[Bug c++/12277] Warn on dynamic casts with known NULL results.

[Bug c++/12277] Warn on dynamic casts with known NULL results.

[paolo.carlini at oracle dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=12277

Paolo Carlini <paolo.carlini at oracle dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|ASSIGNED                    |NEW

[Bug c++/12277] Warn on dynamic casts with known NULL results.

[gcc at dixie dot net.nz]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=12277

Andrew Dixie <gcc at dixie dot net.nz> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |gcc at dixie dot net.nz

--- Comment #7 from Andrew Dixie <gcc at dixie dot net.nz> ---
With final, I believe the following dynamic_cast must always return NULL:

struct a1 { virtual ~a1() {} };
struct a2 final { virtual ~a2() {} };

bool test(a1 *x)
{
        return dynamic_cast<a2 *>(x) != 0;
}

[Bug c++/12277] Warn on dynamic casts with known NULL results.

[gcc at dixie dot net.nz]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=12277

--- Comment #8 from Andrew Dixie <gcc at dixie dot net.nz> ---
Created attachment 34130
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=34130&action=edit
dynamic_cast warning

[Bug c++/12277] Warn on dynamic casts with known NULL results.

[ville.voutilainen at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=12277

Ville Voutilainen <ville.voutilainen at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |ville.voutilainen at gmail dot com

--- Comment #9 from Ville Voutilainen <ville.voutilainen at gmail dot com> ---
(In reply to Andrew Dixie from comment #7)
> With final, I believe the following dynamic_cast must always return NULL:
> 
> struct a1 { virtual ~a1() {} };
> struct a2 final { virtual ~a2() {} };
> 
> bool test(a1 *x)
> {
>         return dynamic_cast<a2 *>(x) != 0;
> }

If a1 is final, sure. Whether a2 is final is irrelevant.

[Bug c++/12277] eliminate and warn on dynamic casts with known NULL results

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=12277

Martin Sebor <msebor at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|2017-01-11 00:00:00         |2021-2-11
      Known to fail|7.0                         |11.0, 7.5.0

--- Comment #11 from Martin Sebor <msebor at gcc dot gnu.org> ---
No improvement in GCC 11.

[Bug c++/12277] eliminate and warn on dynamic casts with known NULL results

[egallager at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=12277

Eric Gallager <egallager at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |egallager at gcc dot gnu.org

--- Comment #12 from Eric Gallager <egallager at gcc dot gnu.org> ---
Would this warning go under an existing flag, or a new one?

[Bug c++/12277] Warn on dynamic casts with known NULL results.

[gdr at gcc dot gnu dot org]

-- 

gdr at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         AssignedTo|unassigned at gcc dot gnu   |gdr at gcc dot gnu dot org
                   |dot org                     |
             Status|NEW                         |ASSIGNED
   Last reconfirmed|2005-09-10 19:11:30         |2005-11-26 08:05:53
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=12277

[Bug c++/12277] Warn on dynamic casts with known NULL results.

[pinskia at gcc dot gnu dot org]

------- Comment #4 from pinskia at gcc dot gnu dot org  2007-06-11 11:18 -------
*** Bug 32282 has been marked as a duplicate of this bug. ***


-- 

pinskia at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |Woebbeking at web dot de


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=12277

[Bug c++/12277] Warn on dynamic casts with known NULL results.

[jason at gcc dot gnu dot org]

------- Comment #5 from jason at gcc dot gnu dot org  2009-07-06 20:45 -------
This is only known to be NULL if we know the dynamic type of p.  Consider:

struct Foo: Base {};
struct Bar: Derived, Foo {};

Bar bar;
Base *p2 = (Foo*)&bar;

A dynamic_cast to Derived from p2 should succeed.  But it still seems
reasonable to give a warning for pinskia's testcase.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=12277

[Bug c++/12277] Warn on dynamic casts with known NULL results.

[bangerth at gmail dot com]

------- Comment #6 from bangerth at gmail dot com  2009-07-06 20:56 -------
(In reply to comment #3)

I had to stare at the testcase in comment #3 for a little while and thought
others may have to as well: it fails because the inheritance from Base is
*private*, not because the dynamically created object is not derived a Derived
object at all.

W.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=12277

[Bug c++/12277] New: Warn on dynamic casts with known NULL results.

[tilps at hotmail dot com]

PLEASE REPLY TO gcc-bugzilla@gcc.gnu.org ONLY, *NOT* gcc-bugs@gcc.gnu.org.

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=12277

           Summary: Warn on dynamic casts with known NULL results.
           Product: gcc
           Version: 3.4
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P2
         Component: c++
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: tilps at hotmail dot com
                CC: gcc-bugs at gcc dot gnu dot org

While this can not be done generally - if the inheritence relation between the
type being cast from and the type being cast to is known at the time of use,
there are some cases where a dynamic cast can be sure to return NULL.  I can not
think of any valid use of a dynamic cast which will always return NULL - so
therefore I suggest that a warning be given.
The specific case which bothered me (and hence why i'm writing this) is
upcasting through private inheritence always gives NULL for dynamic cast.

[Bug c++/12277] Warn on dynamic casts with known NULL results.

[pinskia at gcc dot gnu dot org]

PLEASE REPLY TO gcc-bugzilla@gcc.gnu.org ONLY, *NOT* gcc-bugs@gcc.gnu.org.

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=12277


pinskia at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |diagnostic

[Bug c++/12277] Warn on dynamic casts with known NULL results.

[bangerth at dealii dot org]

PLEASE REPLY TO gcc-bugzilla@gcc.gnu.org ONLY, *NOT* gcc-bugs@gcc.gnu.org.

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=12277


bangerth at dealii dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
     Ever Confirmed|                            |1
   Last reconfirmed|0000-00-00 00:00:00         |2003-09-15 00:15:55
               date|                            |


------- Additional Comments From bangerth at dealii dot org  2003-09-15 00:15 -------
Seems reasonable. 
W.

[Bug c++/12277] Warn on dynamic casts with known NULL results.

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2004-05-19 13:30 -------
*** Bug 15541 has been marked as a duplicate of this bug. ***

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |bangerth at dealii dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=12277

[Bug c++/12277] Warn on dynamic casts with known NULL results.

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2004-05-19 13:31 -------
And here is an example:
struct Base 
  { 
    virtual ~Base () {} 
  }; 
 
  class Derived : Base 
  { 
    friend int main (); 
  }; 
 
  #include <cassert> 
 
  int main () 
  { 
    Base * p = new Derived; 
 
    assert(dynamic_cast<Derived *>(p)); // fails 
  } 

-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=12277