Re: [boost] [optional] rebinding references

From: Fernando Cacciola (fernando_cacciola_at_[hidden])
Date: 2005-10-25 13:12:57

Yuval Ronen wrote:
> Fernando Cacciola wrote:
>> So my current proposal would be:
>>
>> Remove direct-assignment.
>> Let copy-assigment be the only way to define the wrapped value
>> Add make_optional() and make_optional_ref() as shorcuts to help with
>> copy-assignment
>> Keep none-assignment as the only wat to clear the wrapped value.
>
> Does this mean that construction from T is also suggested to be
> removed?
> I assume not...

You assume correct.

>
> Anyway, If I'm required to write
>
> optional<T> o = x;
> ...
> o = optional<T>(y);
>
> or
>
> optional<T> o = x;
> ...
> o = make_optional(y);
>
> then it should be considered that this generates an additional
> temporary instance of y (of type T).
> Direct assignment from T doesn't
> suffer from
> this.

Good point.
Still, I'm not sure this is significant enough to go back to direct 
assigment though.
Specially given that this is precisely the situation rvalue references are 
for (and I think we can safely bet they will be part of C++ even before 
optional<>).

> Working around this makes me write
>
> optional<T>(y).swap(o);
>
> or
>
> make_optional(y).swap(o);
>
> which is completely different...
>
Ya, that's reasonable alternative.
I wouldn't worry too much about it though, chances are C++ will soon 
overcome clearly unnecesary temporaties like that without such code 
gymnastics.

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