[Development] templated QObjects [was: Re: We are planning to upgrade qdoc to use clang for parsing C++]

[Milian Wolff]

On Mittwoch, 2. März 2016 10:14:18 CET Ziller Eike wrote:
> > On Feb 29, 2016, at 1:21 PM, Milian Wolff <milian.wolff at kdab.com> wrote:
> > 
> > On Friday, February 26, 2016 3:56:08 PM CET Thiago Macieira wrote:
> > 
> >> On sexta-feira, 26 de fevereiro de 2016 20:30:28 PST Milian Wolff wrote:
> >> 
> >>>> The main problems of templated QObject are captured more or less in
> >>>> this
> >>>> 
> >>>> thread:
> >>>> http://lists.qt-project.org/pipermail/development/2013-March/010288.htm
> >>>> l
> >>>> 
> >>>> Personally I still think it would be a fancy feature, a bit dangerous
> >>>> to
> >>>> 
> >>>> implement maybe even dangerous to use, but really cool :-D
> >>> 
> >>> 
> >>> Thanks for the link. How often is the MOC layout changed in an ABI
> >>> incompatible way?
> >> 
> >> 
> >> There's no historical pattern. There was a major break from Qt 3 to 4
> >> and
> >> smaller one from 4 to 5. Qt 4 did have updates that didn't break the
> >> ABI;
> >> the Qt 5.0 update removed the ability to read meta objects created with
> >> Qt
> >> 4 moc. I don't remember how the 1→2 and 2→3 updates happened (Qt 3 still
> >> used register-on-load meta objects).
> >> 
> >> But since the meta object itself has a version number and the only code
> >> that
 ever reads the internal data is inside QtCore, so we could make
> >> changes that change the layout.  We haven't done that: all changes
> >> between major releases have added things without changing the layout.
> >> 
> >> That's the structure layout though. The file itself compares the
> >> Q_MOC_OUTPUT_REVISION macro for equality (not ordering).
> > 
> > 
> > OK, but changing the layout between major versions is fine as we break ABI
> > 
 anyways then, no?
> > 
> > 
> >>> I.e. what problems would we get from having to install the
> >>> moc files?
> >> 
> >> 
> >> Lots.
> > 
> > 
> > <snip>
> > 
> > 
> >> Have I convinced you? I'm only getting warmed up. I'm sure I can find
> >> more
> >> issues.
> > 
> > 
> > Thanks for the exhaustive, and educative list! I think this should be put
> > on 
 the Wiki somewhere for future reference.
> > 
> > 
> >>> Alternatively: couldn't moc re-create the required data from included
> >>> files
> >>> when they contain templated objects? That would solve the problem as
> >>> well,
> >>> no?
> >> 
> >> 
> >> I have no idea what you meant here.
> >> 
> >> Or, well, I do have one, but I don't think that I understood correctly
> >> what
 you're suggesting. I understood that we keep a copy of the header
> >> file's source in the target application and run moc at runtime to
> >> dynamically create the meta object, on the fly.
> >> 
> >> Since I don't think that's what you're suggesting and since the above has
> >> so
 many problems (starting with the fact that it doesn't resolve the
> >> problem), I'm not even going to analyse it.
> >> 
> >> Can you clarify what you meant?
> > 
> > 
> > Yes, what I had in mind from my outsiders POV was the following, and I
> > have no 
 clue whether it is feasible at all:
> > 
> > We have the following structure:
> > 
> > $path1/lib/foo.h
> > template<typename T> Foo : QObject { ...};
> > 
> > moc is not doing anything here as the templated QObject is not fully 
> > specialized.
> > 
> > $path2/app/bar.h:
> > #include <some_templated_qobject.h>
> > using Bar = Foo<int>;
> 
> 
> That would break if any other code in the application (possibly in a
> different library linked to it) has e.g.
 using Blah = Foo<int>;
> right?
> That sounds pretty fragile. Especially if an application loads plugins.

Can you clarify what would break here? Isn't it exactly the same as doing 
QVector<int> in multiple TUs?

Thanks
-- 
Milian Wolff | milian.wolff at kdab.com | Software Engineer
KDAB (Deutschland) GmbH&Co KG, a KDAB Group company
Tel: +49-30-521325470
KDAB - The Qt Experts
-------------- next part --------------
A non-text attachment was scrubbed...
Name: smime.p7s
Type: application/pkcs7-signature
Size: 5903 bytes
Desc: not available
URL: <http://lists.qt-project.org/pipermail/development/attachments/20160302/4f9ca1dc/attachment.bin>