Re: [boost] Formal review: Optional library

From: Peter Dimov (pdimov_at_[hidden])
Date: 2002-12-11 15:47:30

From: "Fernando Cacciola" <fernando_cacciola_at_[hidden]>
[...]
> (1) deep-constantness:
>
> I Followed David Abraham's suggestion and decoupled
> constantness of the optional<> object with that of
> the value being wrapped.
> This is how pointers and most smart pointers work, so
> the pointer semantic should suffice to explain this behaviour.

Hm. Did Dave really suggest that? :-)

To reiterate, my opinion is that deep constness is much more appropriate for
optional<>. Constness and copying go hand in hand: deep copy corresponds to
deep constness, and shallow copy corresponds to shallow constness.
Optional<> does a deep copy. Even smart pointers that do deep copy use deep
const.

> template<class T>
> class optional
> {

[...]

>     void swap ( optional& rhs ) ;

Doesn't the standard std::swap do the job, by the way?

[...]

> } ;
>
> template<class T> inline T* get ( optional<T> const& opt ) ;

Why the free function?

>
> template<class T> inline void swap ( optional<T>& x, optional<T>& y ) ;

[...]
> This is utterly important now that is has safe_bool,
> because IMO, Tanton Gibbs interpretation is correct;
> given safe_bool:
>
> optional<int> opt0 ;
> optional<int> opt1 ;
>
> (opt0 == 0 ) // true
> (opt1 == 0 ) // true
>
> therefore....
>
> (opt0 == opt1) HAS TO BE TRUE.
>
>
> If the comparison above is properly understood,
> that is, if it is clear that the initialized states
> are being compared and not the values, then there is
> no problem, but considering that:
>
> optional<int> opt0(1);
> optional<int> opt1(2);
> (opt0 == opt1) // true
>
> is also true even though the values are different,
> I think that allowing direct comparions will
> create unneccesary confusion.

I think that the comparisons we were discussing as a possible candidate
required both the initialized states and the held values to match.

This is consistent with the "optional as a container of zero/one" and the
"optional as a value with an additional unitialized state" models.