Re: [boost] Variant implementation change (was re: unions)

From: E. Gladyshev (egladysh_at_[hidden])
Date: 2003-10-08 19:36:10

--- Eric Friedman <ebf_at_[hidden]> wrote:
> To mitigate the concerns of Eugene Gladyshev and Dave Abrahams, I've 
> modified variant to search through its bounded types for one that is 
> nothrow default-constructible and may be used as a "fallback" type in 
> the event of variant assignment failure. The change is now in CVS.
> 
> Thus, for example, variant<int,T> and variant<T,int> now both operate 
> without any dynamic allocation, irrespective of the presence or behavior 
> of a default constructor for T.

Thanks, Eric. I'll do a get tonight.  
Some people argured that it is not possible to ensure
basic exception safety w/o backup heap.
I guess it is not true?

> 
> However, this change still does not resolve the question with respect to 
> generic code, where it is not known if any of the types Ti provided to 
> some generic code are nothrow default-constructible.
> 

If the user doesn't provide a nothrow default-constructible type,
perhaps variant should use an internal (1 char or 0 size) 
nothrow default-constructible type and set an 'undefined'
flag similar to the 'backup' flag that you already have.

Eugene

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