[Development] Disabling exception support in QtCore?

[Knoll Lars]

On 02.10.13 09:09, "Olivier Goffart" <olivier at woboq.com> wrote:

>On Tuesday 01 October 2013 23:32:00 Thiago Macieira wrote:
>> On quarta-feira, 2 de outubro de 2013 05:42:24, Knoll Lars wrote:
>> > On 01.10.13 23:23, "Thiago Macieira" <thiago.macieira at intel.com>
>>wrote:
>> > >On terça-feira, 1 de outubro de 2013 20:00:56, Knoll Lars wrote:
>> > >> Yes, signal/slot connections between user code should IMO still be
>>able
>> > >> to pass through exceptions. I am afraid removing that will break
>>code
>> > >>that's out there.
>> > >
>> > >This is already forbidden since 5.0.
>> > 
>> > Is there any good reason why we don't support this, as we did in Qt
>>4? My
>> > goal of still compiling Qt Core with exceptions enabled was to allow
>>for
>> > exactly this use case.
>> 
>> Because we don't test it, so there's no guarantee that it works. In
>>fact,
>> I'd be surprised if it works at all. What's more, our types aren't
>> exception-safe, even the container types. It's entirely unknown what
>>will
>> happen during a stack unwind.
>> 
>> Personally, I don't want to maintain that functionality.
>> 
>> And then there's the question: is it worth the 7% increase in code size?
>
>It is working. We even got bug report for some corner case where it did
>not, 
>and I fixed those. (so they are used)
>There is no test because you did not want to have one. But we could
>easily add 
>more auto tests. (It is really not that difficult)
>And it is "forbidden" only because you decided so, but it is not actually
>forbidden.
>
>I personally think letting exception go from the slot to the signal is a
>good 
>feature (for the DirectConnection case).
>I am willing to maintain this functionality.
>
>Of course it is not allowed to throw an exception with QueuedConnection,
>or if 
>the caller of the signal is not exception safe (all signals in Qt).
>And if a signal is connected to several slots, further slots are not
>going to 
>be called.
>
>The only stack frame it needs to crosses are QMetaObject::activate, the
>moc 
>generated code for signals or qt_metacall, and the templated helpers in
>qobject(defs)_impl.h.  Could we perhaps only compile part of QtCore with
>exception support?
>
>Exceptions are not only about std::bad_alloc.
>I think it is unfortunuate that we removed support for user thrown
>exceptions 
>from our container.  (I agree that supporting bad_alloc is useless).
>But supporting throwing constructor and copy constructor in all the
>containers 
>is a bit harder than in signals and slots. (And also less usefull)

+1. It's our decision not to use exceptions in Qt code, but I see quite a
bit of value in being able to throw exceptions from a slot if that's the
pattern a developer chooses to use. We've been doing quite a bit of work
to allow this in Qt 4, and it's the main place where exception safety in
Qt Core really makes sense IMO.

Cheers,
Lars