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Subject: Re: [boost] Boost.Outcome review - First questions
From: Vicente J. Botet Escriba (vicente.botet_at_[hidden])
Date: 2017-05-24 16:29:02

Le 24/05/2017 à 17:31, Niall Douglas via Boost a écrit :
>>>> Back at the beginning when designing Outcome, one of my design ideas
>>> was > a variety which called std::terminate on destruction if an error
>>> had > never been retrieved. So, the idea behind this family of
>>> Outcomes was > that they were "single shot", .error() and .value()
>>> could be called > exactly once after which the outcome became empty.
>>> On destruction, if an > error had never been retrieved, it was fatal
>>> exit time.
>> I wouldn't go as far as calling terminate(), but one could log.
> I'd make what happens user definable of course, but the default
> implementation would be std::terminate. After all, these be rigorous
> Outcomes, failure to collect state is a logic error - it equals "your
> code is wrong".
I don't think we should manage this with complex implementation. We
should use attributes and static analysis.
>> But for
>> this to work, the outcome must be noncopyable (move-only). Otherwise if
>> you make a few copies, none of it could know whether you inspected the
>> error of some of the other copies or not.
> I had been thinking of copy constructor disabled, but there would be a
> clone() member function.
You see. Managing this kind of possible issues make the implementation
more complex than necessary.
E.g. Would you remove new because the user could not store it in a
variable and delete it later on?
>> This doesn't require an empty state though, just a bit that is set (or
>> maybe cleared) when error() is called.
> I think the source of our disagreement is over publicly exposing the
> empty state. You appear to be happy if it's an internal only thing, but
> not available to end users. I take the view that if you're implementing
> the empty state anyway, might as well open it up for public use.
> Can you give a reason why public availability of an empty state with
> well defined semantics and behaviours is a bad idea? Does it naturally
> lead to incorrect code or something?
Because it makes the user code more complex. This is the task of static

We have a lot of examples maintaining publicly this empty state because
the cost of ensuring this empty (partial state) is not expensive. We
have it with unique_ptr, exception_ptr.
However, how many lines of code have you seen that checks for whether
exception_ptr contains an exception or not?

Anyway, I have not understood yet when we need this empty state on the
implementation and les yet when does the user needs it. I'll persevere.


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