[Bug c++/15531] New: dynamic_cast fails (at runtime) when dependant on friendship of invoker

[Bug c++/15531] New: dynamic_cast fails (at runtime) when dependant on friendship of invoker

[gccbugs at contacts dot eelis dot net]

Consider the following snippet:

  struct Base
  {
    virtual ~Base () {}
  };

  class Derived : Base
  {
    friend int main ();
  };

  #include <cassert>

  int main ()
  {
    Base * p = new Derived;

    assert(dynamic_cast<Derived *>(p)); // fails
  }

When Derived is changed to publically inherit Base, the dynamic_cast succeeds.

-- 
           Summary: dynamic_cast fails (at runtime) when dependant on
                    friendship of invoker
           Product: gcc
           Version: 3.2.3
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: c++
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: gccbugs at contacts dot eelis dot net
                CC: gcc-bugs at gcc dot gnu dot org
  GCC host triplet: i486-slackware-linux
GCC target triplet: i486-slackware-linux


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=15531

[Bug c++/15531] dynamic_cast fails (at runtime) when dependant on friendship of invoker

[bangerth at dealii dot org]

------- Additional Comments From bangerth at dealii dot org  2004-05-18 23:29 -------
Of course it fails. You can't use dynamic_cast to work around access 
control. 
 
W. 

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|                            |INVALID


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=15531

[Bug c++/15531] dynamic_cast fails (at runtime) when dependant on friendship of invoker

[gccbugs at contacts dot eelis dot net]

------- Additional Comments From gccbugs at contacts dot eelis dot net  2004-05-18 23:32 -------
Which access control are you referring to?
main() has been granted friendship by Derived, and in the snippet even creates a
Derived instance.

-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=15531

Re: [Bug c++/15531] New: dynamic_cast fails (at runtime) when dependant on friendship of invoker

[Gabriel Dos Reis]

"gccbugs at contacts dot eelis dot net" <gcc-bugzilla@gcc.gnu.org> writes:

| Consider the following snippet:
| 
|   struct Base
|   {
|     virtual ~Base () {}
|   };
| 
|   class Derived : Base
|   {
|     friend int main ();
|   };
| 
|   #include <cassert>
| 
|   int main ()
|   {
|     Base * p = new Derived;
| 
|     assert(dynamic_cast<Derived *>(p)); // fails

As required by the C++ standard.  There is no way you can successfully
dynamic_cast from base class to derived class that does not publically
inherit. 

|   }
| 
| When Derived is changed to publically inherit Base, the dynamic_cast succeeds.

Yes, that is what is required by the ISO C++ standard.

The PR is invalid.

-- Gaby

[Bug c++/15531] dynamic_cast fails (at runtime) when dependant on friendship of invoker

[bangerth at dealii dot org]

------- Additional Comments From bangerth at dealii dot org  2004-05-18 23:35 -------
Maybe I should be a little more explicit: the cast actually 
happens inside the dynamic_cast operator, but that isn't a 
friend of the two classes (and can't be made a friend). Thus, 
it can't do the cast and returns zero. The fact that the 
call to the dynamic_cast is inside a class that _can_ do this 
is immaterial here. 
 
W. 

-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=15531

[Bug c++/15531] dynamic_cast fails (at runtime) when dependant on friendship of invoker

[gdr at integrable-solutions dot net]

------- Additional Comments From gdr at integrable-solutions dot net  2004-05-18 23:38 -------
Subject: Re:  New: dynamic_cast fails (at runtime) when dependant on friendship of invoker

"gccbugs at contacts dot eelis dot net" <gcc-bugzilla@gcc.gnu.org> writes:

| Consider the following snippet:
| 
|   struct Base
|   {
|     virtual ~Base () {}
|   };
| 
|   class Derived : Base
|   {
|     friend int main ();
|   };
| 
|   #include <cassert>
| 
|   int main ()
|   {
|     Base * p = new Derived;
| 
|     assert(dynamic_cast<Derived *>(p)); // fails

As required by the C++ standard.  There is no way you can successfully
dynamic_cast from base class to derived class that does not publically
inherit. 

|   }
| 
| When Derived is changed to publically inherit Base, the dynamic_cast succeeds.

Yes, that is what is required by the ISO C++ standard.

The PR is invalid.

-- Gaby


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=15531

[Bug c++/15531] dynamic_cast fails (at runtime) when dependant on friendship of invoker

[gccbugs at contacts dot eelis dot net]

------- Additional Comments From gccbugs at contacts dot eelis dot net  2004-05-18 23:43 -------
I see, thanks for clearing that up.
In this case, would it not be desirable to have gcc emit an error/warning
indicating that the cast will always fail since Base is not a public base class
of Derived?

-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=15531

[Bug c++/15531] dynamic_cast fails (at runtime) when dependant on friendship of invoker

[gdr at integrable-solutions dot net]

------- Additional Comments From gdr at integrable-solutions dot net  2004-05-19 01:15 -------
Subject: Re:  dynamic_cast fails (at runtime) when dependant on friendship of invoker

"gccbugs at contacts dot eelis dot net" <gcc-bugzilla@gcc.gnu.org> writes:

| I see, thanks for clearing that up.
| In this case, would it not be desirable to have gcc emit an error/warning
| indicating that the cast will always fail since Base is not a public
| base class of Derived?

yes, it could -- not in all cases -- but this is certainly a case when
the answer is known at translation time.  The PR could be open as
diagnostic enhancement and the subject changed to reflect the outcome
ot this discussion.

-- Gaby


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=15531

[Bug c++/15531] dynamic_cast fails (at runtime) when dependant on friendship of invoker

[bangerth at dealii dot org]

------- Additional Comments From bangerth at dealii dot org  2004-05-19 13:16 -------
I opened PR 15541 for this. 
 
W. 

-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=15531

[Bug c++/15531] dynamic_cast fails (at runtime) when dependant on friendship of invoker

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2004-05-19 13:34 -------
Which was a dup of bug 12277.

-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=15531