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 Posted: Tue May 15, 2007 12:41 am Post subject: Casting from a virtual base class to a parent in a diamond |
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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!
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Sebastian Redl
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| Posted: Tue May 15, 2007 4:52 am Post subject: Re: Casting from a virtual base class to a parent in a diamo |
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On Mon, 14 May 2007 lindahlb (AT) hotmail (DOT) com wrote:
| Quote: | 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:
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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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Seungbeom Kim
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| Posted: Tue May 15, 2007 4:52 am Post subject: Re: Casting from a virtual base class to a parent in a diamo |
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lindahlb (AT) hotmail (DOT) com wrote:
| Quote: | class a {};
class b : public a {};
class c : public a {};
class d : public virtual b, public virtual c {};
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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 {};
| Quote: | 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
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
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