Re: general c++ question

[Shai Ayal]

On 6/30/07, Michael Goffioul <address@hidden> wrote:
> On 6/30/07, Shai Ayal <address@hidden> wrote:
> > This is a general c++ question, but I came upon it while trying to
> > patch octave, so I figured I'll try asking here:
> >
> > I want to declare a function returning the figure::figure_properties
> > class. This function will have to be declared before
> > figure::figure_properties is declared. This is usually done using
> > forward declarations -- i.e. I would expect he following to work:
> >
> > class figure;
> > class figure::figure_properties;
> > figure::figure_properties test();
> >
> > however gcc comes back with the error:
> > 'figure_properties' in class 'figure' does not name a type
> > for both lines where figure::figure_properties is mentioned. Trying
> > the following also does not work:
> >
> > class figure;
> > class figure_properties;
> > figure::figure_properties test();
> >
> > with the same error where figure::figure_properties is mentioned.
> >
> > any ideas besides moving figure_properties outside figure?
>
> I'm not sure this is the actual problem you get (and I'm not a C++ expert),
> but I think that you can only deal with pointers to forward-declared classes,
> because the compiler does not know the object size when you use it.
>
> For example:
>
> class A;
> A test(); /* probably not OK */
> A* test(); /* OK */

I actually tried it and it is OK. In any case you are right in that it
is silly to pass the calss by value -- I should pass a reference:

figure::figure_properties& test();

> In your case, you have the additional problem to forward-declare nested
> classes; I've never tried that.


Yes -- this is my problem -- does anybody have a clue?


> Michael.