[boost] Re: In-place construction for variant

From: Anthony Williams (anthony.williamsNOSPAM_at_[hidden])
Date: 2003-10-27 05:17:12

Eric Friedman <ebf_at_[hidden]> writes:
> Double storage has been replaced by the above-described temporary heap
> backup strategy so as to incur overhead only when/if variant assignment
> occurs (vs. the constant space inefficiency incurred by double storage).

I obviously missed the discussion where this was decided; I don't like this
solution. I felt that a strong part of the design was that no heap storage was
used, and so using it for assignment violates that principle, and is IMO
unacceptable.

> Heap backup will not be used for assignment to a variant if the content type
> of the right-hand side is nothrow copyable.
>
> As well, heap backup will never be used for assignment to a
> variant<T1,...,TN> if *any* of its bounded types Ti are nothrow
> default-constructible. (Note, this is regardless of the content of the
> right-hand side.)

This, OTOH, seems thoroughly sensible. You don't want overhead that you don't
need, and you only want the basic guarantee.

If users don't want double-storage, then they can supply a
no-throw-default-constructible type (such as boost::empty) as a valid variant
type, or they can use something like boost::any if heap storage is acceptable.

Anthony
-- 
Anthony Williams
Senior Software Engineer, Beran Instruments Ltd.
Remove NOSPAM when replying, for timely response.