[Bug c++/51283] New: g++ yields different behaviour for postfix increment and decrement operators with fundamental types vs. classes

[Bug c++/51283] New: g++ yields different behaviour for postfix increment and decrement operators with fundamental types vs. classes

[dicomj23 at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51283

             Bug #: 51283
           Summary: g++ yields different behaviour for postfix increment
                    and decrement operators with fundamental types vs.
                    classes
    Classification: Unclassified
           Product: gcc
           Version: 4.6.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: dicomj23@gmail.com


dico:~$ gcc -v
Using built-in specs.
COLLECT_GCC=gcc
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)



dico:~$ cat ./main.cpp 
#include <vector>
#include <assert.h>
using namespace std;

int main ()
{
  int i = 0;
  assert(i++ == i++); // OK

  int* p = 0;
  assert(p++ == p++); // OK

  vector<int> v(10, 10);
  vector<int>::iterator it = v.begin();
  assert(it++ == it++); // FAILS

  return 0;
} 



dico:~$ g++ ./main.cpp && ./a.out
a.out: ./main.cpp:15: int main(): Assertion `it++ == it++' failed.
Aborted


GCC should calculate expression (it++ == it++) with an object copy returned by
the overload it.operator ++ (int) method like for the first two examples.

[Bug c++/51283] g++ yields different behaviour for postfix increment and decrement operators with fundamental types vs. classes

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51283

Jonathan Wakely <redi at gcc dot gnu.org> changed:

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

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> 2011-11-23 13:00:06 UTC ---
(In reply to comment #0)
>   int i = 0;
>   assert(i++ == i++); // OK

Undefined behaviour, try using -Wsequence-point

>   int* p = 0;
>   assert(p++ == p++); // OK

Undefined behaviour, try using -Wsequence-point

>   vector<int> v(10, 10);
>   vector<int>::iterator it = v.begin();
>   assert(it++ == it++); // FAILS

If vector::iterator is a pointer that's undefined behaviour.
If it's a class type (as in libstdc++) then the assertion is meant to fail
because it's equivalent to:

auto it1 = it++;
auto it2 = it++;
assert( it1 == it2 );  // obviously wrong

> GCC should calculate expression (it++ == it++) with an object copy returned by
> the overload it.operator ++ (int) method like for the first two examples.

No it shouldn't.