Multiple inheritance and casting

[Simon Elliott]

If I have a class hierarchy as follows:

class barBase
{
};

class barDerived:public barBase
{
};

class fooBase
{
};

class fooBarDerived:public 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(myFooBase);

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]

"Simon Elliott" <Simon at ctsn.co.uk> wrote in message
news:41***********************@news.gradwell.net.. .

If I have a class hierarchy as follows:

class barBase
{
};

class barDerived:public barBase
{
};

class fooBase
{
};

class fooBarDerived:public fooBase, barDerived
{
};

And if I create an instance as follows:

fooBase* myFooBase = new fooBase;
I think you mean

fooBase* myFooBase = new fooBarDerived;

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(myFooBase);

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.

Assuming my correction above is right, then

barBase* myBarBase = dynamic_cast<barBase*>(myFooBase);

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

john

[JKop]

Simon Elliott posted:

If I have a class hierarchy as follows:

class barBase
{
};

class barDerived:public barBase
{
};

class fooBase
{
};

class fooBarDerived:public 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?
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_object = *reinterpret_cast<barBase* const>
(&foobase_object);

Intuitively I'd think there ought to be, because myFooBase points to an
object which is, in part, a barBase.

Incorrect.

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

fooBarDerived fb_derived_object;

barBase& blah1 = fb_derived_object;

fooBase& blah2 = fb_derived_object;

and also:

barDerived& blah3 = fb_derived_object;
-JKop

[Simon Elliott]

On 01/10/2004, John Harrison wrote:

And if I create an instance as follows:

fooBase* myFooBase = new fooBase;
I think you mean

fooBase* myFooBase = new fooBarDerived;

Well spotted!
Assuming my correction above is right, then

barBase* myBarBase = dynamic_cast<barBase*>(myFooBase);

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

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