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Subject: Re: [boost] several messages
From: John Maddock (boost.regex_at_[hidden])
Date: 2012-08-07 08:41:17

>> OK I'll have to rethink, and make sure everything other than arithmetic
>> is supported for moved-from types.
> Actually, in principle, a moved-from object should be a valid object,
> which just has an unspecified value. But computing x-x should be legal,
> unless it is documented that the type has a valid singular state
> (accessible by being moved from or swapped (=move-assigned)) and that all
> operations other than assignment have a precondition that the arguments
> are not singular. And then it is strange not to have a way to test if a
> value is singular.
> Most uses of a moved-from object I can think of other than destruction and
> assignment are rather contrived. For a container, it can make sense to
> look at its size and assign to the elements. For a bignum, maybe if you
> are interested in an arbitrary number < y (to pass it to a nextafter-like
> function), before setting x to y-1, you could check if x<y and not want
> that to crash the program. If you are going to use a bignum as a bitfield,
> the same argument as for the container could be made.

The problem with your comparison example, is it's not clear whether the
singular state is less than a non-singular value or not. Basically all
operations are undefined.

I think unless there are strong objections, I'll only support
destruct-or-assign-to moved-from objects, and document that as a limitation.
I can add asserts for the other operations, but basically I think that
should be enough for the types to interoperate with the std lib.

> Your remark about this making rvalue references useless is one that has
> been made a number of times and usually leads to the destructive move
> semantics proposal.
> In this case, ideally gmp should have an empty state ("lazy allocation" in
> the projects page). In general, what to do with types that do not have an
> empty state is still a bit unclear. There is a proposal to add an
> overloadable library function that does a destructive move, but it
> wouldn't help here.

Understood, John.

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