On Monday, May 2, 2016 at 9:49:24 AM UTC+10, Jim Blandy wrote:
> No, we don't know that [moved-from strings become empty].
> The contract of a move in C++ is simply that the
> source object is safe to destruct, but otherwise in an undefined state*. You
> must not make any assumptions about its value.
* (To be fair, in that context I think you were only arguing there that we
can't assume anything about the value, but didn't explicitly rule out a
reassignment.)
Sorry to revive this old thread, but I've just noticed that our MFBT Swap() is
written in terms of Move()'s:
> template<typename T>
> inline void
> Swap(T& aX, T& aY)
> {
> T tmp(Move(aX));
> aX = Move(aY);
> aY = Move(tmp);
> }
This assumes that a moved-from object can safely be assigned to -- Which used
to seem reasonable to me, but I got confused after our discussions here!
Clang's std::swap does the same! (With extra checks that the type is move
constructible&assignable.)
So can we assume that any moved-from object should be left in a state that is
safe to destruct AND safe to be assigned to?
But no assumption can be made about the actual contained value, i.e.: It could
be the same, it could be empty, or it could be in some unsafe-to-use state
until destroyed or overwritten.
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