::weak_ptr<anything>...)) until I specialized boost::get_pointer() myself. (I'm using Boost 1.34.1 augmented with the Signals2 library.) In any case, though, requiring coders to remember to explicitly wrap any boost::smart_ptr instance in boost::weak_ptr feels like the wrong solution.) Perhaps I could build a sufficiently smart boost::visit_each() visitor to reconstruct the original boost::bind() expression, but substituting a plain pointer for any smart_ptr? I'm not confident I can do that without relying on too many boost::bind implementation details. I'm hoping one of you will suggest a better approach. :-)
Just a note: you need to be careful about connecting these transient objects, since the signal can't employ shared_ptr to prevent them from destructing in mid-signal invocation.
Thanks for the warning. I'd prefer to handle bound shared_ptrs, if I can do so without making the referenced objects immortal.