Re: [boost] Re: In-place construction for variant

From: E. Gladyshev (egladysh_at_[hidden])
Date: 2003-10-28 17:34:17

--- Eric Friedman <ebf_at_[hidden]> wrote:

[...]
> Here is my idea:
> 
>    variant assignment (lhs = rhs):
>    * copy content of lhs (onto stack or heap, as appropriate)
>    * set "temporary content" pointer in variant to copied content
>    * destroy content in storage of lhs
>    * copy content of rhs into storage of lhs (***)
>    * unset "temporary content" pointer, and indicate new content type
>    * destroy copied content (via delete or stack unwinding, as needed)

This is exactly how I thought it would work.

[...]
> Since such backup is typically not needed -- indeed, only in the 
> specific case that the variant being modified is accessed from the copy 
> constructor of a type in the variant -- I think you'll agree such 
> overhead is a bit excessive.

Agreed.

> >>A better solution, I believe, is simply to document that accessing a 
> >>variant *during* assignment will result in undefined behavior. That way, 
> >>unless the user really needs to do something like the above, such (IMHO, 
> >>excessive) overhead will be avoided.
> > 
> > I think that documenting the special case is almost as 
> > good as to say that variant doesn't really 
> > provide basic guarantees?
> 
> It always provides basic guarantees except in the following (peculiar) 
> situation:
> 
>    A variant's state is undefined if accessed *during* its assignment.

I guess there is no much we can do about it.

[...]
> The other possibility is that the library provide a global_variant class 
> template for such situations. The most elegant solution is probably a 
> specialized assignment policy class. I'll keep this in mind when I 
> design the policy-based variant for 1.32.
> 

You suggested to add const_/fixed_variant 
and use it in the 'variant' implementation.

Have you considered adding singular_variant
instead in use it in the 'variant' implementation?
I believe that singular_variant will be more
useful in practice by itself than the const_variant and
it should provide all the basic functionality for 'variant'.

Eugene

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