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

reply Bill Baxter <wbaxter gmail.com> writes:
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
Sep 21 2006
next sibling parent reply Fredrik Olsson <peylow gmail.com> writes:
First some rech stuff :).

How Qt S&S does it:

Each object can have signals and slots. A signal is an event that can be 
signaled to the "runtime", and a slot is a method that is performed in 
response to a signal.

You connect signals and slots like this:
connect(&obj1, SIGNAL(valueChanged(int)), &obj2, SLOT(setValue(int)));
You are free to hook up a signal to as many slots as you like, and to 
different objects.

And you may then later emit this signal (From obj1) using:
emit valueChanged(42);
which will cause the setValue(int) slot to be performed.


How Obj-C Cocoa does it:

Each object can have an action and a target. An action is an event that 
is signaled at run-time, and the target is an object instance to call. 
If target is nil (null) then the action will be sent to the responder 
chain, instead of a predestined object.

The responder chain is implemented in the NSResponder protocol 
(Interface) and will send the action down the controllers is logical 
order acording to how windows are stacked, ending up in the 
NSApplication instance if no one else is interested. This make sure that 
if a single menu item (OSX == one menu you know :) ) has nil as target, 
then the topmost window will get a chance to respond first, and yet the 
same item will work for all windows.

You connect an action/target like this:
[obj1 setAction: selector(valueChanged:)];
[obj1 setTarget:obj2];

And you can later "emit" the action using:
[obj1 sendAction:[obj1 action] to:[obj1 target]];
which will cause the valueChanged: method of obj2 to be performed. In 
reality most classes have shortcuts such as sendAction, and even more so 
you rarely need to care about sending manually at all.


Bill Baxter skrev:
<sbip>
 - Is Qt S&S a subset of Obj-C's messaging?

many slots, while Obj-C have only one target. In short, one button click invokes a single action, in Obj-C, while in Qt it could invoke a series of actions. This is more of a design limitation in Cocoa, not in Objective-C. On the plus-side of Cocoa is that a "signal"/"action" do not need to have a target set, but can dynamically find it's target using a responder chain, for example targetting the top-most windows document controller from a single menu item.
 - Could Qt S&S be implemented using Obj-C messaging?

 - Could Obj-C-like messaging be implemented using Qt S&S?

implemented.
 - Qt S&S includes an observer pattern in that emit() will call all 
 connected slots.  Is this built into Obj-C messaging too?

would be performAction. But it can only be hooked up to a single target/slot.
 - Qt requires derivation from QObject to participate in S&S.  Does Obj-C 
 also require deriving from a particular base object?  Is it explicit?

those subclasses will let you use say [foo sendAction], and such shorter things. In the same way subclasses of NSResponder will automatically fit in the responder chain. But in practice, Obj-C will not care as much of the classes, but of what the classes implement. So if you send the action foo: to bar what the run-time will do is a: if ([bar respondsToSelector: selector(foo:)]) { [bar performSelector: selector(foo:)]; } And yes, if it does not respond, it is no error, unless you want it to be, it is perfctly legal to call methods that are not implemented :).
 - 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?

it, each "method call" is a message dispatch. And there is a overhead as compared to C++. The run-time uses caching and other neat tricks, so in practice a Obj-C method call cost about 2.5 C++ method calls.
 - Do people use Obj-C messaging for anything besides GUIs?

messages. Best of all is what is called categories. You can extend a class by subclassing, or by adding a category. NSArray is a base class for static arrays. NSMutableArray is a subclass of NSArray that is mutable (dynamic). If you wanted to add a method say something useful as reversedArray to them, then in C++ (And D) you would need to make two subclasses, one to NSArray, and one to NSMutableArray, and it would be dead on impossible make a function accept both without typecasting (Ok, add an interface then). In Obj-C you can instead add a category to NSArray, and it will automatically be added to all subclasses as well. Like this: interface NSArray (ReversedCat) - (NSArray *)reversedArray; end And named arguments rocks, for code readability: myFoo.write("bar.txt", true); vs [myFoo writeToFile: "bar.txt" atomically:YES]; Same thing cn be done with lots of comments, but is rarely done. And short names have already become a bad habit/convention anyway.
 
 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 see no real problem in linking with the GNU Obj-C runtime. And Obj-C as such is very tiny, the power lies in the flexibility of the little there is. Should: Not too sure. I would like it, my only problem with Obj-C is the C part, so replacing that with D would be nice. But I seriously doubt it is worth the trouble, mostly because it is a quite different paradigm, and before you understand it you cannot appreciate it.
 
