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[Bug c++/11867] [3.3/3.4 Regression] static_cast ignores ambiguity
- From: "mark at codesourcery dot com" <gcc-bugzilla at gcc dot gnu dot org>
- To: gcc-bugs at gcc dot gnu dot org
- Date: 7 Sep 2003 19:17:31 -0000
- Subject: [Bug c++/11867] [3.3/3.4 Regression] static_cast ignores ambiguity
- References: <20030809122531.11867.alfred.minarik.1@aon.at>
- Reply-to: gcc-bugzilla at gcc dot gnu dot org
PLEASE REPLY TO gcc-bugzilla@gcc.gnu.org ONLY, *NOT* gcc-bugs@gcc.gnu.org.
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=11867
------- Additional Comments From mark at codesourcery dot com 2003-09-07 19:17 -------
Subject: Re: [3.3/3.4 Regression] static_cast ignores
ambiguity
> struct A {};
>
> struct B : private A {};
>
> int
> main ()
> {
> B b;
> A* ap = (A*) &b;
> B* bp = static_cast<B*> (ap); //Error
> }
This program is invalid.
The reason is that the only static_cast alternative that would allow
this conversion is the:
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 (clause _class.derived_) from B, if a valid standard conver-
sion from "pointer to D" to "pointer to B" exists (_conv.ptr_), cv2 is
the same cv-qualification as, or greater cv-qualification than, cv1,
and B is not a virtual base class of D.
The "valid standard conversion" clause must be read to imply that this
restriction from [conv.ptr] applies to the conversion:
If B is an inaccessible (clause _class.access_) or ambiguous
(_class.member.lookup_) base class of D, a program that necessitates
this conversion is ill-formed.
Otherwise, there is nothing in the static_cast language to eliminate a
conversion from an ambiguous base to a derived class.
In fact, this was examined by the committee back in 1998, and is Core
Issue 54:
=====================
54. Static_cast from private base to derived class
Section: 5.2.9 expr.static.cast Status: NAD Submitter: Steve
Adamczyk Date: 13 Oct 1998
Is it okay to use a static_cast to cast from a private base class to a
derived class? That depends on what the words "valid standard
conversion" in paragraph 8 mean â?? do they mean the conversion exists, or
that it would not get an error if it were done? I think the former was
intended â?? and therefore a static_cast from a private base to a derived
class would be allowed.
Rationale (04/99): A static_cast from a private base to a derived class
is not allowed outside a member from the derived class, because 4.10
conv.ptr paragraph 3 implies that the conversion is not valid. (Classic
style casts work.)
=====================