[Bug libstdc++/51823] New: reverse iterator instantiated with POD type returns uninitialized values + work around

["jakobsybren at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51823

             Bug #: 51823
           Summary: reverse iterator instantiated with POD type returns
                    uninitialized values + work around
    Classification: Unclassified
           Product: gcc
           Version: 4.6.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: jakobsybren@gmail.com


Created attachment 26299
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=26299
*.i* output of minimal example that reproduces the bug

System info:
$ uname -a 
Linux ****** 3.0.0-14-generic #23-Ubuntu SMP Mon Nov 21 20:28:43 UTC 2011
x86_64 x86_64 x86_64 GNU/Linux

$ g++ -v
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/4.6.1/lto-wrapper
Target: x86_64-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 --disable-werror --with-arch-32=i686 --with-tune=generic
--enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu
--target=x86_64-linux-gnu
Thread model: posix
gcc version 4.6.1 (Ubuntu/Linaro 4.6.1-9ubuntu3)


Attachment:
 * ostream_iterator_reverse_iterator.ii: output of:
 g++ -save-temps -o ostream_iterator_reverse_iterator -Wall
ostream_iterator_reverse_iterator.cc
 where ostream_iterator_reverse_iterator.cc is a minimal example that
reproduces the bug.

Headerfiles involved:
 <iterator>

Compile command: g++ -o ostream_iterator_reverse_iterator -Wall
ostream_iterator_reverse_iterator.cc

Description:
Consider an iterator class that implements the requirements of a
forward_iterator. We underlying datatype is always POD and the operator*()
member of the iterator returns the results by value, since it is POD anyway.

Upon creating a reverse_iterator from this class a simple for-loop produces the
expected results:
 Returning 9 9
 Returning 8 8
 Returning 7 7
 Returning 6 6
 Returning 5 5
 Returning 4 4
 Returning 3 3
 Returning 2 2
 Returning 1 1
 Returning 0 0

but using the reverse_iterator in std::copy() with std::ostream_iterator
produces:
 Returning 9 32767
 Returning 8 32767
 Returning 7 32767
 Returning 6 32767
 Returning 5 32767
 Returning 4 32767
 Returning 3 32767
 Returning 2 32767
 Returning 1 32767
 Returning 0 32767

It seems that the internal value does not get copied. When using -O3 both the
for-loop and the copy() construct produce 0 (zero) for each call to operator*()
in the reverse_iterator, but not in the normal iterator. 

The 'work-around' forces the use of forwarding. I put it as comment in the
minimum example. If I should have attached a separate source file with just the
operator*()-line replaced, I wish to express my apologies in advance.

The source file with the work-around instead of the original version of
operator*() needs an additional compiler option though: -std=c++0x
However, even with this version, the result with -O3 is still all zeros; not
what I expected, but since that is the same as before, I guess that is more
likely to be me misunderstanding what is supposed to happen with -O3 switched
on.