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

From: Fernando Cacciola (fernando_cacciola_at_[hidden])
Date: 2005-10-11 12:47:32

Joel de Guzman wrote:
> Howard Hinnant wrote:
>> On Oct 10, 2005, at 10:36 AM, Fernando Cacciola wrote:
>>
>>>>   Dubious of use of references.
>>>
>>> Does this mean they don't see that as useful or that is not well
>>> supported?
>>
>>
>> I believe there was specific concern about reference rebinding under
>> optional<T&>.  There were also several comments that went like:  I
>> use optional all the time (gives use case), but I've never needed to
>> use it with reference types.
>
> [...]
>
>> You might explore a interface here on boost which did not have the
>> pointer interface, and which dropped reference support.  Then perhaps
>> submit a follow on paper reflecting that experience.  Said paper may
>> either modify your current proposal, or more fully support it.
>
> I use optional reference but I do not like the way rebinding works.
> I still insist that it follow the way boost::tuple does it. IMO,
> the same way with tie, when you need it, you'll really need it.
> I think it was I who requested for optional references, so let me
> present my use case:
>
> In Spirit, a parser may return an attribute. This attribute should
> only be constructed on a successful match. Thus, optional<T>.
> A symbol table is a parser in Spirit. Each symbol has a corresponding
> data slot. On a successful match, the attribute returned is a
> reference to the data slot. Thus optional<T&>.
>
Ya, if you use optional<T> generically, chances are T is really a reference, 
so you do want it to support references.
And I don't think we should drop that support because of the rebinding issue 
since I don't see it as much of an issue..  IMO, is about wrapping the mind 
around the nature of an "optional reference"... once you do that, the 
current binding rules just comes naturally IMO.

The nature of an optional object is that it can be absent... and when an 
optional reference is absent, there is not much of a choice about what to 
do: you just have to bind to the rvalue.

Since this expression:

opt = ref ;

cannot but to bind 'opt' to 'ref' if 'opt' happens to be absent, then IMO it 
just better do that _always_, no matter if 'opt' is absent or not.
The other choice of semantic would be inconsistent and depending on the 
previous state of 'opt'.

> > Zen of Python:
> ...
> There should be one-- and preferably only one --obvious way to do it.
> ...
>

Zen of Boost: (can I invent one in the name of Boost? :)

"When in the fog, follow the path of least surprise"

Best

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