Casting from a virtual base class to a parent in a diamond inheritance? - c++

On Mon, 14 May 2007 lindahlb@hotmail.com wrote:

> If I have a pointer to class a, how do I turn that into a pointer to
> class d? I get an ambiguity error for the ptr2 conversion, and I get
> an implied error for the ptr3 conversion. Compiler is MSVC++ 8.0.5.

What's an implied error?

Anyway. 5.2.9/9 says:

-----
An rvalue of type "pointer to cv1 B," where B is a class type, can be
converted to an rvalue of type "pointer to cv2 D," where D is a class
derived from B, if a valid standard conversion from "pointer to D" to
"pointer to B" exists, cv2 is the same cv-qualification as, or greater
cv-qualification than, cv1, and B is neither a virtual base class of D nor
a base class of a virtual base class of D.
-----

The last clause is what prevents your cast. The reason is that the offset
from an object and one of its virtual bases (or a subobject thereof) is
not known at compile-time.

Sebastian Redl

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lindahlb@hotmail.com wrote:
> class a {};
> class b : public a {};
> class c : public a {};
> class d : public virtual b, public virtual c {};

This is a suspicious usage of virtual inheritance.
To make the "diamond" hierarchy, it's the class to be shared
that needs to be virtually inherited; in this case, class a.

class a {};
class b : public virtual a {};
class c : public virtual a {};
class d : public b, public c {};

> a * ptr = 0;
> d * ptr2 = static_cast<d *>(ptr);
> d * ptr3 = static_cast<d *>(static_cast<b *>(ptr));
>
> If I have a pointer to class a, how do I turn that into a pointer to
> class d? I get an ambiguity error for the ptr2 conversion, and I get
> an implied error for the ptr3 conversion. Compiler is MSVC++ 8.0.5.

Given a pointer to a virtual base, you don't know the exact layout of
the object statically. Make class a polymorphic (by adding a virtual
function), and use dynamic_cast.

--
Seungbeom Kim

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lindahlb@hotmail.com wrote:
> class a {};
> class b : public a {};
> class c : public a {};
> class d : public virtual b, public virtual c {};
>
> a * ptr = 0;
> d * ptr2 = static_cast<d *>(ptr);
> d * ptr3 = static_cast<d *>(static_cast<b *>(ptr));
>
> If I have a pointer to class a, how do I turn that into a pointer to
> class d? I get an ambiguity error for the ptr2 conversion, and I get
> an implied error for the ptr3 conversion. Compiler is MSVC++ 8.0.5.

Put a virtual function in a and use dynamic_cast.

>

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On May 15, 8:41 am, linda...@hotmail.com wrote:
> class a {};
> class b : public a {};
> class c : public a {};
> class d : public virtual b, public virtual c {};
>
> a * ptr = 0;
> d * ptr2 = static_cast<d *>(ptr);
> d * ptr3 = static_cast<d *>(static_cast<b *>(ptr));
>
> If I have a pointer to class a, how do I turn that into a pointer to
> class d? I get an ambiguity error for the ptr2 conversion, and I get
> an implied error for the ptr3 conversion. Compiler is MSVC++ 8.0.5.
>
> Thanks!
>

class a {};
class b : public a {};
class c : public a {};
class d : public virtual b, public virtual c {};
The structure seems not to be a diamond inheritance.
It may be look like:
class a {};
class b : virtual public a {};
class c : virtual public a {};
class d : public virtual b, public c {};
do you mean?








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