Re: [boost] Re: basic guarantees in variant

From: E. Gladyshev (egladysh_at_[hidden])
Date: 2003-10-09 18:13:37

--- David Abrahams <dave_at_[hidden]> wrote:
> "E. Gladyshev" <egladysh_at_[hidden]> writes:
[...]
> 
> Do you really think I missed that sentence?  You can prove almost
> anything you want to about C++ by picking a single sentence out of
> the standard on its own.
> 
> > It doesn't define the process of how exactly the storage gets
> > reused, does it?  "reusing storage" what does it mean?
> >
> > When you do an assignment in variant it is the process
> > of reusing the storage that is not defined in the standard.
> 
> You have to read and think about the whole clause to understand the
> problems.  That one sentence doesn't stand by itself.
> 
> Once you reuse the storage for the object, its lifetime has ended.
> The first problem is that clients of variant should have the right to
> depend on side-effects of that object's destruction (anything else
> makes variant a very strange beast to use, indeed), but if they do,
> it's undefined behavior (see the last sentence of the clause).
> 
> The second problem, even if the program doesn't depend on the
> side-effects of the object's destructor, is: copying the saved bits
> back to that location do not constitute beginning the lifetime of a
> new object if it has a non-trivial constructor (see the beginning of
> the clause -- you have to call a constructor do that).  It should be
> obvious that you can't call the destructor of an object whose lifetime
> has never begun.  I'll leave you to find that in the standard if it
> isn't obvious to you.  The destructor call you use after copying away
> the bits of the newly-constructed object resulting from assignment
> causes undefined behavior.
> 
> That is about as clear as it is possible to make this issue.  If you
> have any argument with my analysis, please go to comp.std.c++ or some
> other forum and get the opinions of other standards experts before
> bringing them to me because I am not going to try to explain it any
> more and nothing you can say without the strength of a standards
> expert's authority to back it up, at this point, will impress me.

Why do you think I want to impress you? Ok, ok, I want. :)

> 
> > FYI I don't really care about your comments other than technical.
> > You can relax a bit.
> 
> If you would like me to be relaxed, you'll stop making arrogant
> pronouncements, treat your fellow Boosters as though they may have
> done a little thinking about the issues and might even know something
> you don't, especially lay off remarks like "stop kidding yourself",
> and approach the issue with a genuine spirit of inquiry rather than as
> though you already know all the answers.  There's more to having a
> productive technical discussion than just making "technical comments."
> 

Then you drop your "silliness" and "I am not going blah, blah..."
remarks. Didn't you notice I have been very forgiving
to your "non-technical" remarks?

Ok, back to the point.

You are not answering my question.
How does the standard defines "reusing storage"?

Eugene

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