Re: [boost] [outcome] Possible extensions/changes to std::experimental::expected

Subject: Re: [boost] [outcome] Possible extensions/changes to std::experimental::expected
From: Andrzej Krzemienski (akrzemi1_at_[hidden])
Date: 2017-05-25 22:04:23

2017-05-25 23:57 GMT+02:00 Peter Dimov via Boost <boost_at_[hidden]>:

> Andrzej Krzemienski wrote:
>
>> 2017-05-25 23:34 GMT+02:00 Peter Dimov via Boost <boost_at_[hidden]>:
>>
>> > Andrzej Krzemienski wrote:
>> >
>> >> Instead, rewrite observer functions like this:
>> >>
>> >> bool has_value() const
>> >> {
>> >>   if (BOOST_UNLIKELY(_is_in_empty_state()))
>> >>     __builtin_unreachable();
>> >>
>> >
>> > Please no. This is horrible. Actually it's doubly horrible; merely >
>> horrible would be putting the above in value(). In an observer >
>> function... it's just evil.
>>
>>
>> Can you explain?
>>
>
> What is there to explain? This makes calling has_value undefined if empty
> ("narrow contract"), which makes the user responsible of checking empty()
> before doing ANYTHING with a result/outcome object. This applies to each
> and every result/outcome object you receive from somewhere.
>
>    auto r = function();
>    if( r.empty() { /* always have to do this*/ }
>    else if( r.has_value() ) { /* otherwise UB here */ }
>
> As I said, I don't like undefined behavior for its own sake, and I most
> certainly don't like putting a "Requires: not empty" contract on every
> function, including observers.
>
> Putting a contract is not even a good fit for value(), because it already
> checks and throws, to enable
>
>    auto r = function().value();
>
> where you go from noexcept land to exception land. If UB on empty, goodbye
> one liner, you still need to save the outcome and check for empty first,
> before retrieving value().
>

Yes, it would be wrong if the message was, "whoever gets an `expected`
object needs to check for the empty state". The ideal solution would be to
put this responsibility on someone who produces the `expected` object.

Also, they way I look at this solution is not "when I get this value I have
to check ...", but "when I produce this value I have to make sure...". Is
there no way to acheive this in the language?

Regards,
&rzej;