Signals & Slots

[Sean Kelly]

Bill Baxter wrote:
> David B. Held wrote:
>> I know much has been said about this topic over the years, and several 
>> libraries have been written.  Can anyone summarize the state of the 
>> art for S&S in D?  I know there is a std.signals, but I can see that 
>> it is not the end of the story...
> 
> I'd like to know what the current story is too, both in Phobos and 
> Tango.  Mainly I mean the current story with regard to weak references.

Tango does not currently support weak references.  This was a deliberate 
decision on my part because I do not feel that weak references are 
compatible with garbage collection in D.  In a multithreaded program, 
the only way to safely use the callback mechanism built to support the 
weak reference concept is by using lock-free programming (using 
synchronized blocks instead is a sure way to deadlock an application 
when using "stop the world" collections).  However, lock-free 
programming is quite complicated and utterly non-portable, given its 
reliance on specialized knowledge of the target memory model. 
Therefore, I can't rationalize adding a feature that is so prone to 
error and misuse unless it provides an extremely compelling reason for 
its presence.  And I'll admit that I question the point of needing the 
weak reference feature for S&S in the first place.  I feel that doing so 
encourages bad programming style, and frankly, I can't envision a 
programming model that requires it anyway.  But then I'm not a Qt 
person, so perhaps someone could explain the need for this to me?

If it's any consolation, the Tango/Phobos merger will basically require 
that weak reference support be added to the runtime unless Phobos 
decides this feature is no longer necessary.  So my opinion in the 
matter may ultimately be rendered unimportant.  Unfortunately, this will 
complicate the means by which object monitors are overridden in Tango, 
but I don't think there's any way around this.


Sean