[Qt-interest] signals and slots....revisited...

[Scott Aron Bloom]

Very easily...

 

The SIGNAL and SLOT macro normalize the interfaces...

 

Then the QObject connect function (at the high level) creates a mapping
from the slot (or signal) to  the signal..

 

Its all in the code

 

Scott

 

From: qt-interest-bounces+scott.bloom=onshorecs.com at qt.nokia.com
[mailto:qt-interest-bounces+scott.bloom=onshorecs.com at qt.nokia.com] On
Behalf Of sarvesh saran
Sent: Monday, March 14, 2011 2:44 AM
To: qt-interest at trolltech.com
Subject: Re: [Qt-interest] signals and slots....revisited...

 

Hi,

 

which brings up this question again.....how is the connection made at
run time? 

 

thanks,

Sarvesh

On Mon, Mar 14, 2011 at 2:14 PM, Bo Thorsen <bo at fioniasoftware.dk>
wrote:

Den 14-03-2011 09:22, sarvesh saran skrev:

	 

	Hi Bo,
	
	thank you for your answer. There was one question that is still
	unanswered...
	
	   5. QT stores the names slots and signals of an object in its
	     meta object? in what format.....<name,function pointer> ?
	
	
	/Something like it, yes. But you don't need to know. Just use
the SLOT
	and SIGNAL macros to access them./
	/
	/
	I would like to know...at least at a higher abstraction. The
SLOT and
	SIGNAL macros would generate a string representation of the
signal and
	the slot containing the name, arguments etc (kind like a mangled
C++
	method name?). These names are then looked up (where?) and the
	appropriate slots are called ( i guess through the meta
	object?..qt_metacall).

 

I did this:

QLOG_DEBUG() << "Slot:" << SLOT(onFirmwareUpdateAvailable(QString)) <<
"signal:" << SIGNAL(customContextMenuRequested(QPoint));

And got this output:

Slot: 1onFirmwareUpdateAvailable(QString)
signal: 2customContextMenuRequested(QPoint)

These really are just strings with a 1 for a slot and 2 for a signal.
connect() compares these to the lists in the generated moc code and
finds the proper signal or slot.

	 

	I want this info for a variety of reasons...i was under the
belief that
	signals and slots involve function pointers until some one very
recently
	contradicted this...so i would like some clarification as to how
the
	signals/slots are mapped,looked up and called?

 

You need to read the generated moc code.

On Mon, Mar 14, 2011 at 12:40 PM, Bo Thorsen <bo at fioniasoftware.dk

<mailto:bo at fioniasoftware.dk>> wrote:

   Hi Sarvesh,

   Den 13-03-2011 19:00, sarvesh saran skrev:

       I have been a Qt programmer for quite some time now and i
understand
       most of the general features of Qt. I am still confused  about
how
       signals and slots work...

       this is what i know (correct me if i am wrong)

          1. slots are like ordinary member functions.however they can
be

             called via signals irrespective of their access specifier.


   Yes. A slot is just a method.

          2. emit is a macro that expands to nothing.


   Correct. Your code would work without "emit" in front of signals,
   but it's useful for Qt programmers to communicate that this is a
   signal emit, so you have no way of knowing how long time it will
take.

          3. when you write emit mysignal() you are just calling a
function

             called mysignal which has been implemented in the
       generated moc file.


   Correct.

          4. for every object (instantiated from a class containing
       Q_OBJECT

             macro and inheriting QObject) an addition meta object is
             instantiated ?


   Only if your QObject subclass also have the Q_OBJECT macro.

   class X : public QObject { ... }

   If you don't have Q_OBJECT in the ... there is no additional meta
   object.

          5. QT stores the names slots and signals of an object in its
         meta

             object? in what format.....<name,function pointer> ?


   Something like it, yes. But you don't need to know. Just use the
   SLOT and SIGNAL macros to access them.

          6. connections between objects are made at runtime...how??
       not sure

             how and where a slot is searched for....


   The SLOT and SIGNAL macros generates a string based on name, type
   (slot or signal) and arguments. Then it does a string compare until
   it finds the one it's looking for.

          7. but the arguments in the connect call are matched at
       compile time?


   No, it's all runtime. If you need compile time matching, boost
   signals is the solution.

          8. while connecting, only the arguments of the slot is

             checked...surplus arguments are discarded.


   Correct. You can have fewer arguments on the slots than there are on
   the signals, and the generated moc code will just disregard those.
   But you can't have more arguments on the slots than there are on the
   signals.

          9. SIGNAL and SLOT are macros......not sure what their
       purpose is....


   They generate the string from the slot/signal signature.


       i tried to look into the generated moc file but things are still
       murky..any help will be greatly appreciated.


   I hope this helps :)

   Bo Thorsen,
   Fionia Software.

   --

   Expert Qt and C++ developer for hire
   Contact me if you need expert Qt help
   http://www.fioniasoftware.dk





Bo Thorsen,
Fionia Software.

-- 

Expert Qt and C++ developer for hire
Contact me if you need expert Qt help
http://www.fioniasoftware.dk

 

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