Re: virtual table off by three problem

[Aubrey Holland]

Here's what I've learned....

1) This only happens if the object being called is allocated on the
heap.  Declaring a local variable and then making calls to it works
just fine.

2) The problem can be fixed by maintaining a consistent ordering of
virtual methods throughout the class hierarchy.  For example, if we
saw this problem in the following example:

class A
{
        virtual void a();
        virtual void b();       
};

class B : public A
{
        virtual void c();
        virtual void b();
        virtual void a();
};

then we could solve the problem simply by reorganizing the declaration
of a, b, and c in class B so that they follow the order declared in
class A with new ones at the end.  e.g.

class B : public A
{
        virtual void a();
        virtual void b();
        virtual void c();
};

I attempted to write a simple test application that would exhibit this
problem with no luck, so I think there must be something else
involved.  Also, we have plenty of classes where we haven't worried
about the ordering and there is only one place where I've seen this
problem.

Mark, I'd be interested to hear if you learn anything new about this
or if it's fixed in the 3.0 builds.  I'm concerned about finding
further problems if we move to 3.0 at this point.

Aubrey Holland



address@hidden (Marc Brett) wrote in message news:<address@hidden>...
> Me too.
> We're getting the same behaviour with g++ 2.95.2 and Redhat 6.2,
> SGI workstation with single processor Pentium III.
> Trying to recompile with g++ 3.0.2, but this may take some time.
> 
> Would be interested to hear if you make any progress.
> 
> Aubrey Holland <address@hidden> wrote:
> > In one of my libraries, I have a rather deep inheritance tree where
> > the classes in this tree contain quite a few virtual functions.  I
> > have noticed recently that there seems to be a problem with the
> > virtual tables that get created for some of these classes. 
> > Specifically, when I try to call one of the virtual methods in this
> > class, the result is that I get a call to the virtual method that is
> > three virtual methods down in the header file.  I.e. in the following
> > overly-simplified example, a call to a() would result in d() actually
> > being called.  This happens regardless of where I place the function
> > in the header file... the third one down is always called.
>  
> > class example
> > {
> >     virtual void a();
>  
> >     void nonvirtual();
>  
> >     virtual void b();
> >     virtual void c();
> >     virtual void d();
> > };
>  
> > I saw the following previous similar posting, but it seems to indicate
> > the the bug is already fixed:
>  
> > http://groups.google.com/groups?q=g%2B%2B+virtual+table+off+by&hl=en&selm=Pine.OSF.3.91j.950911160602.5228N-100000%40saul6.u.washington.edu&rnum=3
>  
> > I'm working on trying to reproduce this in a smaller, more portable
> > example program but haven't had much luck as of yet.
>  
> > Sysyem info:
>  
> > Dual processor Dell Workstation, running Pentium Xeon processors.
> > RedHat 7.2
> > gcc and g++ 2.96-98
>  
> > TIA,
> > Aubrey Holland