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

[Milian Wolff]

On Thursday, March 3, 2016 2:05:18 PM CET Dominik Haumann wrote:
> Hi Milian,
> 
> On Thu, Feb 25, 2016 at 7:22 PM, Milian Wolff <milian.wolff at kdab.com> wrote:
> > On Donnerstag, 25. Februar 2016 09:02:11 CET Thiago Macieira wrote:
> >> On quinta-feira, 25 de fevereiro de 2016 17:33:52 PST Cristian Adam 
wrote:
> >> > This might be a burden for some of the Qt developers (Windows ones).
> >> > 
> >> > But all the Qt users get a modern / flexible moc, see this thread:
> >> > https://www.reddit.com/r/cpp/comments/470ama/qt_moc_myths_debunked/d09c
> >> > 90e
> >> 
> >> I don't think we need a more flexible moc. What do we want to do that we
> >> can't do with the current one?
> >> 
> >> Don't say "template QObjects". That has other reasons for being a bad
> >> idea,
> >> currently.
> > 
> > Can you explain what those reasons are? I'd really love to write a generic
> > QAbstractTableModel implementation that operates using concepts. Currently
> > that would require type erasure and thus another set of virtual function
> > calls...
> > [...]
> > If we'd have templates QObjects, the above could easily be written. I bet
> > there are other valid use-cases.
> 
> It's not directly related, but instead of templated QObjects you
> could also find worksarounds like this:
> template <typename T>
> class Foo {
> public:
>     // ...
>     QObject * notificationObject();
> private:
>     QObject * m_notificationObject;
> };
> 
> In the constructor, you can create the m_notificationObject
> and even to connects etc.
> 
> I once used this trick (in a non-templated version) to obtain
> value-semantics for a class that can still have signals and
> slots etc (be careful with assignment operator, though).
> Of course, you still might not have the metadata moc
> typically creates for you. So I'm not claiming this is a
> totally cool solution, but sometimes this workaround
> comes in very handy.

Note that this won't work for the use-case I have in mind, namely templated 
QAIM implementations for efficient, and correct, list and table models on 
QVector<T>.

Cheers

-- 
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
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