Re: [boost] [optional] operator<(optional<T>, T) -- is it wrong?

Subject: Re: [boost] [optional] operator<(optional<T>, T) -- is it wrong?
From: Vladimir Batov (vb.mail.247_at_[hidden])
Date: 2014-11-24 01:42:36

Andrzej Krzemienski wrote
> ...
> User defining its own function
>>
>>   void f(optional
> <T>
> , optional
> <T>
> );
>>
>> would need to add the following?
>>
>>   void f(T, optional
> <T>
> ) { BOOST_STATIC_ASSERT(); }
>>   void f(optional
> <T>
> , T) { BOOST_STATIC_ASSERT(); }
>>
>> What if there are 3 optional parameters? We can not say to the user that
>> they need to program this way.
> 
> I do not think it has been addressed.

The problem here (as I can see it) is that you supply an abstract fun(T,
optional<T>) with no context whatsoever and then ask -- how it can be
addressed -- promote T to optional<T> or poison... or something else?..
Obviously, no matter what the choice is it'll be correct for one set of real
functions and wrong for others. So, the answer is "it depends on the
context". Granted, it's very tempting to have one absolute truth, one simple
answer to all fun(T, optional<T>) under the sun... Say, "shall not kill"
sounds like a good rule to live by... unless one is attacked... then "kill
in defence" might sound like a reasonable approach... unless one is at
war... then "kill plenty" sounds like an expectation... I guess, all I am
trying to convey with all that killing and mandarins and oranges is that we
need to look at things in context and address those in context so that is,
in the end, beneficial to the user.  From that perspective (usefulness,
user-friendliness, rather than some hypothetical library purity) there
cannot possibly be any doubts about sensible/unquestionable implicit
promotion of T to optional<T>, about what is more natural, better for the
user:

foo->set_time("11:55PM") or
foo->set_time(boost::optional<string>("11:55PM"))

As for op<(T, optional<T>), then the library writer'd like to be as helpful
as possible and to provide as much functionality as possible. Unfortunately,
there is just not enough context to go one way or the other. So, stating
that by prohibiting the operator is the safest and most honest solution.

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