Subject: Re: [boost] [gsoc-2013] Boost.Expected
From: Pierre T. (ptalbot_at_[hidden])
Date: 2013-04-23 15:28:00
Le 23/04/2013 18:55, Gottlob Frege a écrit :
> On Tue, Apr 23, 2013 at 10:18 AM, Krzysztof Czainski <1czajnik_at_[hidden]>wrote:
>> 2013/4/23 Pierre T. <ptalbot_at_[hidden]>
>>> std::expected<int, Exception> exp;
>>>> int x = exp.value(); // has no value - does this throw
>>>> or does it throw Exception()?
>>>> Actually, expected contains an error and we don't know anything about
>>> the type of the error so we cannot throw it.
> We can throw anything we want. I'm not sure it is a good idea, but you can
> throw anything. It doesn't need to derive from std::expection, for
> example. You can throw an int. etc.
Of course, but I guess it's a bad practice ?
>> It's what I called
>>> expected_base<T, Error> in my previous mails, the get() on expected_base
>>> doesn't throw and if expected_base doesn't contain a value, then
>>> behavior occurs.
>> Hmm, couldn't the original error be thrown anyway (if the wrapping type had
>> a virtual function that would throw the error)?
> I think we'd like to avoid virtual functions. I'm not even sure the base
> class is needed. I think we only want one expected<>, not 2 or 3 variants.
> (Although maybe I don't understand what all the variants are, I haven't
> looked at it thoroughly.)
> What we did for optional<> is allow you to access the value in 2 ways:
> optional<int> oi;
> int x = oi.value(); // throws bad_optional_access
> int y = *oi; // undefined behaviour
> Similar to vector [n] vs at(n).
We could use something similar in expected. But we must keep in mind
that expected has a value and an error, is it clear that *expect return
the value and not the error ?
> I think expected<> should work the same way.
> The only question, to me, is what it throws. Either it throws the Error,
> or it throws bad_expected_access. Or it throws bad_expected_access<Error>
> which derives from bad_expected_access_base, or something like that. (ie a
> well defined exception, but I can still get my custom Error value back from
> it if I want.)
I guess that the idea of error_wrapper_exception<Error> is interesting.
Mainly because it enables us to have an unique variant of expected.
However, if Error is an exception, it shouldn't be encapsulated into a
What others think about this behavior and interface ?
Thanks for any comments.