 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.
 

object that will have a method called the reciever. What C++/D does is very simply put: classes are structs with hidden function pointers, when calling a method it looks up the function pointer at a static offset from the reciever, and jumps. And the class better make sure there is a valid function at the other end. What Obj-C does is that it takes the reciever, and requested method message and send to objc_msgSend(), that function looks up the correct function pointer for the given reciever and method pair, if found it jumps there. If not found it can raise en exception, pass control to a "unhandled message" method, or silently ignore. Oh well, hopefully you have less question now at least :). // Fredrik Olsson
Sep 25 2006
next sibling parent reply Bill Baxter <wbaxter gmail.com> writes:
Fredrik,
Thanks for this very informative reply.
It definitely seems from your explanations that just going for "Qt-style 
signals and slots" in D is "aiming low", and that a basic dynamic 
messaging system is more the level of abstraction you'd want.  And from 
there you can implement Qt-style S&S easily, as you demonstrated.

However, I think taking a 2.5x speed hit (worse for a poorly optimized 
implementation) for all method calls will be difficult for D users to 
swallow.

It sounds like basically what is required in terms of per-class storage 
is a second sort of vtable per class, a dynamic vtable.  Maybe more like 
a dictionary/hash-table, but anyway you need some place to store 
per-class mappings from messages to actual functions.  It could probably 
be pointed to from the regular vtable in some manner so that each class 
instance still only has the overhead of one vtable pointer.

You can go ahead and put every method in the dynamic dispatch vtable. 
It's just per-class, so not a huge overhead.  Kind of like how every 
non-private method in D is virtual.  Then I guess it needs to be the 
caller's choice whether to use dynamic dispatch or static.  So a 
separate syntax for making "message"-based calls, like Objective C uses.

Gotta go, but I think some sort of built-in dynamic dispatch mechanism 
like this would have to be part of my "dream language".

--bb

Fredrik Olsson wrote:
 First some rech stuff :).
 
 How Qt S&S does it:
 
 Each object can have signals and slots. A signal is an event that can be 
 signaled to the "runtime", and a slot is a method that is performed in 
 response to a signal.
 
 You connect signals and slots like this:
 connect(&obj1, SIGNAL(valueChanged(int)), &obj2, SLOT(setValue(int)));
 You are free to hook up a signal to as many slots as you like, and to 
 different objects.
 
 And you may then later emit this signal (From obj1) using:
 emit valueChanged(42);
 which will cause the setValue(int) slot to be performed.
 
 
 How Obj-C Cocoa does it:
 
 Each object can have an action and a target. An action is an event that 
 is signaled at run-time, and the target is an object instance to call. 
 If target is nil (null) then the action will be sent to the responder 
 chain, instead of a predestined object.
 
 The responder chain is implemented in the NSResponder protocol 
 (Interface) and will send the action down the controllers is logical 
 order acording to how windows are stacked, ending up in the 
 NSApplication instance if no one else is interested. This make sure that 
 if a single menu item (OSX == one menu you know :) ) has nil as target, 
 then the topmost window will get a chance to respond first, and yet the 
 same item will work for all windows.
 
 You connect an action/target like this:
 [obj1 setAction: selector(valueChanged:)];
 [obj1 setTarget:obj2];
 
 And you can later "emit" the action using:
 [obj1 sendAction:[obj1 action] to:[obj1 target]];
 which will cause the valueChanged: method of obj2 to be performed. In 
 reality most classes have shortcuts such as sendAction, and even more so 
 you rarely need to care about sending manually at all.
 
 
 Bill Baxter skrev:
 <sbip>
 
 - Is Qt S&S a subset of Obj-C's messaging?

I think Qt's S&S is more of a superset. Qt allows a signal to trigger many slots, while Obj-C have only one target. In short, one button click invokes a single action, in Obj-C, while in Qt it could invoke a series of actions. This is more of a design limitation in Cocoa, not in Objective-C. On the plus-side of Cocoa is that a "signal"/"action" do not need to have a target set, but can dynamically find it's target using a responder chain, for example targetting the top-most windows document controller from a single menu item.
 - Could Qt S&S be implemented using Obj-C messaging?

Yes, no problem. And no hacks :).
 - Could Obj-C-like messaging be implemented using Qt S&S?

Yes, with some problem. The responder chain mechanism needs to be implemented.
 - Qt S&S includes an observer pattern in that emit() will call all 
 connected slots.  Is this built into Obj-C messaging too?

As I mentioned Cocoa only use one action. So the equivalent of emit() would be performAction. But it can only be hooked up to a single target/slot.
 - Qt requires derivation from QObject to participate in S&S.  Does 
 Obj-C also require deriving from a particular base object?  Is it 
 explicit?

