Re: [boost] Report from Mont Tremblant C++ Committee meeting

From: Fernando Cacciola (fernando_cacciola_at_[hidden])
Date: 2005-10-17 14:55:11

Joel de Guzman wrote:
>>
>> Of course we would specialize swap for optional<T> to call swap(*x,
>> *y) if both x and y are initialized, but I think the fact that this
>> is not equivalent to the unspecialized version is an argument in
>> favor of the rebind semantics.
>
> Interesting point!
>
Very!!
This is another problem caused by non-rebinding semantics.
Which of course, being a con of non-rebinding, turns into an argument in 
favor of rebinding.

> Now consider this:
>
>     int a = 1;
>     int b = 2;
>     tuple<int&> x(a);
>     tuple<int&> y(b);
>     swap(x, y);
>
> What happens after the swap? The same thing happens!
>
> Yet, does tuple have to change and assume a rebinding behavior
> as optional did? Definitely not!
>
Why definitely not!?

> (Aside: Keep in mind that we never touched on the nullability of
> optionals. There's no nulled optional at all anywhere in this
> thought experiment).
>
Which only means that I have now yet _another_ reason for the rebinding 
semantics.

>
> At most, I might be inclined to think that boost::reference_wrapper
> does the right swap and rebind behavior (**):
>
> [SNIP]
>
> (**) reference_wrapper rebinds only on explicit copy construction
> and assignment from another reference_wrapper. Never from
> assignment from a T (its embedded type).
>
(***) Because it doesn't support direct assignment from a T.
The cloest match to what optional<> has is:

*opt=value

which does not rebind, just as reference_wrapper()

Best

-- 
Fernando Cacciola
SciSoft
http://fcacciola.50webs.com/