Re: [boost] Re: unions

From: E. Gladyshev (egladysh_at_[hidden])
Date: 2003-10-08 13:50:46

--- David Abrahams <dave_at_[hidden]> wrote:

[...]
> 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
> 
>   - implement a moderately-complex type sequence reordering algorithm,
>     which surely costs far more in compilation resources than the
>     simple search that would be implemented by the library to provide
>     the optimization whenver possible.
> 
> IOW, we make the library designer choose between safety and efficiency
> at compile-time just to get runtime efficiency.

I would add to what David just said.

According to Eric, the current variant implementation doesn't 
provide a *strong* exception safety anyway.
So I would argue that the *weak* safety is not really usefull.
I would also argue that in 99.9% cases given a choice 
between the optimization and weak exception safety, 
the user will choose the optimization.
So 99.9% of all users will need to add the empty type
to the begining of all their variants.

It seems that the only justification
was O(N) vs. O(2N) compile-time improvement that
as David just pointed out, is not really an 
improvement at all.

My vote is to remove the *weak* exception safety
from variant all together. Come on guys, variant 
should be imlemented as a very basic type with
a clear and intuitive behaviour.
Don't try to be too smart with it. :)

Eugene

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