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Subject: Re: [boost] [outcome] Exception safety guarantees
From: Andrzej Krzemienski (akrzemi1_at_[hidden])
Date: 2017-05-30 11:13:33

2017-05-29 6:15 GMT+02:00 Howard Hinnant via Boost <boost_at_[hidden]>:

> On May 28, 2017, at 6:36 PM, Andrzej Krzemienski via Boost <
> boost_at_[hidden]> wrote:
> >
> > 2017-05-28 2:10 GMT+02:00 Peter Dimov via Boost <boost_at_[hidden]>:
> >
> >>
> >> (It's the same for move, by the way. There are people who prefer
> >> destroy-only for moved-from objects. The standard library does not agree
> >> with them, this time because Howard.)
> >>
> >
> > Acknowledged. But what do the STL containers/algorithms do with the
> > moved-from objects other than to destroy them or reset them?
> They are allowed to do everything with moved-from objects that they are
> allowed to do with objects that haven’t been moved from. For example sort
> is allowed to move construct from, move assign to, move assign from, swap,
> and compare objects, whether or not they are in a moved from state.
> std::sort requires objects to satisfy strict weak ordering. This means
> among other things that x < x is always false, even if x is in a moved-from
> state.
> In summary, being in a moved-from state does not excuse an object from
> meeting the requirements of the algorithm it is being used with. It only
> excuses the object from having a specified state (in most cases).
> Real-world examples:
> An old std::reverse implementation, when given a range with an odd length,
> would swap the middle element with itself. This results in a moved-from
> object being self-move-assigned if the generic std::swap is being used. It
> just has to work. By work, I mean that the self-move-assignment of a
> moved-from-object has to leave the object in a valid but unspecified
> state. If it does this, then self-swap is a non-modifying operation, and
> the reverse algorithm gives the correct result.
> The std::remove algorithm returns moved-from objects back to the client
> (those at the end of the sequence that have been “removed”). The client is
> free to do anything he wants with those objects as long as that operation
> does not have a precondition. Although the typical use case is for the
> client to delete those “removed” elements, that is by no means required, or
> the only use case.
> I know of no sort implementation that compares moved-from objects (though
> I have not done a rigorous survey either). But it is quite possible that a
> sort algorithm could compare a moved-from object against itself, and still
> be a correct algorithm, as long as x < x (or comp(x, x)) _always_ returns
> false.

Ok, I think I asked the wrong question. I asked:

 what do the STL containers/algorithms do with the moved-from objects

But what I actually meant was:

"when an STL algorithm *causes* an object to obtain a valid-but-unspecified
*during its operation* , what does it later do with this object other than
to destroy it or re-set it (or leave it like this for the caller -- for the
caller to destroy it or reset it)?"

Because, if I myself am passing objects in valid-but-unspecified state to
STL algorithms, I am already doing something wrong (I should have probably
destroyed or reset these objects).


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