c++/111: Re: Member of virtual classes incorrectly accessed in methods with no objet specification

["Martin v. Loewis" <martin at loewis dot home dot cs dot tu-berlin dot de>]

>Number:         111
>Category:       c++
>Synopsis:       Member of virtual classes incorrectly accessed in methods with no objet specification
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    unassigned
>State:          analyzed
>Class:          wrong-code
>Submitter-Id:   net
>Arrival-Date:   Wed Mar 15 01:16:00 PST 2000
>Closed-Date:
>Last-Modified:
>Originator:     "Casteyde" <casteyjp@club-internet.fr>
>Release:        2.95.2
>Organization:
>Environment:
>Description:
 Date: Sat, 14 Aug 1999 12:55:38 +0200
 Original-Message-ID: <01bee643$8b944560$LocalHost@casteyjp>

 Hello, here are what I think is a bug. Tested and reproduced both on
 Linux and W32 :

 Win32 conf :

 gcc 2.95 (mingw32 port)
 gas 2.9.4 (mingw32 port)
 Windows 95 OSR2 (K6, 32Mo)
 No Options ("gcc bug.cpp")

 Linux conf :

 gcc 2.95
 gas 2.9.1 (SuSE Linux 6.1)
 SuSE Linux 6.1, kernel 2.2.10 (gcc 2.95 recompiled with =
 -fno-strict-aliasing)
 No Options ("gcc bug.C")

 Description of the problem :

 The members of a virtual base class are incorrectly accessed when I
 specify only a part of the path in the inheritance graph (that is,
 "B::i" and not "B::A::i"). In fact, it seems that gcc assumes that
 the object is in the derived class B so specified, and doesn't see
 that it is in the base class A.  If I give the object pointer
 explicitly, (with "this->B::"), the member is correctly accessed.
 Under Linux, I got a SIGSEGV (once, not reproduced), so memory is
 really incorrectly accessed.

 There should be at least a warning, if my construct is incorrect
 (none even with -Wall)...  Nevertheless, this bug is not very
 important, since I see no application of partial specification of
 inheritance path and I can use this pointer.

 Regards,

 C. Casteyde
>How-To-Repeat:
#include <iostream.h>

class A
{
public :
     int i;
};

class B : virtual public A
{
};

class C : virtual public A
{
};

class D : public B, public C
{
public :
     int f(void);
     int g(void);
};

int D::f(void)
{
     return B::i;          // Doesn't work.
}

int D::g(void)
{
     return this->B::i; // Works because there is "this->".
}

D d;

int main(void)
{
    d.C::i=325;                    // Wroks because there is "d."
    cout << d.f() << endl;     // Does not work, as mentionned upper.
    cout << d.B::i << endl;   // Works with "d" specified.
    cout << d.g() << endl;    // Works fine ("this" is specified).
    // Therefore, B::i works only when the object is specified (with "this" in a method).
    return 0;
}
>Fix:
>Release-Note:
>Audit-Trail:
>Unformatted: