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Subject: Re: [boost] several messages
From: Marc Glisse (marc.glisse_at_[hidden])
Date: 2012-08-07 08:11:32

On Sun, 5 Aug 2012, John Maddock wrote:

>> I believe Marc is correct. You must be able to destruct *and* assign-to a
>> moved-from object.
> Just checked the std and you're both correct.
> 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.

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.

Marc Glisse

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