From mboxrd@z Thu Jan  1 00:00:00 1970
From: gbr@netel.bg
To: gcc-gnats@gcc.gnu.org
Subject: c++/3608: Virtual funtions called with wrong this pointer when having virtual inheritance
Date: Sun, 08 Jul 2001 06:56:00 -0000
Message-id: <20010708134638.30843.qmail@sourceware.cygnus.com>
X-SW-Source: 2001-07/msg00198.html
List-Id: <gcc-prs.sourceware.org>

>Number:         3608
>Category:       c++
>Synopsis:       Virtual funtions called with wrong this pointer when having virtual inheritance
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    unassigned
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Sun Jul 08 06:56:00 PDT 2001
>Closed-Date:
>Last-Modified:
>Originator:     Vladimir Panov
>Release:        2.95.3 20010315 (release)
>Organization:
>Environment:
i386-slackware-linux
>Description:
When invoking a virtual function via a base class pointer the function receives a wrong this pointer. This happens only when having virtual inheritance. I suppose it receives the this pointer for the most-derived object.


This causes no problem (i.e. both this pointers in the ctor and the virtual method are equal), because object b is the most-derived one:
#include <iostream>
#include <iomanip>
using namespace std;

class A
{
public:
  A (void)
  {
  }
  virtual ~A (void)
  {
  }
  virtual void t (void)
  {
  }
};

void T (A *a)
{
  a->t ();
}

class B : public virtual A
{
public:
  B (void)
  {
    cerr << "Ctor: " << hex << (unsigned)this << endl;
    T (this);
  }
  virtual ~B (void)
  {
  }
  virtual void t (void)
  {
    cerr << "t:    " << hex << (unsigned)this << endl;
  }
};

int main (void)
{
  B b;
  return 0;
}


This causes no problem, because class C does not have extra members over class B and (C *) is equivalent to (B *):
#include <iostream>
#include <iomanip>
using namespace std;

class A
{
public:
  A (void)
  {
  }
  virtual ~A (void)
  {
  }
  virtual void t (void)
  {
  }
};

void T (A *a)
{
  a->t ();
}

class B : public virtual A
{
public:
  B (void)
  {
    cerr << "Ctor: " << hex << (unsigned)this << endl;
    T (this);
  }
  virtual ~B (void)
  {
  }
  virtual void t (void)
  {
    cerr << "t:    " << hex << (unsigned)this << endl;
  }
};

class C : public B
{
public:
  C (void)
  {
  }
  virtual ~C (void)
  {
  }
};

int main (void)
{
  C c;
  return 0;
}


This causes the problem, because (I suppose) the virtual inheritance of class C adds some hidden extra fields:
#include <iostream>
#include <iomanip>
using namespace std;

class A
{
public:
  A (void)
  {
  }
  virtual ~A (void)
  {
  }
  virtual void t (void)
  {
  }
};

void T (A *a)
{
  a->t ();
}

class B : public virtual A
{
public:
  B (void)
  {
    cerr << "Ctor: " << hex << (unsigned)this << endl;
    T (this);
  }
  virtual ~B (void)
  {
  }
  virtual void t (void)
  {
    cerr << "t:    " << hex << (unsigned)this << endl;
  }
};

class C : public virtual B
{
public:
  C (void)
  {
  }
  virtual ~C (void)
  {
  }
};

int main (void)
{
  C c;
  return 0;
}


This causes the problem, because class C has extra fields over class B:
#include <iostream>
#include <iomanip>
using namespace std;

class A
{
public:
  A (void)
  {
  }
  virtual ~A (void)
  {
  }
  virtual void t (void)
  {
  }
};

void T (A *a)
{
  a->t ();
}

class B : public virtual A
{
public:
  B (void)
  {
    cerr << "Ctor: " << hex << (unsigned)this << endl;
    T (this);
  }
  virtual ~B (void)
  {
  }
  virtual void t (void)
  {
    cerr << "t:    " << hex << (unsigned)this << endl;
  }
};

class C : public B
{
  int i;
public:
  C (void)
  {
  }
  virtual ~C (void)
  {
  }
};

int main (void)
{
  C c;
  return 0;
}

Both the erroneous examples are OK with Micro$oft Visual C++ 6.0. Shame on you! :-)))
In the following days I will try to test these examples on Solaris 7 ULTRASPARC with gcc 2.9x.xx to find out if this is an architecture-specific or general bug.
>How-To-Repeat:
g++ example.cpp
and run the resulting a.out.
>Fix:
None.
>Release-Note:
>Audit-Trail:
>Unformatted: