Re: [Boost-users] Valgrind false positive in make_shared -- possible solution

1 Dec 2010

      On Tuesday, November 30, 2010 03:41:29 pm Dave Steffen wrote:
...
On Tuesday, November 30, 2010 01:46:39 pm Dave Steffen wrote:
...
On Tuesday, November 30, 2010 03:40:14 am Peter Dimov wrote:
...
Dave Steffen wrote:
...
Hi
Coming back to this issue after a week or two...  trying the above
[changing to std::forward]
...
made no difference: no compiler warnings, and no change in
  Valgrind's error messages.
Looks like a g++ bug then. You should try reproducing it in a
self-contained example and report it to the GCC Bugzilla. I'm not sure
what we can do on our end to avoid it. (I'll switch to std::forward in
any case, since this seems a good idea in general, but it won't help in
this specific case.)
I reported this as GCC bug 46732.  You can see the whole discussion
  at
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=46732
but the short version is:
    There was a defect in the language, core issue 664
    http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#664
GCC 4.4 predates that issue being resolved, so the only real
    workaround
is to
avoid using -std=c++0x with GCC 4.4
(thanks to Jonathan Wakely for getting to this so quickly)
Jonathan Wakely has proposed a fix; since I'm guessing nobody out
  there is watching the bug report, I thought I'd pass this along.

----------------------------------------------------------------------
...
GCC 4.4 predates that issue being resolved, so the only real workaround is 
to
avoid using -std=c++0x with GCC 4.4
Or do something like I did for std::move in my initial implementation of
std::future, which returned scalars by value rather than by rvalue-reference:

  // workaround for CWG issue 664 and c++/34022
  template::value>
    struct _Move_future_result
    {
      typedef _Result&& __rval_type;
      static _Result&& _S_move(_Result& __res) { return std::move(__res); }
    };

  // specialization for scalar types returns rvalue not rvalue-reference
  template<typename _Result>
    struct _Move_future_result<_Result, true>
    {
      typedef _Result __rval_type;
      static _Result _S_move(_Result __res) { return __res; }
    };

Boost could use something like this (for GCC 4.4 with -std=c++0x only) to 
avoid
the problem

----------------------------------------------------------------------

  Thanks very much.

-- 
Dave Steffen, Ph.D. - Software Engineer

Numerica Corporation http://www.numerica.us
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Loveland, Colorado 80538
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Email: dave.steffen@numerica.us
fax (970) 461-2004

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