[Qt-interest] Objective-C like message-passing

[Ross Bencina]

Hi Peter

I use QMetaObject::invokeMethod a little bit for this kind of thing. It's 
pretty easy to use once you realise that it doesn't use the same method 
signature syntax as connect():
http://doc.trolltech.com/4.5/qmetaobject.html#invokeMethod

I'm not sure what advantages/disadvantages your idea has over this though...

Ross.



----- Original Message ----- 
From: "Peter Kümmel" <syntheticpp at gmx.net>
To: <qt-interest at trolltech.com>
Sent: Tuesday, September 15, 2009 9:14 AM
Subject: [Qt-interest] Objective-C like message-passing


> One main feature of Objective-C/Smalltalk is their message-passing.
> Using this mechanism all OOP is done on a dynamic typing and late-binding 
> basis:
> http://en.wikipedia.org/wiki/Objective-C
> Very different here, is C++ with its compile time polymorphism.
>
> Having Qt it isn't very hard to add Objective-C like message-passing to 
> C++/Qt.
>
> I've pushed such a extension to gitorious:
> http://qt.gitorious.org/~syntheticpp/qt/syntheticpp/commits/message-passing
>
>
> What is the best way to do such message-passing using C++?
> First I had the idea to also use [] and to overload operator[] on QObject, 
> but
> this could break existing code where already operator[] is overloaded.
> After several other ideas I ended here:
>
> - passing a message to a QObject which doesn't takes arguments and returns 
> nothing:
>
>    obj->take("slotname")(); or
>
>    qmsg("slotname").to(obj)(); or
>
>    qmsg(obj, "slotname")();
>
>
> - passing a message to a QObject which takes a argument and returns 
> nothing:
>
>    obj->take("slotname")(111); or
>
>    qmsg("slotname").to(obj)(111); or
>
>    qmsg(obj, "slotname")(111);
>
>
> - passing a message to a QObject which takes a argument and returns a 
> value:
>
>    Type t = obj->take("slotname").reply<Type>(1111); or
>
>    Type t = qmsg("slotname").to(obj).reply<Type>(1111); or
>
>    Type t = qmsg(obj, "slotname").reply<Type>(1111);
>
>
> I wonder if there are better ways to do these calls, any ideas?
>
> Example code could be seen here:
> http://qt.gitorious.org/~syntheticpp/qt/syntheticpp/blobs/message-passing/examples/messagepassing/simple/main.cpp
>
> And typedefing QMetaMessage to qmsg allows it to use unpatched Qts by 
> simply including
> http://qt.gitorious.org/~syntheticpp/qt/syntheticpp/blobs/message-passing/src/corelib/kernel/qmetamessage.h
>
>
>
> It is very interesting to have such non-C++ way of invoking methods on
> objects where at runtime only is known that they are a QObjects (Qt's 
> NSObject).
> Using message-passing makes it possible 'think different' about 
> architectures.
> it's at the other end of compile-time type-checking.
>
> It would also be interesting to implement more Objective-C like behavior 
> with Qt.
>
>
> Cheers,
> Peter
>
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