on Wed Aug 08 2012, Marc Glisse <marc.glisse-AT-inria.fr> wrote:
On Wed, 8 Aug 2012, Dave Abrahams wrote:
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.
This sounds kind of like programming with iostreams whose badbit has been set. Lots of times it's possible to write the code so that it can barrel ahead as though nothing is wrong and only check for a problem much later. But it sounds kind of implausible that you'd be able to pick a value for empty bignums that would allow that in most or even many cases.
Hi Dave,
I am not sure what you mean here. The safe solution is that whenever you move-construct from a bignum, you set it to zero (move-assignment is a swap). I am not sure, but it sounds like you are saying this wouldn't be safe?
Not exactly. With floats, we have non-signalling NaN, which IIUC is used in exactly that way: you might end up with a few NaNs in your big linear algebra solution but they might well turn out to be unimportant. What you do know is that anything that touches them will also yield a NaN, sort of like anything that uses a stream with badbit set yields a stream with badbit set. Zero doesn't work that way: after all, it's an additive identity. -- Dave Abrahams BoostPro Computing Software Development Training http://www.boostpro.com Clang/LLVM/EDG Compilers C++ Boost