More or less. All existing senders are subclasses of NSControl, so using those subclasses will let you use say [foo sendAction], and such shorter things. In the same way subclasses of NSResponder will automatically fit in the responder chain. But in practice, Obj-C will not care as much of the classes, but of what the classes implement. So if you send the action foo: to bar what the run-time will do is a: if ([bar respondsToSelector: selector(foo:)]) { [bar performSelector: selector(foo:)]; } And yes, if it does not respond, it is no error, unless you want it to be, it is perfctly legal to call methods that are not implemented :).
 - 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?

The Obj-C language is build upon this message dispatch, so no way around it, each "method call" is a message dispatch. And there is a overhead as compared to C++. The run-time uses caching and other neat tricks, so in practice a Obj-C method call cost about 2.5 C++ method calls.
 - Do people use Obj-C messaging for anything besides GUIs?

Oooh, yes. The real power here is in the dynamic dispatching of messages. Best of all is what is called categories. You can extend a class by subclassing, or by adding a category. NSArray is a base class for static arrays. NSMutableArray is a subclass of NSArray that is mutable (dynamic). If you wanted to add a method say something useful as reversedArray to them, then in C++ (And D) you would need to make two subclasses, one to NSArray, and one to NSMutableArray, and it would be dead on impossible make a function accept both without typecasting (Ok, add an interface then). In Obj-C you can instead add a category to NSArray, and it will automatically be added to all subclasses as well. Like this: interface NSArray (ReversedCat) - (NSArray *)reversedArray; end And named arguments rocks, for code readability: myFoo.write("bar.txt", true); vs [myFoo writeToFile: "bar.txt" atomically:YES]; Same thing cn be done with lots of comments, but is rarely done. And short names have already become a bad habit/convention anyway.
 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.

Could: YES! The first Obj-C compilers where simple preprocessors for C. I see no real problem in linking with the GNU Obj-C runtime. And Obj-C as such is very tiny, the power lies in the flexibility of the little there is. Should: Not too sure. I would like it, my only problem with Obj-C is the C part, so replacing that with D would be nice. But I seriously doubt it is worth the trouble, mostly because it is a quite different paradigm, and before you understand it you cannot appreciate it.
 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.

object that will have a method called the reciever. What C++/D does is very simply put: classes are structs with hidden function pointers, when calling a method it looks up the function pointer at a static offset from the reciever, and jumps. And the class better make sure there is a valid function at the other end. What Obj-C does is that it takes the reciever, and requested method message and send to objc_msgSend(), that function looks up the correct function pointer for the given reciever and method pair, if found it jumps there. If not found it can raise en exception, pass control to a "unhandled message" method, or silently ignore. Oh well, hopefully you have less question now at least :). // Fredrik Olsson

Sep 26 2006
parent reply Lucas Goss <lgoss007 gmail.com> writes:
Bill Baxter wrote:
 Gotta go, but I think some sort of built-in dynamic dispatch mechanism 
 like this would have to be part of my "dream language".
 
 --bb
 

I think I'm of the same opinion. I'm just starting to learn Obj-C now... lucas
Sep 27 2006
parent Fredrik Olsson <peylow gmail.com> writes:
Lucas Goss skrev:
 Bill Baxter wrote:
 Gotta go, but I think some sort of built-in dynamic dispatch mechanism 
 like this would have to be part of my "dream language".

 --bb

I think I'm of the same opinion. I'm just starting to learn Obj-C now... lucas

I want to agree as well. But hmm... how D and Obj-C tackles OOP is so very different that I can not see any way to merge the two. So just as you can not seamlessly mix C++ classes and Obj-C classes in Objective-C++, I can not see how D classes and Obj-C classes could be mixes seamlessly in any practical way. So if (a big IF) Obj-C type classes should ever be added to D I think an Objective-D dialect, just as Objective-C++. As a bonus there would: 1. Not be reinventing the wheel. 2. Reuse the GNU Obj-C runtime (Already optimized). 3. Reuse/link against GNUStep I think it would be just like C<->D where code is binary compatible, and similar enough for anyone to learn. And it has already been done, with Objective Modula-2, so there is a test case to learn from. And as I have said before; my only problem with Obj-C is C. So the thought of Objective-D appeals to me :). Garbage collection, foreach, etc. Hmm... all of which are features mentioned by Apple to be part of Objective-C 2.0 that comes next spring with OS X 10.5. But well, backward compatible can be a heavy anchor. // Fredrik Olsson
Sep 27 2006
prev sibling next sibling parent reply Lutger <lutger.blijdestijn gmail.com> writes:
Fredrik Olsson wrote:
<snip>
 How Obj-C Cocoa does it:
 
 Each object can have an action and a target. An action is an event that 
 is signaled at run-time, and the target is an object instance to call. 
 If target is nil (null) then the action will be sent to the responder 
 chain, instead of a predestined object.
 
