Re: [boost] Re: unions

From: Daniel Wallin (dalwan01_at_[hidden])
Date: 2003-10-08 15:34:24

At 20:18 2003-10-08, David Abrahams wrote:
>Daniel Wallin <dalwan01_at_[hidden]> writes:
>
> >>At that point, it might not contain any of the types you wanted.  It almost
> >>becomes equivalent to a design Eric rejected, where the variant can be 
> empty.
> >
> > I don't yet buy that this is a problem for generic programming;
> >
> > If variant is part of the program interface, the type shouldn't be changed
> > anyway, so "wanting the benefit of the optimization" becomes the users
> > problem.
> >
> > If the variant is part of the implementation, where the type
> > sequence is part of the interface, the invariant type is just an
>                                          ^^^^^^^^^???

I meant the state in which the variant ends up when an exception
is thrown in the assignment operator.

> > implementation detail, or part of the program documentation.
>
>Let me try again:
>
>- the variant design has a nice invariant that it contains one of the
>   types that were passed as template arguments
>
>- if a library takes a user's list at face value, the library
>   designer gets the benefit of that invariant.
>
>- if a library designer wants the optimization in the cases where
>   it's possible, she either has to:
>
>   - prepend empty_type to the list in order to ensure it, and give up
>     on the nice invariant

I guess the problem is I can't really think of a situation where it makes
any difference if the variant contains the prepended empty_type or
a type from the original list. I'm guessing you can though, so please
help me understand.

---
Daniel Wallin