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Subject: Re: [boost] [move][container] Review Request (new versions ofBoost.Move and Boost.Container in sandbox and vault)
From: Howard Hinnant (howard.hinnant_at_[hidden])
Date: 2009-09-07 17:29:11

On Sep 7, 2009, at 5:20 PM, Christopher Jefferson wrote:

> On 7 Sep 2009, at 21:44, Peter Dimov wrote:
>>>> I'm actually not 100% positive what the intended semantics of x =
>>>> move(x) are, but clearly as a QOI issue it should behave.
>>>> Most people put:
>>>> if(&__x != this)
>>>> or something similar in normal assignment operators, adding it also
>>>> to all move operators seems very sensible.
>> This is not enough in general. clear() may end up indirectly
>> destroying the contents of x, even when &x != this. (Although...
>> now that I think of it... this may be a problem with the vector
>> copy assignment as well.)
>>> My hope has been to make x = move(x) undefined behavior.
>> I find this a bit hard to justify.
>>> Rationale:
>>> The move assignment operator should be fast enough that an extra if-
>>> test may adversely impact performance (above the noise level).
>> The problem with making it UB is that y = move(x) now needs to be
>> guarded with if( &x != &y ) everywhere in client code. Sometimes
>> the user will know that x is not y, but is this really the case
>> most of the time?
> Actually thinking about it, I wonder if when this occurs it is a
> mistake -- should instead a debug mode assert if &x == &y?
> x = move(x) actually seems like it would be a logic error. Certainly
> having now implemented a reasonable amount of rvalue code, including
> a standard library implementation, I can't think of where it would
> arise without an error. move(x) implies we do not care about the
> value of x, but then the assignment means that we do care.
> Certainly I'd want to see an assert in debug mode, to catch this, as
> I imagine errors it causes could be quite subtle.

<nod> Along the same line of thought a debug mode vector/deque::insert
(p, T&& t) could detect/assert if &t is within the range of those
objects that need to be moved to make room for t.


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