[C++-sig] newbie questions

[David Abrahams]

Stefan Seefeld <seefeld at sympatico.ca> writes:

> David Abrahams wrote:
> 
> > 1. Are you trying to wrap an interface without changing it? In other
> >    words, is it OK to change the definition of Foo, Bar, and BarImpl,
> >    or is this wrapping job to be non-intrusive?
> 
> ideally it's non-intrusive. I'm all for cleaning it up (and it needs
> some cleanup !), but that's not my (current) mandate...
> 
> > 2. What is the semantics of GetBar()? Does it just return a pointer to
> >    some BarImpl, to be used as a kind of reference, or is it the
> >    caller expected to delete (or otherwise manage) the pointer it gets
> >    back?
> 
> in this particular case Foo remains the owner.
> 
> > 3. If the BarImpl* is just a kind of reference, which object actually
> >    manages the lifetime of the BarImpl object?
> 
> Foo.
> 
> I guess in all other cases I would use the factory approach we
> discussed yesterday.
> 
> Regards,
> 		Stefan

Okay, revisiting your code in that light:

    class Bar {};
    class BarImpl : public Bar {};
    class Foo
    {
     public:
       BarImpl *GetBar();
    };

you need this part no matter what:

   class_<Bar>("Bar")
       ...
       ;

option 1: expose the BarImpl class too:

   class_<BarImpl, bases<Bar> >("BarImpl", no_init)
       ;

   class_<Foo>("Foo")
       .def("GetBar", &Foo::GetBar
              , return_value_policy<return_internal_reference>())
       ...
       ;

The only downside of that is that Python users might detect that
there's a BarImpl class as well as a Bar class. But so what?

option 2: use a thin wrapper to hide the existence of BarImpl:

    Bar* foo_GetBar(Foo& f) { return f.GetBar(); }

    class_<Foo>("Foo")
       .def("GetBar", foo_GetBar
              , return_value_policy<return_internal_reference>())
       ...
       ;


HTH,
Dave

P.S. I suggest you read up on CallPolicies in the documentation I sent
you pointers to.

-- 
           David Abrahams * Boost Consulting
dave at boost-consulting.com * http://www.boost-consulting.com