 The responder chain is implemented in the NSResponder protocol 
 (Interface) and will send the action down the controllers is logical 
 order acording to how windows are stacked, ending up in the 
 NSApplication instance if no one else is interested. This make sure that 
 if a single menu item (OSX == one menu you know :) ) has nil as target, 
 then the topmost window will get a chance to respond first, and yet the 
 same item will work for all windows.
 
 You connect an action/target like this:
 [obj1 setAction: selector(valueChanged:)];
 [obj1 setTarget:obj2];
 
 And you can later "emit" the action using:
 [obj1 sendAction:[obj1 action] to:[obj1 target]];
 which will cause the valueChanged: method of obj2 to be performed. In 
 reality most classes have shortcuts such as sendAction, and even more so 
 you rarely need to care about sending manually at all.

Sounds good, but isn't it quite possible to implement this in D with a more flexible messaging system than templated S&S?
 - Qt requires derivation from QObject to participate in S&S.  Does 
 Obj-C also require deriving from a particular base object?  Is it 
 explicit?

those subclasses will let you use say [foo sendAction], and such shorter things. In the same way subclasses of NSResponder will automatically fit in the responder chain. But in practice, Obj-C will not care as much of the classes, but of what the classes implement. So if you send the action foo: to bar what the run-time will do is a: if ([bar respondsToSelector: selector(foo:)]) { [bar performSelector: selector(foo:)]; }

This is also a benefit of templates, no need for a superclass and free functions, literal functions and delegates work too.
 And yes, if it does not respond, it is no error, unless you want it to 
 be, it is perfctly legal to call methods that are not implemented :).
 
 - 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?

it, each "method call" is a message dispatch. And there is a overhead as compared to C++. The run-time uses caching and other neat tricks, so in practice a Obj-C method call cost about 2.5 C++ method calls.

According to QT, templated S&S is approximately 4x the cost of a function call and QT's S&S is 10x the cost. So 2.5 certainly looks acceptably to me, but this cost is paid for all calls right? And if I understood this correctly, this also means Obj-C cannot inline member functions?
 - Do people use Obj-C messaging for anything besides GUIs?

messages. Best of all is what is called categories. You can extend a class by subclassing, or by adding a category. NSArray is a base class for static arrays. NSMutableArray is a subclass of NSArray that is mutable (dynamic). If you wanted to add a method say something useful as reversedArray to them, then in C++ (And D) you would need to make two subclasses, one to NSArray, and one to NSMutableArray, and it would be dead on impossible make a function accept both without typecasting (Ok, add an interface then). In Obj-C you can instead add a category to NSArray, and it will automatically be added to all subclasses as well. Like this: interface NSArray (ReversedCat) - (NSArray *)reversedArray; end

Thanks for this explanation,I'm going to look into this Objective C thing as it seems interesting.
<snip>

 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.


Yes, you could say they fulfill different purposes. This article compares them and even makes a point of using them side by side: http://www.scottcollins.net/articles/a-deeper-look-at-signals-and-slots.html
 - 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.

object that will have a method called the reciever. What C++/D does is very simply put: classes are structs with hidden function pointers, when calling a method it looks up the function pointer at a static offset from the reciever, and jumps. And the class better make sure there is a valid function at the other end. What Obj-C does is that it takes the reciever, and requested method message and send to objc_msgSend(), that function looks up the correct function pointer for the given reciever and method pair, if found it jumps there. If not found it can raise en exception, pass control to a "unhandled message" method, or silently ignore.

Putting aside performance, it looks to me that the main difference is that Objective-C has good support for reflection. Maybe I'm wrong, but I'd think better reflection (wherever it comes from) support would enable to write the things you'd want to do in 'Objective-D' in D. Or not? Excuse my ignorance of Obj-C.
Sep 27 2006
parent reply Josh Stern <josh_usenet phadd.net> writes:
On Wed, 27 Sep 2006 17:55:03 +0200, Lutger wrote:

I found this comparison of different C++ approaches to signal slots
with a web search.

http://slotsig.sourceforge.net/doc/features-compare.html

Can't vouch for the validity of the summary, but it might be a good
starting point for further investigation.
Sep 27 2006
parent reply Lutger <lutger.blijdestijn gmail.com> writes:
Josh Stern wrote:
 On Wed, 27 Sep 2006 17:55:03 +0200, Lutger wrote:
 
 I found this comparison of different C++ approaches to signal slots
 with a web search.
 
 http://slotsig.sourceforge.net/doc/features-compare.html
 
 Can't vouch for the validity of the summary, but it might be a good
 starting point for further investigation.
 
