From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <sourceware-bugzilla@sourceware.org>
Received: by sourceware.org (Postfix, from userid 48)
 id B453938515FF; Wed, 30 Mar 2022 23:55:09 +0000 (GMT)
DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org B453938515FF
From: "evan_l00 at qq dot com" <sourceware-bugzilla@sourceware.org>
To: gdb-prs@sourceware.org
Subject: [Bug python/29011] New: Python Value.dynamic_cast does not work as
 expected
Date: Wed, 30 Mar 2022 23:55:09 +0000
X-Bugzilla-Reason: CC
X-Bugzilla-Type: new
X-Bugzilla-Watch-Reason: None
X-Bugzilla-Product: gdb
X-Bugzilla-Component: python
X-Bugzilla-Version: 11.1
X-Bugzilla-Keywords: 
X-Bugzilla-Severity: normal
X-Bugzilla-Who: evan_l00 at qq dot com
X-Bugzilla-Status: UNCONFIRMED
X-Bugzilla-Resolution: 
X-Bugzilla-Priority: P2
X-Bugzilla-Assigned-To: unassigned at sourceware dot org
X-Bugzilla-Target-Milestone: ---
X-Bugzilla-Flags: 
X-Bugzilla-Changed-Fields: bug_id short_desc product version bug_status
 bug_severity priority component assigned_to reporter target_milestone
Message-ID: <bug-29011-4717@http.sourceware.org/bugzilla/>
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable
X-Bugzilla-URL: http://sourceware.org/bugzilla/
Auto-Submitted: auto-generated
MIME-Version: 1.0
X-BeenThere: gdb-prs@sourceware.org
X-Mailman-Version: 2.1.29
Precedence: list
List-Id: Gdb-prs mailing list <gdb-prs.sourceware.org>
List-Unsubscribe: <https://sourceware.org/mailman/options/gdb-prs>,
 <mailto:gdb-prs-request@sourceware.org?subject=unsubscribe>
List-Archive: <https://sourceware.org/pipermail/gdb-prs/>
List-Help: <mailto:gdb-prs-request@sourceware.org?subject=help>
List-Subscribe: <https://sourceware.org/mailman/listinfo/gdb-prs>,
 <mailto:gdb-prs-request@sourceware.org?subject=subscribe>
X-List-Received-Date: Wed, 30 Mar 2022 23:55:09 -0000

https://sourceware.org/bugzilla/show_bug.cgi?id=3D29011

            Bug ID: 29011
           Summary: Python Value.dynamic_cast does not work as expected
           Product: gdb
           Version: 11.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: python
          Assignee: unassigned at sourceware dot org
          Reporter: evan_l00 at qq dot com
  Target Milestone: ---

The document says that Value.dynamic_cast works like dynamic cast in C++, b=
ut
in the following experiment, it shows that the Value.cast works in C++, and
Value.dynamic_cast gives some value I don't understand.

I have asked on the stackoverflow in
https://stackoverflow.com/questions/71644182/gdb-python-api-dynamic-cast-no=
t-work-as-expected,
but received a comment that says this might be a bug in the gdb. So I open =
this
ticket.

This is the sample code


class Base {
public:
  virtual void p() {}
  int i =3D3;
};
class Base2 {
public:
  virtual void f() {}
  int i2 =3D4;
};
class Derived: public Base, public Base2{
  public:
  void p() override{}
  int t =3D4;
};
int main() {
   auto* p =3D new Derived();
   Base2* pb2 =3D p;
   Base* pb =3D p;
   return 0;
}
Run the code, and print the values of p, pb, and pb2

(gdb) p p
$1 =3D (Derived *) 0x613c20
(gdb) p pb //=20
$2 =3D (Base *) 0x613c20
(gdb) p pb2
$3 =3D (Base2 *) 0x613c30
Then use python script to print the value as follows, the Value.dynamic_cast
will give

(gdb) py print(ppb2.dynamic_cast(ppb2.dynamic_type))
0x400808 <vtable for Derived+16>
And the Value.cast gives the correct derived pointer,

(gdb) py print(ppb2.cast(ppb2.dynamic_type))
0x613c20
pp, ppb, and ppb2 are the corresponding values but in Python env, got from =
the
following code.

(gdb) py pp =3D gdb.parse_and_eval('p')
(gdb) py print(pp)
0x613c20
(gdb) py ppb2 =3D gdb.parse_and_eval('pb2')
(gdb) py print(ppb2)
0x613c30
(gdb) py print(ppb2.cast(ppb2.dynamic_type))
0x613c20
(gdb) py print(ppb2.dynamic_cast(ppb2.dynamic_type))
0x400808 <vtable for Derived+16>
Why Value.dynamic_cast will give 0x400808 <vtable for Derived+16> instead of
0x613c20 (the derived pointer)?

--=20
You are receiving this mail because:
You are on the CC list for the bug.=