Re: [boost] [signals] connection death notification

From: Douglas Gregor (doug.gregor_at_[hidden])
Date: 2006-12-15 08:27:31

On Mon, 2006-12-04 at 11:40 -0600, Bryan Silverthorn wrote:
> Is there any mechanism in boost::signals that could provide notification 
> when a slot/connection dies (is disconnected)?
> 
> This functionality is needed to "chain" signals effectively. Imagine 
> wrapping an existing signal-based library with a signal-based interface 
> of some other type: a user connects to some signal on the wrapper, which 
> then connects to some corresponding signal in the inner library. If the 
> user disconnects from the wrapper's signal, the wrapper library would 
> like to disconnect its associated connection to the inner library 
> signal---but it has no way of knowing when the user's connection dies.
> 
> Am I missing some trick that would give me this functionality?

I think it can be done... the slot can contain an arbitrary function
object. When the slot is disconnected, that function object will be
destructed (there might be a delay in the case of recursive signal
calls). So if you had a shared_ptr stored in that function object, which
pointed to something that disconnected the other connection when it was
destroyed, you could get nearly the same behavior.

There might be a slightly cleaner way using trackable objects and
perhaps overriding visit_each in a clever way... but I'm not sure.

>  Would a 
> patch implementing this functionality be accepted? Any suggestions for 
> such an interface?

I'd definitely like to see what the interface would look like. This
seems like generally useful functionality, but I'm a little nervous
about making the Signals interface much bigger.

  Cheers,
  Doug