[Development] Disabling exception support in QtCore?

[Knoll Lars]

On 10/10/13 6:02 PM, "Olivier Goffart" <olivier at woboq.com> wrote:

>On Thursday 10 October 2013 08:22:44 BRM wrote:
>
>> I have personnally maintained a 400k+ SLOC codebase based on QT.
>> It made extensive use of Signals/Slots between objects. Even though I
>>was
>> pretty much the only developer working on it, I still had to quite often
>> track through signals/slots to make sure I understood things. Not
>>because
>> the codebase was unclear, but because it was quite non-trivial. It would
>> have been an extensive PITA to try to know which ones I could or could
>>not
>> use exceptions with.
>> 
>> I'm no compiler expert - but without help from the compiler to make sure
>> that kind of things doesn't happen - e.g. marking signals/slots as
>> Q_EXCEPTION_THROW and Q_EXCEPTION_SAFE and having some part of the
>>compiler
>> chain detect and at minimal warn of issues - it will not be feasible.
>
>
>Programming is not easy.
>Now, our goal is to make programming with Qt as easy as possible.
>
>Tell me what makes programming easier:
>
>1) We forbid any use of exception in any slot connected to any signal.
>     - Programmers that don't use exceptions don't care because it does
>not 
>impact them.
>     - Programmers that uses exception in their application to forward
>errors 
>cannot use signals and slots to propagate exceptions from one object of
>their 
>application to another anymore.
>
>2) We allow slots to forward the exception to the signal.
>    - Programmers that don't use exceptions still don't care because it
>does 
>not impact them.
>    - Programmers that use exceptions can hapilly let exceptions go from
>the 
>slot to where the signal is emitted.
>
>
>If you are not using exception now as I understand from your text above,
>then 
>nothing changes for you.
>
>However, someone who is using currently exceptions, they will have to
>change 
>their code for more complicated code because we decided to forbid it
>("for 
>their own good")
>
>May I recall that exceptions through signals and slots always worked, and
>was 
>used.    And now you need to give a good reason to forbid it.
>Reducing the size of QtCore might be a good reason,  but just saying
>"it's for 
>your own good" is not.

Agree with Olivier.

Let's keep the level of support we had in Qt 4.x for throwing exceptions
from slots:

* It's undefined behaviour unless the place that emits the signal is
prepared to catch the exception.
* If a slot throws, no further slots connected to the signal will get
called
* throwing from any place that's connected to a signal defined in the Qt
libraries leads to undefined behaviour

The size reduction of QtCore is not a valid argument IMO. First of all
we're talking about a number below 10%, and secondly this number could be
reduced to almost 0 by making the build system for Qt Core only compile a
few selected files with exception support enabled.

Cheers,
Lars