[Development] d_ptr and q_ptr

[David Laing]

Hi all,

I was just reading this:
   http://herbsutter.com/gotw/_100/
and got to wondering about the pimpl idiom in Qt with respect to 
forwards compatibility with C++11.

Should there be some discussion of the relative merits of using
   QFooClass* QFooClassPrivate::q_ptr
to refer to public classes from inside the private implementation 
classes versus using
   QFooClassPrivate::barMethod(QFooClass *parent, <<bar method args>>)
for the methods that need to use the parent reference.

At the moment we tend to use the first technique.  The downside is that 
we would then need to make sure that q_ptr is kept up to date if the 
parent class is acted on by the move operators from C++11.

This could be remedied by documenting that you need to update the 
pointers after a move and providing examples (preferably including 
examples on how to detect if you have support for the move operators as 
well).  We'd also need to update the existing code if we want to avoid 
surprises there.

With the second technique we'd avoid the need to update the pointer, 
although we'd still need to document the new idiom and then update the 
existing code.

There are a few other issues that are also probably relevant (how to 
deal with grandparents, for instance) , but I figured it's worth 
mentioning the basic issue to stimulate some discussion and see what 
falls out.

Cheers,

Dave