[Development] QList

[Marc Mutz]

Hi Thiago,

You misunderstand me.

I listed _the_ three use-cases where inheritance is the tool of choice:

1. modelling is-a
2. inheriting to reuse
3. reimplementing virtual functions

If your use-case is not one of the three, then inheritance is not the right 
tool.

Clearly, by now, it's neither (1) nor (3). So if anything, it needs to be (2).

And I'm arguing that it's not (2), either, because you can only re-use a 
subset of QStringView API, and need to wrap the other half anyway. (2) is 
really only interesting for pure OO languages with no concept of free 
functions. In C++, we have free functions.

On Thursday 23 March 2017 22:22:36 Thiago Macieira wrote:
> > > A naïve implementation of QString::mid() could be:
> > > 
> > >
> > > QString QString::mid(int start, int len) const
> > > {
> > >     return QString(QStringView::mid(start, len));
> > > }
> >
> > Which is no different from
> > 
> >    return QString(QStringView(*this).mid(start, len));
> > 
> >
> > so does not provide a case supporting using inhertance.
> 
> Actually, the code generation could be slightly different. If 
> QStringView::mid() isn't inlined, then the difference is whether we need
> to  create a temporary QStringView in the stack and pass its this pointer
> as a parameter.

If QStringView::mid() isn't inline, then it doesn't matter which 
implementation we choose. The implicit this in QStringView::mid is still 
there, forcing the object on the stack.

So what I take away from this discussion is "avoid having to go though *this 
(into out-of-line code)". And to avoid having to go through *this, you need 
free functions, not member functions of some private base class.

And this is what I intend to do: QStringView::mid is inline. And I'm proposing 
to make all QString/View member functions inline non-exported, either 
implemented directly or by calling a free function (exported or not).

As an aside: going through *this has obviously so far not been an issue with 
QString's myriads of member functions, so I guess even on Win64 calling 
performance will increase with QStringView taken by value.

Ville: are there any proposals for making *this passable by value? Like

   struct QLineF {
       Q_CORE_EXPORT QPointF intersection(QLineF other) =;
        // =: pass *this by value, mimicking & and && for (l|r)value ref
   };

Thanks,
Marc

-- 
Marc Mutz <marc.mutz at kdab.com> | Senior Software Engineer
KDAB (Deutschland) GmbH & Co.KG, a KDAB Group Company
Tel: +49-30-521325470
KDAB - The Qt, C++ and OpenGL Experts