 

Nice list, there are also a few benchmarks there. Depending on interpretation of course, my sigslots library scores equal points feature-wise as slotsig. There are some features in slotsig which might be worth investing, thank you for the link. These lists are always biased, but the biggest advantages of QT are not listed: dynamic connections which make it possible to, for example, script connections, load from xml resource, etc. This is where string based sigslots or - as I understand - Objective C is superior to the template-based implementations.
Sep 27 2006
parent reply Josh Stern <josh_usenet phadd.net> writes:
On Wed, 27 Sep 2006 20:44:05 +0200, Lutger wrote:

 Josh Stern wrote:
 On Wed, 27 Sep 2006 17:55:03 +0200, Lutger wrote:
 
 I found this comparison of different C++ approaches to signal slots
 with a web search.
 
 http://slotsig.sourceforge.net/doc/features-compare.html
 
 Can't vouch for the validity of the summary, but it might be a good
 starting point for further investigation.
 
 

Nice list, there are also a few benchmarks there. Depending on interpretation of course, my sigslots library scores equal points feature-wise as slotsig. There are some features in slotsig which might be worth investing, thank you for the link. These lists are always biased, but the biggest advantages of QT are not listed: dynamic connections which make it possible to, for example, script connections, load from xml resource, etc. This is where string based sigslots or - as I understand - Objective C is superior to the template-based implementations.

The issues of what has to be decided at design time, compile time, and run time in order to use signals and slots are important. Requiring inheritance from QObject for some things is one kind of strong restriction, and requiring connections specified at compile time is another. There are language vs. library issues involved here. A (the?) key language feature seems to be the question of what kind of support does the language provide for making it easy to attach a "hook" to a given function (where hook means roughly "now Mr.Ffunction, from now on, before you exit, please call this other function I'm about to tell you about with your return value as its input and maybe some other info like a pointer to your object in case you are a member function"). I agree with the implicit point made by the slotsig guy that part of this mechanism ideally should give the user options about thread-safety and thread-locality. Seems like if the language provides a really good hook mechanism, a lot of convenient library stuff can be built on top to taste. Intuitively it seems like D's system of Boxs and delegates could be put to good use towards finding attractive possibilities within the library problem design space, and having GC to eliminate most ownership issues makes many solutions a lot simpler.
Sep 27 2006
parent Lutger <lutger.blijdestijn gmail.com> writes:
Josh Stern wrote:
<snip>
 The issues of what has to be decided at design time, compile time, and 
 run time in order to use signals and  slots are important. 
 Requiring inheritance from QObject for some things is one kind
 of strong restriction, and requiring connections specified at
 compile time is another.  There are language vs. library issues
 involved here.  A (the?) key language feature seems to be the question of
 what kind of support does the language provide for making it easy to
 attach a "hook" to a given function (where hook means roughly "now
 Mr.Ffunction, from now on, before you exit, please call this other
 function I'm about to tell you about with your return value as
 its input and maybe some other info like a pointer to your object in case
 you are a member function"). I agree with the implicit point made by the
 slotsig guy that part of this mechanism ideally should give the user
 options about thread-safety and thread-locality. Seems like if the
 language provides a really good hook mechanism, a lot of convenient
 library stuff can be built on top to taste. Intuitively it seems like D's
 system of Boxs and delegates could be put to good use towards finding
 attractive possibilities within the library problem design space, and
 having GC to eliminate most ownership issues makes many solutions a lot
 simpler.   

Yes, although I would not see requiring connections at compile time as a restriction, I would file this under 'feature' as it enables compile-time type checking. It's a choice really, connections at runtime are more flexible but less safe and incur performance cost. Requiring inheritance (but more so custom preprocessor) in QT is needed to make up for lack of introspection in C++. Ideally you'd have both systems available to choose the appropiate one for what you want.
Sep 27 2006
prev sibling parent reply Bill Baxter <wbaxter gmail.com> writes:
Howdy.

Fredrik Olsson wrote:
 How Obj-C Cocoa does it:
 
 Each object can have an action and a target. 

By this you mean each GUI widget can have an action and target, right?
 An action is an event that 
 is signaled at run-time, and the target is an object instance to call. 
 If target is nil (null) then the action will be sent to the responder 
 chain, instead of a predestined object.

 The responder chain is implemented in the NSResponder protocol 
 (Interface) and will send the action down the controllers is logical 
 order acording to how windows are stacked, ending up in the 
 NSApplication instance if no one else is interested. 

So it forms a kind of tree or funnel with leaves being widgets, and the root being the NSApplication instance.
 This make sure that 
 if a single menu item (OSX == one menu you know :) ) has nil as target, 
 then the topmost window will get a chance to respond first, and yet the 
 same item will work for all windows.

