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Gmane
From: Stephan T. Lavavej <stl <at> exchange.microsoft.com>
Subject: Re: C++ announcements coming tomorrow
Newsgroups: gmane.comp.lib.boost.devel
Date: Monday 5th November 2012 02:06:42 UTC (over 4 years ago)
[Andrey Semashev]
> +1, I had reported a memory leak bug [1] to MS and had been told that
> the behavior is correct as it doesn't contradict the Standard. I can't
> trust the vendor that treats users this way and declares a memory leak
> as a valid behavior. I doubt I will ever bother reporting any other
> bugs in MSVC.
> [1] The leak appeared in STL streams, if initialized multiple times.
> Here's a code snippet:
> https://sourceforge.net/apps/trac/boost-log/ticket/2#comment:4
> I didn't keep a reference to the MS bug tracker and I can't find it now.

This was Dev10#831920/Connect#518512. I can't load the Connect link anymore
(it is supposed to be http://connect.microsoft.com/VisualStudio/feedback/details/518512/memory-leak-in-ostream-init
and I don't know why it's broken) but I can still see the comments through
Team Foundation Server. This behavior was really, truly conformant to the
Standard - I was surprised too, so I had to ask P.J. Plauger for an
explanation. Here are the comments:

--
Posted by ildjarn on 12/7/2009 at 3:21 AM
Reproduced using VC++ 2010 beta 2. I traced the leak back to
std::ios_base::_Init(), which effectively does the following (where _Ploc
is a std::locale*): _Ploc = 0; // other code _Ploc = new locale; I would
think that before assigning NULL to _Ploc, it should be doing 'delete
_Ploc;'.

Posted by Microsoft on 12/7/2009 at 6:39 PM
Thank you for your feedback, we are currently reviewing the issue you have
submitted. If this issue is urgent, please contact support directly(http://support.microsoft.com)

Posted by Microsoft on 12/8/2009 at 10:36 PM
Thanks for your feedback. We are rerouting this issue to the appropriate
group within the Visual Studio Product Team for triage and resolution.
These specialized experts will follow-up with your issue. Thank you

Posted by Microsoft on 12/9/2009 at 3:11 PM
Hi, Thanks for reporting this issue. I've resolved it as By Design because
section 27.4.4.1 [lib.basic.ios.cons] of the 2003 C++ Standard does not
allow init() to be called on a constructed object. (Yes, this is strange.)
If you have any further questions, feel free to E-mail me at
[email protected] . Stephan T. Lavavej Visual C++ Libraries Developer

Posted by AndySem on 12/9/2009 at 9:42 PM
Sorry, but what particular statement in the Standard leads to this
conclusion? I did not find such a restriction in 27.4.4.1 (and in
particular, in point 3, where init effects are described).

Posted by Microsoft on 12/10/2009 at 4:47 PM
See 27.4.4.1/2: basic_ios() "Constructs an object of class basic_ios
(27.4.2.7) leaving its member objects uninitialized. The object must be
initialized by calling its init member function. If it is destroyed before
it has been initialized the behavior is undefined." Because init() is
called when data members are uninitialized, it must assume that any
pointers are garbage, and can't delete them. That's why it leaks memory
when called on a fully initialized object. Stephan T. Lavavej Visual C++
Libraries Developer

Posted by AndySem on 12/10/2009 at 8:24 PM
First, this does not mean that init cannot be called more than once.
Technically speaking, it may track whether it was called or not in an
internal data member that is not covered by the Standard. Second, the
Standard does not describe data members at all in the first place. It does
not say that basic_ios shall have raw pointers instead of, say, auto_ptrs.
The constructor cannot be called without default initializing these
members. So the whole statement about "leaving its member objects
uninitialized" is a moot and should be understood as "the members are
default initialized and in that state the basic_ios object may not be
usable". That is why I still consider it as a bug. I think it should be
fixed.

Posted by Microsoft on 12/11/2009 at 1:24 PM
The Standard allows an implementation to provide very weak guarantees on
when init() may be called; therefore, VC's implementation conforms to the
Standard. Although we could probably provide stronger guarantees, strictly
conforming programs couldn't take advantage of them. Our implementation
(licensed from Dinkumware) has behaved this way for over 20 years, and
given our finite resources we don't believe that changing our design here
is worth the time necessary. (There are some places where we believe that
exceeding the Standard's guarantees is worthwhile - e.g. we supported
stateful allocators long before C++0x required it.) I know that this isn't
what you want to hear, but this is why the Standard exists in the first
place - to provide a contract between implementers and users, saying w
 hat implementers are required to provide and what users are allowed to
depend on. If you would like the Standard to provide stronger guarantees,
you can file a Library Issue with the Committ
 ee. Stephan T. Lavavej Visual C++ Libraries Developer
--

STL


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