Re: [boost] Re: Variant implementation change (was re: unions) final summary

From: E. Gladyshev (egladysh_at_[hidden])
Date: 2003-10-10 01:09:25

I forgot to mention

1.7.1
"An object has a storage duration which influences its lifetime"

Eugene

--- "E. Gladyshev" <egladysh_at_[hidden]> wrote:
> 
> --- Douglas Gregor <gregod_at_[hidden]> wrote:
> > On Friday 10 October 2003 12:02 am, E. Gladyshev wrote:
> > > char *p = new( (void*)&m.x );
> > >
> > > I am using the part of already constructed object 'm' to construct
> > > another object so is 'm' lifetime is over?
> > 
> > Apparently you did not read the clause David A. cited. Part of it says:
> >   12.6.2 describes the lifetime of base and member subobjects
> > 
> > Did you read 12.6.2?
> 
> 
> Of course I read the standard and I reread 12.6.2 
> it is not relevant to reusing storage.
> What is relevant is 3.7.4
> 
> "The storage duration of member subobjects, base class  subobjects  and
>   array elements is that of their complete object "
> 
> Now let go back to my example.
> 
> According 3.8.4 (IF WE ASSUME THAT  'new(obj)' is "reusing storage") when I call
> char *p = new( (void*)&m.x ) char;
> 
> I end lifetime of m.x, so I end the storage duration of a member subobject.
> 
> According to 3.7.4
> The storage duration of m.x is the same as the its complete object.
> 
> Do you see what I am talking about? "reusing storage" has to be
> consistently defined to fit into the whole picture.
> 
> > I have: http://lists.boost.org/MailArchives/boost/msg53635.php
> 
> I just proved you wrong.
> 
> > Summary: you ended the lifetime of an object, then tried to resurrect it by 
> > copying the bits back. It doesn't even take *any* deduction to see that the 
> > code invokes undefined behavior, because the standard explicitly states what 
> > happens, and we've quoted the relevant clause. 
> 
> The standard used undefined concepts when it states what happens.
> 
> [...]
> > 
> > Wait, so we're supposed to rewrite an international standard first, and then 
> > come back to this discussion? Even if this were even remotely feasible (it 
> > isn't), this is the wrong forum for such things. We develop C++ libraries; we 
> > do not write standards.
> 
> No, you are not supposed to rewrite anything. It is not the point of 
> the discussion. The point is to reach a trully correct conclusion.
> How to deal with it is another story.
> 
> > At Boost we try to keep an open mind, we hope to learn from each other, and we 
> > don't mind teaching from time to time. We do enjoy a fresh perspective on an 
> > old problem, and some problems that were considered unworkable have indeed 
> > been solved. But they have not been solved by ignoring the old arguments, the 
> > ones that prove ways in which the problem can't be solved. You have 
> > categorically refused to do your share
> 
> What do you mean exactly by "refused to do your share"?
> 
> Eugene
> 
> 
> 
> 
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