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

[sarvesh saran]

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 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?

thanks,
Sarvesh

On Mon, Mar 14, 2011 at 12:40 PM, Bo Thorsen <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
>
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