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

From: Fernando Cacciola (fernando_cacciola_at_[hidden])
Date: 2005-10-18 12:57:43

Reece Dunn wrote:
> Fernando Cacciola wrote:
>> Sam Partington wrote:
>>> [...]
>
> Why not use partial specialization so that optional< T > has the
> semantic sugar, but optional< T & > does not? Or is this already the
> case?
>
I'm not too fond of template classes that support different interfaces 
according to the properties of the template parameters. That just doesn't 
play along with generic programming where the type you optionalize can or 
cannot be a reference.
Think of vector<bool> for instance.
I rather keep looking for a consistent solution.

>> A related note about the use of operator *
>>
>> You said that, even for optional<T&>, it is clear that this:
>>
>>  *o = i;
>>
>> is UB if 'o' is uninitialized. And so there is no problem defining
>> that assignment as really just the assignment of the underlying
>> type, which in
>> the case of optional<T&> doesn't rebind.
>>
>> Now I wonder, if instead of operator*, we used value() (or
>> whatever), would
>> it be just as clear?
>>
>> // Nullable interface
>>
>> *o = rb ; // assing 'b' to 'a'
>> o = none ; // releases 'a'
>> *o = rc ; // !!! UB !!! Cleary IMO
>>
>> // Container interface
>>
>> o.value() = rb ; // assing 'b' to 'a'
>> o.reset() ; // releases 'a'
>> o.value() = rc ; // !!! Still UB !!! ¿But cleary enough?
>
> I personally prefer the nullable interface as it is more intuitive
> w.r.t.
> smart pointer usage.
>    o.value() = b
> reads to me like value() is returning a (temporary) return value,
> then you
> are assigning rb to that.
>
Me too!

> Q: If o == none, does *o = foo throw? (I am not familiar with
> optional<>)
>
Yes..
Although strictly speaking is Undefined Behaviour.
Depending on the compilation options and your boost setup that can throw or 
core dump.

> optional<bool> side, is it posible to provide a specialization
> for it that doesn't break the optional interface, but still provides
> the
> functionality that is wanted from an optional<bool> type as discussed
> elsewhere?
>
Is possible, but I don't like it for the reasons expressed before.

Best

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