Re: [boost] [optional] Safe optional

Subject: Re: [boost] [optional] Safe optional
From: Gottlob Frege (gottlobfrege_at_[hidden])
Date: 2014-11-24 13:12:22

On Thu, Nov 20, 2014 at 4:29 AM, Andrzej Krzemienski <akrzemi1_at_[hidden]> wrote:
> 2014-11-19 22:36 GMT+01:00 Vladimir Batov <Vladimir.Batov_at_[hidden]>
> :
>
>>
>> On 11/20/2014 08:22 AM, Vladimir Batov wrote:
>>
>>>
>>> On 11/19/2014 07:37 PM, Andrzej Krzemienski wrote:
>>>
>>>  ...
>>>> optional<double> Aircraft::weight()
>>>> {
>>>>    if (Impl* impl = get_impl())
>>>>      return impl->compute_weight();
>>>>    else
>>>>      return none;
>>>> }
>>>>
>>>> double Aircraft::max_weight()
>>>> {
>>>>    return 10000.0;
>>>> }
>>>>
>>>> bool too_heavy(Aircraft& ac)
>>>> {
>>>>    return ac.weight() > ac.max_weight();
>>>>    // in "safe" optional compiler would spot the bug
>>>> }
>>>>
>>>
>>> There is no bug here! Not from the human perspectives. You are comparing
>>> two extremely closely related types! As long as we agree on how "none" is
>>> treated, then all kosher. We do compare "ints" with "doubles", don't we? No
>>> "safety" concerns.
>>>
>>
>> On the second thought I might probably agree that op<() might be
>> questionable... Can we address that differently then?
>>
>
> No. Allowing this comparison to work is the right thing to do. It is a
> natural consequence of Optional's conceptual model. You should look at
> optional<T> as a T plus one additional value, less than any other value.
> No-one stops you from adopting any other model (like container of size
> 0-or-1), but then you are risking that you will be surprised by the result.
>

The "correct" model is actually initialized-or-not.  For non-Regular
types (&T or others), anything else is surprising.
In particular, the nature of optional assignment is
construct-or-assign.  For Regular types, there is no real difference,
for custom types there might be.

Its behaviour of assignment is, to me, the fundamental essence of optional.

Seeing it as may-be-uninitialized would suggest that
less-than-comparison doesn't make much sense.
(Equality still makes sense, IMO)

Tony