Don't really follow you there. Why does the topmost window get the first chance? Just before you said it went up a chain, so it seems like the immediate container window of the item would get the first chance, and eventually the topmost window will get a chance. Anyway, this part of your description seems much more Cocoa than Objective C. I.e. it seems independent of the language itself, more or less just the classic "chain of responsibility" pattern. Just something implemented on top of the messaging sys provided by Obj-C.
 You connect an action/target like this:
 [obj1 setAction: selector(valueChanged:)];
 [obj1 setTarget:obj2];

 And you can later "emit" the action using:
 [obj1 sendAction:[obj1 action] to:[obj1 target]];
 which will cause the valueChanged: method of obj2 to be performed. In 
 reality most classes have shortcuts such as sendAction, and even more so 
 you rarely need to care about sending manually at all.

Ok, I'm trying to grok these message things now. It looks like there's no distinction between method names and call parameters. Like here:
 [obj1 setAction: selector(valueChanged:)];

But then here:
 [obj1 sendAction:[obj1 action] to:[obj1 target]];

And I take it [obj1 action] is a named attribute lookup, similar to a QProperty? Fredrik Olsson wrote:
 Lucas Goss skrev:
 Bill Baxter wrote:

 Gotta go, but I think some sort of built-in dynamic dispatch
 mechanism like this would have to be part of my "dream language".


I want to agree as well. But hmm... how D and Obj-C tackles OOP is so very different that I can not see any way to merge the two.

Is it really so difficult? Maybe I just don't understand how Obj-C works well enough, but in Obj-C you have both regular C function calls (do_something(foo)) and message passing ([obj doSomething:foo]) and the user decides which paradigm to use at what time. Similarly in an Obj-D you could have regular method calls (obj.do_something(foo)) and messaging calls ([obj doSomething:foo]). It seems like it could work more or less the way slots do in Qt. If I have: # slot: # void callMeLater(int val); In a Qt program, then callMeLater is a valid method that can be called like any other method. But it is *also* callable via dynamic lookup using QObject's under-the-hood moc-generated mechanisms. I don't see why a similar idea wouldn't work for adding a touch of Obj-C to D.
 So just as you can not seamlessly mix C++ classes and Obj-C classes in
 Objective-C++, I can not see how D classes and Obj-C classes could be
 mixes seamlessly in any practical way.

Maybe I'm just not seeing the full scope of how Obj-C works yet. But nothing I've heard so far seems like an insurmountable hurdle. It just doesn't seem a very attractive proposition to me to have to decide at initial design time whether my class will have regular methods or use dynamic messaging. I may know I want to start out with one or the other, but then I want my static class to recieve a particular message from a GUI. I guess these guys at Apple have given it some thought with Obj-C++, but my initial reaction was that keeping the types of classes completely separate must be some kind of corner cutting to get the thing out the door quickly, or perhaps having something to do with preserving C++ compatibility. --Bill
Sep 27 2006
parent reply Fredrik Olsson <peylow gmail.com> writes:
Bill Baxter skrev:
 Howdy.
 
 Fredrik Olsson wrote:
 How Obj-C Cocoa does it:

 Each object can have an action and a target. 

By this you mean each GUI widget can have an action and target, right?

A && T = Will trigger A in T. A && !T = Will trigger A in responder chain. !A = Will rigger nothing, ignores T.
 An action is an event that is signaled at run-time, and the target is 
 an object instance to call. If target is nil (null) then the action 
 will be sent to the responder chain, instead of a predestined object.

 The responder chain is implemented in the NSResponder protocol 
 (Interface) and will send the action down the controllers is logical 
 order acording to how windows are stacked, ending up in the 
 NSApplication instance if no one else is interested. 

So it forms a kind of tree or funnel with leaves being widgets, and the root being the NSApplication instance.

 This make sure that if a single menu item (OSX == one menu you know :) 
 ) has nil as target, then the topmost window will get a chance to 
 respond first, and yet the same item will work for all windows.

Don't really follow you there. Why does the topmost window get the first chance? Just before you said it went up a chain, so it seems like the immediate container window of the item would get the first chance, and eventually the topmost window will get a chance. Anyway, this part of your description seems much more Cocoa than Objective C. I.e. it seems independent of the language itself, more or less just the classic "chain of responsibility" pattern. Just something implemented on top of the messaging sys provided by Obj-C.

on Obj-C's ability to let objects forward invocations of methods they do not implement themselves. The topmost window needs the first chance as if for example the user presses Command-B for "make bold", then the first object that needs to know about it is probably on the topmost object, and with the cursor in it. The responder chain sends messages like this: 1. The key window’s first responder and successors, including objects added with setNextResponder: 2. The key window itself 3. The key window’s delegate (which need not inherit from NSResponder) 4. The main window’s first responder and successors, including objects added with setNextResponder: 5. The main window itself 6. The main window’s delegate (which need not inherit from NSResponder) 7. The application object, NSApp 8. The application object’s delegate (which need not inherit from NSResponder)
 You connect an action/target like this:
 [obj1 setAction: selector(valueChanged:)];
 [obj1 setTarget:obj2];

 And you can later "emit" the action using:
 [obj1 sendAction:[obj1 action] to:[obj1 target]];
 which will cause the valueChanged: method of obj2 to be performed. In 
 reality most classes have shortcuts such as sendAction, and even more 
 so you rarely need to care about sending manually at all.

