Re: [boost] [optional] rebinding references

From: Fernando Cacciola (fernando_cacciola_at_[hidden])
Date: 2005-10-18 12:52:27

David Abrahams wrote:
> Anthony Williams <anthony_w.geo_at_[hidden]> writes:
>
>> How's this for a radical thought --- optional<T> should not be
>> assignable from T. That way, it doesn't matter whether T is a
>> reference, or not.
>
> There's at least some precedent for types that are "immutable except
> for copy assignment."
>
> The assignment from T is really a kind of mutating operation.
>
Indeed.
I'm leaning fast into dropping assignment form T.
That definitely solves the reference binding issues.

>> Instead, to change the binding, use reset(), and to assign use *.
>
> To make that valid for a null optional, I think operator* would have
> to return a proxy reference... which is not an altogether bad idea,
> IMO.
>
That would actually bring back the problem as assignment to a null reference 
would bind and then there's the question about what to do with assignment to 
a non-null reference (rebind as I proposed or not-rebind as Joel proposed)
But if we do as both Sam Partignton and Antony Williams  suggest, drop 
assignment from T altoghether, the situation looks a lot more clear, at 
least to me.

Antony's proposal goes even beyond Sam's idea: rather than dropping 
assignment from T, we spell it: reset().
That is definetly better then direct assignment from T, but I'm not quite 
sure that is as good a solution as keeping just copy assignment.

Consider these 3 versions:

int a = 1;
int b = 2;
int& ra = a ;
int& rb = b ;

optional<int&> o(ra);

*o = rb ; // Clearly doesn't rebind but is UB if 'o' were null.

// (1) Current case:
o = rb ; // rebind or not??

// (2) Antonty's proposal:
o.reset(rb); // still some room for doubts?

// (3) Sam's proposal:
o = make_optional(rb); // Clearly rebinds, doesn't it?

// I just introduced make_optional() here to get rid of the template 
argument

I think (3) spells the actual semantics (rebinding) even more clearly than 
(2).
OTOH, both (2) and (3) are equivalent, so the difference is merely 
sintatical.
Having said that, I prefer (3) which just looks better to my eyes, but (2) 
could work too.

Best

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