From: "Douglas Gregor" <gregod@[...].edu> 
> 
>  We now have three distinct allocation strategies with different exception
&
>  performance characteristics:
> 
>    1) Stack allocation. May become empty if an exception is thrown during
>  assignment/swap (basic guarantee). Takes minimal stack space & will be
most
>  efficient otherwise.
> 
>    2) Your strategy. Strong guarantee (no ugly "empty" state). Still
>  stack-allocated, but takes twice the minimum space. As efficient as (1)
> 
>    3) Heap allocation. Strong guarantee, but uses heap allocation (less
>  efficient than the other two because of new/delete calls).

Nothrow guarantee for swap, if I'm not mistaken.

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