[PATCH][ADT] Add move-construction and move-assignment from SmallVectorImpl<T> to SmallVector<T,N>.

[David Blaikie]

On Thu, Jan 22, 2015 at 4:17 PM, Duncan P. N. Exon Smith <
dexonsmith at apple.com> wrote:

>
> > On 2015-Jan-22, at 16:04, David Blaikie <dblaikie at gmail.com> wrote:
> >
> > This looks like it's only adding copy, not move - did I miss something?
> >
> > On Thu, Jan 22, 2015 at 3:54 PM, Lang Hames <lhames at gmail.com> wrote:
> > Hi All,
> >
> > The attached patch adds move-construction and move-assignment from
> SmallVectorImpl<T> to SmallVector<T, N>. It enables us to move between
> SmallVectors of different types, which will allow me to fix one of the most
> commented-upon warts from the new JIT APIs.
> >
> > Unit-test included.
> >
> > Cheers,
> > Lang.
> >
>
> Yeah, the tests don't actually confirm that the thing was moved.  In
> particular, you want to check that whenever the source is in big-mode,
> the destination points at the same memory that the source used to (well,
> assuming that's the behaviour you want, which the naive implementation
> will give you IIUC).
>
> There's also a "big mode -> small mode" funny state thing that'll happen
> with the naive implementation.  I'm not sure it's *wrong*, but...
>
> Given:
>
>     SmallVector<int, 1> InBigMode(2);
>     SmallVector<int, 2> InBigModeToo = std::move(InBigMode);
>
> I think this will pass:
>
>     assert(!InBigModeToo.isSmall());
>
> This a new state -- a SmallVector can be in "big" mode with a heap
> allocation, even though its capacity would fit in its "small" mode
> stack allocation.
>
> While weird, I don't really have a problem with it (but maybe others
> do?).  Might be worth testing a few operations in this untested state
> though.


It's probably valuable that this is the outcome - it'd be wasteful to throw
away the dynamic allocation (going down to small mode) - better to keep it
even though it's < small_size. So I'd be in favor of this.
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