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
>