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Date: Sun, 27 Sep 2009 12:22:00 -0000
From: Jan Kratochvil <jan.kratochvil@redhat.com>
To: Tom Tromey <tromey@redhat.com>
Cc: Dragos Tatulea <dragos.tatulea@gmail.com>, keiths@redhat.com,
        archer@sourceware.org, cmoller@redhat.com
Subject: set print objct pros/cons  [Re: [Keith Seitz] Re: [tools-team]
 Status 2008-09-01]
Message-ID: <20090927122147.GA15789@host0.dyn.jankratochvil.net>
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Hi,

just found out `set print object on' is not such a clear win:

(gdb) b 27
Breakpoint 1 at 0x40069b: file cxxinherit.C, line 27.
(gdb) r
Starting program: /home/jkratoch/t/cxxinherit 
derived::m
base::m
Breakpoint 1, main () at cxxinherit.C:27
27	  return 0;
(gdb) show print object 
Printing of object's derived type based on vtable info is off.
(gdb) p derivedp->m
$1 = {void (Derived *)} 0x4006c4 <Derived::m()>
(gdb) p basep->m
$2 = {void (Base *)} 0x4006a2 <Base::m()>
(gdb) call derivedp->m ()
derived::m
(gdb) call basep->m ()
base::m
(gdb) set print object on
(gdb) p derivedp->m
$3 = {void (Derived *)} 0x4006c4 <Derived::m()>
(gdb) p basep->m
$4 = {void (Derived *)} 0x4006c4 <Derived::m()>
(gdb) call derivedp->m ()
derived::m
(gdb) call basep->m ()
base::m


(1) Last "call basep->m ()" should have printed "derived::m"
    because "p basep->m" prints "Derived::m()", shouldn't it?

(2) A lookup can succeed in both "off" and "on" cases with different results.

One should fix (1) as it looks just as a bug to me.

Afterwards for (2) maybe any C++ print should be evaluated both ways and if
the resulting `struct value' differs it should print both values with a warning?

("any C++ print" could be optimized somehow, not the point here.)


Thanks,
Jan

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#include <stdio.h>

class Base
  {
  public:
    void m () { puts ("base::m"); }
    virtual void stub () {}
  };

class Derived : public Base
  {
  public:
    virtual void m () { puts ("derived::m"); }
    virtual void stub () {}
  };

int
main ()
{
  Derived derived_local;
  Derived *derivedp = &derived_local;
  Base *basep = &derived_local;

  derivedp->m ();
  basep->m ();

  return 0;
}

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