[boost] Addressing the variant issue

From: David Abrahams (dave_at_[hidden])
Date: 2002-08-10 09:02:55

It seems to me that if you want a variant that can never be empty (and I'm
sympathetic to the importance of having a simpler invariant!) you need to
do something like the move-away-move-back trick Doug was suggesting. Sounds
like a job for move semantics.

IIUC, Howard's idea of move semantics either constructs or assigns a T,
leaving a valid but possibly changed T behind in the source object. POD
types, for example, are trivially-movable.

Maybe we just need to ask people to give us types which are
nothrow-movable, and to specialize some template for types they want to put
in a variant which aren't known to be nothrow-movable.

As a convenience, we could also allow people to pass trivially_movable<U>
to the variant, which would say "I know memcpy moves this type, and I don't
feel like specializing".

Thoughts?

BTW, Move construction would have to be emulated via something like
move-placement-new.

-----------------------------------------------------------
           David Abrahams * Boost Consulting
dave_at_[hidden] * http://www.boost-consulting.com