Re: [boost] [optional] Safe optional

Subject: Re: [boost] [optional] Safe optional
From: Andrzej Krzemienski (akrzemi1_at_[hidden])
Date: 2014-11-21 05:30:10

 The usage of converting constructor in optional only shows to me that we
>> have not established a good criteria for telling when having a converting
>> constructor is a good idea. I always used to think that if only you are
>> not
>> loosing any information, it is safe. But in the case of optional I can see
>> that even then it can have some negative consequences. Perhaps it is the
>> way the language is designed that is wrong: that you either allow this
>> conversion in every possible situation or never at all.
>>
>
> Oh, c'mon. Cheer up. Things are not as gloomy, are they. :-) Here the
> subtlety (IMO) lies in how reasonable it is to add information. Namely, as
> I indicated, if we bring T into the optional<T> fold, then we apply
> optional<T> rules to T, i.e. apply t to optional<T> conversion. Say, we
> have a mandarin and an orange. When we bring the mandarin to an orange
> factory, then mandarin-to-orange "conversion" is applied and for all
> purposes mandarin is a "small orange". Outside the factory, when we compare
> mandarin to orange, it is not immediately clear what should be treated as
> what. If we apply mandarin-to-orange "conversion", then it'll be "small
> orange vs. big orange". If instead we apply orange-to-mandarin
> "conversion", then it'll be "sweet mandarin vs. sour mandarin". Given the
> library writer does not know, which it is, we ban it. Still not convinced?
> I've spent all munition I had. :-)

I responded to this in another thread. We diverged too much from the
original issue.