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

From: Jaakko Jarvi (jarvi_at_[hidden])
Date: 2005-10-14 15:18:34

On Oct 14, 2005, at 3:01 PM, Ian McCulloch wrote:

> Fernando Cacciola wrote:
>
>> Joel de Guzman wrote:
>>
>>>
>>>
>
> [...]
>
>
>>> The nullability aspect is *besides*
>>> the point and has nothing at all to do with the rebinding semantics.
>>>
>>
>> This is 100% false. Is 100% the opposite.
>>
>> Rebinding semantics are there _precisely_ and _only_ to solve a  
>> problem
>> posed by nullability.
>>
>> Let me say that again:
>>
>> Rebinding semantics are there _precisely_ and _only_ to solve a  
>> problem
>> posed by nullability.
>>
>> So once again, I ask you to deeply consider nullability and the  
>> problem it
>> poses on assignment (I hope I don't have to repeat it again), then  
>> weight
>> my proposed solution (rebinding) with your expectactions. If  
>> possible,
>> propose a better solution or at least show that you don't think the
>> problem is worth a controversial solution like rebinding.
>>
>
> If optional<> is to be useful for references, then it should have  
> the same
> semantics that a class acting as a proxy-reference has.  ie, if I use
> optional<> with a class like
>
> template <typename T>
> struct Proxy
> {
>    explicit Proxy(T& obj) : my_obj(&obj) {}
>
>    Proxy& operator=(T const& obj) { *my_obj = obj; return *this; }
>
>    // ...
>
>    T* my_obj;
> };
>
> then optional<T&> should act as close to optional<proxy<T> > as  
> possible.
> ie., if optional<T&> does something different just because the  
> template
> argument is a reference, then there must be a way to replicate that
> behaviour with optional<proxy<T> > too.  As to what behaviour is most
> useful, I am not sure because I have not yet come across a  
> situation where
> I wanted an optional<T&> - but I do find the rebinding behaviour
> surprising.
>
> I don't see what nullability per-se has to do with this discussion.
>

I agree with Ian and Joel.
I don't see that nullability should be the decisive characteristic.
Much more so optional is defined by:

   In the case that optional holds a value, it should work exactly  
(or as much as possible) like the type it contains.

Rebinding goes counter to that.

   Best, Jaakko

>