[Bug c++/51563] New: dynamic_cast bug

[gausszhch at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51563

 Bug #: 51563
   Summary: dynamic_cast bug
Classification: Unclassified
   Product: gcc
   Version: 4.6.1
Status: UNCONFIRMED
  Severity: normal
  Priority: P3
 Component: c++
AssignedTo: unassig...@gcc.gnu.org
ReportedBy: gaussz...@gmail.com


The following code snippet:


#include 
using namespace std;

struct D1{ virtual void fooD1(){} };
struct D2{ virtual void fooD2(){} };

struct C1: virtual D1{ virtual void fooC1(){} };
struct C2: virtual D1{ virtual void fooC2(){} };

struct C3: virtual D2{ virtual void fooC3(){} };
struct C4: virtual D2{ virtual void fooC4(){} };
// A-pub->C1-vir->D1
// A-pri->C2-vir->D1
// A-pub->C3-vir->D2
// A-pri->C4-vir->D2
struct A: C1, private C2, C3, private C4{ virtual void fooA(){} };

int main(int argc, char** argv){
A* pa = new A;

D1* pd1 = dynamic_cast(pa);
if(pd1) cout << "success 1\n";
elsecout << "FAIL 1\n";
D2* pd2 = dynamic_cast(pd1);
if(pd2) cout << "success 2\n";
elsecout << "FAIL 2\n";

pd2 = dynamic_cast(pa);
if(pd2) cout << "success 3\n";
elsecout << "FAIL 3\n";
pd1 = dynamic_cast(pd2);
if(pd1) cout << "success 4\n";
elsecout << "FAIL 4\n";
return 0;

}
-

Compile it and run:
$ g++ test.cpp -o test
$ ./test
success 1
success 2
success 3
FAIL 4

This is, I believe, a bug.  VC++ (on windows) results in:

success 1
success 2
success 3
success 4

which I think is correct.

The gcc I used is:
$ gcc -v
Using built-in specs.
COLLECT_GCC=g++
COLLECT_LTO_WRAPPER=/usr/lib/gcc/i686-linux-gnu/4.6.1/lto-wrapper
Target: i686-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu/Linaro
4.6.1-9ubuntu3' --with-bugurl=file:///usr/share/doc/gcc-4.6/README.Bugs
--enable-languages=c,c++,fortran,objc,obj-c++,go --prefix=/usr
--program-suffix=-4.6 --enable-shared --enable-linker-build-id
--with-system-zlib --libexecdir=/usr/lib --without-included-gettext
--enable-threads=posix --with-gxx-include-dir=/usr/include/c++/4.6
--libdir=/usr/lib --enable-nls --with-sysroot=/ --enable-clocale=gnu
--enable-libstdcxx-debug --enable-libstdcxx-time=yes --enable-plugin
--enable-objc-gc --enable-targets=all --disable-werror --with-arch-32=i686
--with-tune=generic --enable-checking=release --build=i686-linux-gnu
--host=i686-linux-gnu --target=i686-linux-gnu
Thread model: posix
gcc version 4.6.1 (Ubuntu/Linaro 4.6.1-9ubuntu3) 

The operating system is Ubuntu 11.10 (server 64bit).

I have also tested this code snippet on Debian-6.0 with GCC-4.4.5 and
CentOS-5.4 with GCC-4.1.2, which produces same result.

[Bug c++/51563] dynamic_cast bug

[gausszhch at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51563

--- Comment #3 from gausszhch at gmail dot com 2011-12-15 10:21:21 UTC ---
What does EDG stand for?

Besides, why does dynamic_cast(pd1) success but dynamic_cast(pd2)
fail? 

Here, D1 and D2 are symmetrical in the hierarchy graph. I think they can be
cast to each other according to the ISO/IEC 14882:2003. It is said that

" dynamic_cast(v)
...
if v points (refers) to a public base class sub-object of the most derived
object, and the type of the most derived object has a base class, of type T,
that is unambiguous and public, the result is a pointer (an lvalue referring)
to the T sub-object of the most derived object.
"

(In reply to comment #1)
> A real C++ compiler based on EDG agrees with GCC.