Ok, I'm trying to grok these message things now. It looks like there's no distinction between method names and call parameters. Like here: > [obj1 setAction: selector(valueChanged:)]; setAction looks sort of like a method with parameter being a selector. But then here: > [obj1 sendAction:[obj1 action] to:[obj1 target]]; it looks like there are two methods, 'sendAction' and 'to'. And I take it [obj1 action] is a named attribute lookup, similar to a QProperty?

well :). Lets take this one: [obj1 sendAction:[obj1 action] to:[obj1 target]] It is actually three method calls, two of them nested. [obj1 action] is calling obj1's method action, and that method returns a selector, a selector is a method. [obj1 target] is also a method call, but returning an object instance. So you could rewrite that as: SEL selector = [obj1 action]; id target = [obj1 target]; [obj1 sendAction:action to:target]; This last statement is a single method call, with two arguments, and no result obviously. The name of the method is "sendAction:to:", so if you wanted the selector for that method you would do: SEL selector = selector(sendAction:to:); Methods with : in their names takes parameters, one parameter after each :, and sometimes a list of parameters after the last :. You would for example use the "fileExistsAtPath:isDirectory:" method of a NSFileManager object to check if a file exists, and if it then is a directory. Used like this: id fm = [NSFileManager defaultManager]; // Get the shared instance BOOL isDir = NO; BOOL doesExist = [fm fileExistsAtPath: "/tmp/foo" isDirectory:&isDir]; If this syntax look wierd, then here is the method definition :) : - (BOOL)fileExistsAtPath:(NSString *)path isDirectory:(BOOL *)isDirectory { ... }
 Fredrik Olsson wrote:
  > Lucas Goss skrev:
  >> Bill Baxter wrote:
  >>
  >>> Gotta go, but I think some sort of built-in dynamic dispatch
  >>> mechanism like this would have to be part of my "dream language".
  >>>
  >> I think I'm of the same opinion. I'm just starting to learn Obj-C 
 now...
  >
  > I want to agree as well. But hmm... how D and Obj-C tackles OOP is so
  > very different that I can not see any way to merge the two.
 
 Is it really so difficult?  Maybe I just don't understand how Obj-C 
 works well enough, but in Obj-C you have both regular C function calls 
 (do_something(foo)) and message passing ([obj doSomething:foo]) and the 
 user decides which paradigm to use at what time.  Similarly in an Obj-D 
 you could have regular method calls (obj.do_something(foo)) and 
 messaging calls ([obj doSomething:foo]).
 

Their "bhind the scenes" implementation is vastly different. And I see no easy way to treat a D-method as if it where a Obj-C-method, and vice versa.
 It seems like it could work more or less the way slots do in Qt.  If I 
 have:
 # slot:
 #   void callMeLater(int val);
 
 In a Qt program, then callMeLater is a valid method that can be called 
 like any other method.  But it is *also* callable via dynamic lookup 
 using QObject's under-the-hood moc-generated mechanisms. I don't see why 
 a similar idea wouldn't work for adding a touch of Obj-C to D.

and syntax compatibility with existing D-methods then you will have to make trade offs. And with those tradeoffs, you lose much of the power from Obj-C. And then a template based S&S solution would be best. In the end a template based S&S is probably the way to go anyway, D was never intended to use the Smalltalk school of OOP.
  > So just as you can not seamlessly mix C++ classes and Obj-C classes in
  > Objective-C++, I can not see how D classes and Obj-C classes could be
  > mixes seamlessly in any practical way.
 
 Maybe I'm just not seeing the full scope of how Obj-C works yet.  But 
 nothing I've heard so far seems like an insurmountable hurdle.
 
 It just doesn't seem a very attractive proposition to me to have to 
 decide at initial design time whether my class will have regular methods 
 or use dynamic messaging.  I may know I want to start out with one or 
 the other, but then I want my static class to recieve a particular 
 message from a GUI.  I guess these guys at Apple have given it some 
 thought with Obj-C++, but my initial reaction was that keeping the types 
 of classes completely separate must be some kind of corner cutting to 
 get the thing out the door quickly, or perhaps having something to do 
 with preserving C++ compatibility.
 

