Multiple inheritance and casting

[Simon Elliott]

If I have a class hierarchy as follows:

class barBase
{
};

class barDerived:publ ic barBase
{
};

class fooBase
{
};

class fooBarDerived:p ublic fooBase, barDerived
{
};

And if I create an instance as follows:

fooBase* myFooBase = new fooBase;

Is there any way I can then cast myFooBase to a barBase, without
knowing about fooBarDerived?

(this code should only know about barBase and fooBase)

barBase* myBarBase = some_cast(myFoo Base);

Intuitively I'd think there ought to be, because myFooBase points to an
object which is, in part, a barBase. But I can't see any way of doing
this.

--
Simon Elliott http://www.ctsn.co.uk

[John Harrison]

Re: Multiple inheritance and casting


"Simon Elliott" <Simon at ctsn.co.uk> wrote in message
news:415d16d0$0 $94923$bed64819 @news.gradwell. net...[color=blue]
> If I have a class hierarchy as follows:
>
> class barBase
> {
> };
>
> class barDerived:publ ic barBase
> {
> };
>
> class fooBase
> {
> };
>
> class fooBarDerived:p ublic fooBase, barDerived
> {
> };
>
> And if I create an instance as follows:
>
> fooBase* myFooBase = new fooBase;[/color]

I think you mean

fooBase* myFooBase = new fooBarDerived;
[color=blue]
>
> Is there any way I can then cast myFooBase to a barBase, without
> knowing about fooBarDerived?
>
> (this code should only know about barBase and fooBase)
>
> barBase* myBarBase = some_cast(myFoo Base);
>
> Intuitively I'd think there ought to be, because myFooBase points to an
> object which is, in part, a barBase. But I can't see any way of doing
> this.[/color]

Assuming my correction above is right, then

barBase* myBarBase = dynamic_cast<ba rBase*>(myFooBa se);

but this only works if your classes have at least one virtual function.

john

[JKop]

Re: Multiple inheritance and casting

Simon Elliott posted:
[color=blue]
> If I have a class hierarchy as follows:
>
> class barBase
> {
> };
>
> class barDerived:publ ic barBase
> {
> };
>
> class fooBase
> {
> };
>
> class fooBarDerived:p ublic fooBase, barDerived
> {
> };
>
> And if I create an instance as follows:
>
> fooBase* myFooBase = new fooBase;
>
> Is there any way I can then cast myFooBase to a barBase, without
> knowing about fooBarDerived?[/color]

There's no relationship whatsoever between the classes, "fooBase" and
"barBase", so there's nothing you can do. But... if you want to use brute
force:

barBase &barbase_obj ect = *reinterpret_ca st<barBase* const>
(&foobase_objec t);

[color=blue]
> Intuitively I'd think there ought to be, because myFooBase points to an
> object which is, in part, a barBase.[/color]

Incorrect.

An object of the class "fooBarDeri ved" would contain both a "fooBase" object
and a "barBase" object.

fooBarDerived fb_derived_obje ct;

barBase& blah1 = fb_derived_obje ct;

fooBase& blah2 = fb_derived_obje ct;

and also:

barDerived& blah3 = fb_derived_obje ct;


-JKop

[Simon Elliott]

Re: Multiple inheritance and casting

On 01/10/2004, John Harrison wrote:
[color=blue][color=green]
> > And if I create an instance as follows:
> >
> > fooBase* myFooBase = new fooBase;[/color]
>
> I think you mean
>
> fooBase* myFooBase = new fooBarDerived;[/color]

Well spotted!
[color=blue]
> Assuming my correction above is right, then
>
> barBase* myBarBase = dynamic_cast<ba rBase*>(myFooBa se);
>
> but this only works if your classes have at least one virtual
> function.[/color]

Thanks! I hadn't realised that dynamic_cast could work in cases where
the whole class hierarchy was not visible.

--
Simon Elliott http://www.ctsn.co.uk