Dynamic binding -- Qt's Signals and Slots vs Objective-C

[Bill Baxter]

There seems to be a lot in common between Qt's signals and slots (Qt 
S&S) and Objective-C's messaging system.  Both basically offer a way to 
call a method using a string rather than a pointer, i.e they allow 
run-time binding and dispatch.  Both seem to be considered Really Good 
Things by those who use them, primarily for the purpose of creating GUIs.

So is there anyone out there knowledgeable about both who could compare 
and contrast?  This link from Fredrik, http://tinyurl.com/kojmm, helped 
me understand Obj-C a little, but I've got more questions:

- Is Qt S&S a subset of Obj-C's messaging?
- Could Qt S&S be implemented using Obj-C messaging?
- Could Obj-C-like messaging be implemented using Qt S&S?
- Qt S&S includes an observer pattern in that emit() will call all 
connected slots.  Is this built into Obj-C messaging too?
- Qt requires derivation from QObject to participate in S&S.  Does Obj-C 
also require deriving from a particular base object?  Is it explicit?
- What is the overhead to support Obj-C messaging in a class?  Is there 
a way to create an Obj-C class that doesn't support messaging?
- Do people use Obj-C messaging for anything besides GUIs?

The goal of this questioning, of course, is to encourage discussion 
about whether these or any other form dynamic binding could / should be 
added to D.

I've heard many people in C++ land say things to the effect of "Bah, 
those silly Qt people weren't smart enough realize that you can 
implement signals and slots directly in C++".  But I've not been 
particularly impressed by the pure C++ S&S libraries I've seen.  On the 
other hand, I found Qt's S&S to be simple, clean, straightforward, and 
very easy to use.  And I've not heard anything but praise about making 
GUIs with Obj-C.  Lutger's S&S for D posted earlier looks very nice, but 
there must be something neat that Qt S&S--being dynamic--can do that 
isn't possible with the static S&S solutions.  Like enumerating 
available signals for use in a GUI builder tool.

- Is it possible to mix D-style classes with Obj-C style messaging?
Apple's Objective-C++ apparently doesn't allow it.
http://developer.apple.com/releasenotes/Cocoa/Objective-C++.html
I.e. a class is either a C++ class or an Obj-C class.  They can call 
each other, but they are separate.  You can't send an Obj-C message to a 
C++ class.  I think the ideal, though, would be something closer to Qt 
where you have the ability to make any method you want be a slot.  In Qt 
those methods remain valid C++ methods, but you can also call them by 
name, effectively.

I think what it comes down to is that you want to have as many tools 
available as possible in your toolbox.  Some things are just easier to 
do dynamically (for flexibility and loose coupling), and other things 
are better done statically (for runtime speed and robust compile-time 
type-checking).  I think that explains the growing popularity of hybrids 
like Jython and JRuby.  C# probably fits in there somewhere too.

--bb