Objective-C does not touch anything of how C works. Objective-C is a pure superset on top. So Objective-C++ is in reality a programming language with two OOP implementations. One from the Simula school; the untouched C++, and one from the Smalltalk school; the very same superset as used in Objective-C. The ++-classes have no clue that the Objective-classes exists, and vice versa. "Objective" do only add one syntactic element to C (Or C++); message passing, to invoke methods. And for this it uses the [object message] syntax. For the rest of "Objective" the preprocessor roots can very easily be seen; for one all keywords begin with the character. selector(foo) interface MyClass : SuperClass end etc. And all they do is constructing constants, and "hidden" functions calls to setup the run-time at program start. Something that could easily be stuffed away in static constructors of each module in D. // Fredrik Olsson
Sep 28 2006
next sibling parent "d" <d nomail.com> writes:
 Hmm... now this is strange, and it took a while for me to grasp it as
 well :). Lets take this one:
 [obj1 sendAction:[obj1 action] to:[obj1 target]]
 It is actually three method calls, two of them nested.
 [obj1 action] is calling obj1's method action, and that method returns a
 selector, a selector is a method.
 [obj1 target] is also a method call, but returning an object instance.

The best explanation is that Objective-C is modelled after Smalltalk. A method in C might be: int AddAToX(int& a, int x); where as in Objective C it would be: - (int) Add: (int) a To: (int) x; where the Method signature is Add:To:. Smalltalk would use almost the same structure.
Sep 28 2006
prev sibling parent reply Bill Baxter <wbaxter gmail.com> writes:
Hey there,

Fredrik Olsson wrote:
 Bill Baxter skrev:
  > [obj1 sendAction:[obj1 action] to:[obj1 target]];
 it looks like there are two methods, 'sendAction' and 'to'.
 And I take it [obj1 action] is a named attribute lookup, similar to a 
 QProperty?

well :). Lets take this one: [obj1 sendAction:[obj1 action] to:[obj1 target]] It is actually three method calls, two of them nested. [obj1 action] is calling obj1's method action, and that method returns a selector, a selector is a method. [obj1 target] is also a method call, but returning an object instance.

That's very interesting. Can you also reverse the order of arguments in the call, like to:sendAction:? Or is that a different method? [obj1 to:[obj1 target] sendAction:[obj1 action]] --bb
Sep 30 2006
parent Fredrik Olsson <peylow gmail.com> writes:
Bill Baxter skrev:
 Hey there,
 
 Fredrik Olsson wrote:
 Bill Baxter skrev:
  > [obj1 sendAction:[obj1 action] to:[obj1 target]];
 it looks like there are two methods, 'sendAction' and 'to'.
 And I take it [obj1 action] is a named attribute lookup, similar to a 
 QProperty?

well :). Lets take this one: [obj1 sendAction:[obj1 action] to:[obj1 target]] It is actually three method calls, two of them nested. [obj1 action] is calling obj1's method action, and that method returns a selector, a selector is a method. [obj1 target] is also a method call, but returning an object instance.

That's very interesting. Can you also reverse the order of arguments in the call, like to:sendAction:? Or is that a different method? [obj1 to:[obj1 target] sendAction:[obj1 action]] --bb

- (void)sendAction:(SEL)action to:(id)target; But the method signature is always typeless and is thus: sendAction:to: In practice the type SEL is a char*, and selector(sendAction:to:) will be a char* to the null terminated string "sendAction:to:". What the selector() "macro" does is to guarantee that there is only a single pointer to each method signature string so that if ([item action] == selector(removeUser:)) { ... } is guaranteed to work. Would be nice if you could rearrange the arguments though. But well, they are part of the method signature, not named arguments. // Fredrik Olsson
Oct 02 2006
prev sibling parent Fredrik Olsson <peylow gmail.com> writes:
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
Content-Transfer-Encoding: 7bit

Bill Baxter skrev:
<snip>
 - Could Qt S&S be implemented using Obj-C messaging?

the root class NSObject so all classes get the functionality. And it turned out to require 174 SLOC (Source Lines Of Code), and then much is convenience code that is not really needed. A minimal implementation would easily go under 100 lines of code. The message passing is already build in, so all I did was using some existing classes to emulate the Qt behavior. One for sets to store a set of target objects. And then a hash map to store a dictionary (with the instance as key) of such sets for each instance with connected signals. Oh well, I attach the code, and an example using it. Compiles with Xcode 2.2. Should work with with OSX 10.0 and up, and GNUStep as well.
 - Could Obj-C-like messaging be implemented using Qt S&S?

like messaging, but with less optimization, only implemented for one special case, whereas Obj-C is made for the more general case. I see it as Obj-C messages with a C++ syntax.
 - Qt S&S includes an observer pattern in that emit() will call all 
 connected slots.  Is this built into Obj-C messaging too?

emulated. Just keep the targets in a collection instance such as NSSet, NSArray or NSDictionary, and you can use makeObjectsPerformSelector: to "call all". // Fredrik Olsson
Sep 26 2006