Re: Bogus 'pure virtual method called' abort in all g++ versions

["James A. Bednar" <jbednar at cs dot utexas dot edu>]

|  From: Jan Reimers <janr@molienergy.com>
|  Cc: "'bug-gcc@gnu.org'" <bug-gcc@gnu.org>
|  Date: Tue, 19 Sep 2000 12:54:58 -0700
|  
|  Your storing a reference to a temporary which gets destroyed by the time you
|  call c.g().  When compiling with -Wall the compiler (2.95.2) still gave no
|  warning.  I think it should.

Ah, that's it, thanks!  Yes, I guess I've gotten used to the compiler
warning about that when it's a problem, so it would be good if a
warning could be added.  A warning would be particularly useful since
the symptoms are fairly strange -- seems like it should segfault
instead, since not only is there no object of type B anymore, there
isn't one of type A either.  I wonder why other compilers don't object
either.  Hmm...

Thanks,

Jim 


|  > -----Original Message-----
|  > From: James A. Bednar [mailto:jbednar@cs.utexas.edu]
|  > Sent: Tuesday, September 19, 2000 12:43 PM
|  > To: bug-gcc@gnu.org
|  > Subject: Bogus 'pure virtual method called' abort in all g++ versions
|  > 
|  > 
|  > 
|  > The following code aborts with a message saying 'pure virtual method
|  > called' (see output attached), even though C++ compilers on other
|  > platforms do not complain, and the code looks valid to me:
|  > 
|  >   struct A {
|  >     virtual ~A() { }
|  >     virtual void f() const=0;
|  >   };
|  >   
|  >   strict B : public A {
|  >     virtual ~B() { }
|  >     virtual void f() const { }
|  >   };
|  >   
|  >   struct C {
|  >     const A& a;
|  >     
|  >     C(const A& a_init=B()) : a(a_init) {
|                   ^^^^^^
|  
|  
|  >       a.f();
|  >     }
|  >   
|  >     void g() const {
|  >       a.f(); 
|  >     }
|  >   };
|  >   
|  >   
|  >   int main(int argc, char **argv)
|  >   {
|  >     const C c;
|  >     c.g();
|  >   }
|  > 
|  > The program runs fine under g++ if the second "a.f()" call is
|  > commented out.  Presumably, the type B default argument to C's
|  > constructor is somehow being sliced to its base type A before that
|  > call.  Oddly enough, that slicing appears to happen *after* the
|  > constructor completes, since the first "a.f()" call does not cause any
|  > problems.
|  > 
|  > Perhaps I'm misunderstanding some obscure part of the C++ standard
|  > with respect to default arguments, but this behavior certainly *seems*
|  > incorrect.  The same behavior was observed in egcs 1.1.2, GCC 2.95.2,
|  > and the 20000919 CVS snapshot at
|  > http://www.codesourcery.com/gcc-compile.shtml.
|  > 
|  > By the way, debugging things like this would be a lot easier if we got
|  > a little more debugging information than just "pure virtual method
|  > called", at least when compiled with -g :-).
|  >  
|  > In case this code proves useful as a test case, I hereby place it into
|  > the public domain for anyone to use for any purpose whatsoever.
|  > 
|  > Jim Bednar
|  > 
|  > --------------------------------------------------------------
|  > -----------------
|  > pale:~> g++ -v
|  > Reading specs from /usr/lib/gcc-lib/i386-redhat-linux/2.95.2/specs
|  > gcc version 2.95.2 19991024 (release)
|  > pale:~> cat bug16.c
|  > struct A {
|  >   virtual ~A() { }
|  >   virtual void f() const=0;
|  > };
|  > 
|  > struct B : public A {
|  >   virtual ~B() { }
|  >   virtual void f() const { }
|  > };
|  > 
|  > struct C {
|  >   const A& a;
|  >   
|  >   C(const A& a_init=B()) : a(a_init) {
|  >     a.f();
|  >   }
|  > 
|  >   void g() const {
|  >     a.f(); 
|  >   }
|  > };
|  > 
|  > 
|  > int main(int argc, char **argv)
|  > {
|  >   const C c;
|  >   c.g();
|  > }
|  >   
|  > 
|  > pale:~> g++ bug16.c -o bug16
|  > pale:~> ./bug16 
|  > pure virtual method called
|  > Abort (core dumped)
|  > pale:~> diff -u bug16.c bug16a.c
|  > --- bug16.c     Tue Sep 19 14:30:11 2000
|  > +++ bug16a.c    Tue Sep 19 14:31:49 2000
|  > @@ -16,7 +16,7 @@
|  >    }
|  >  
|  >    void g() const {
|  > -    a.f(); 
|  > +    //    a.f(); 
|  >    }
|  >  };
|  >  
|  > pale:~> g++ bug16a.c -o bug16a
|  > pale:~> ./bug16a
|  > pale:~> 
|  > 
|  >