Re: [boost] expected<T, E...> (Was: [outcome] High level summary of review feedback accepted so far)

Subject: Re: [boost] expected<T, E...> (Was: [outcome] High level summary of review feedback accepted so far)
From: Peter Dimov (lists_at_[hidden])
Date: 2017-06-04 16:35:49

Vicente J. Botet Escriba wrote:
> What do you think of making expected constructor from T explicit?

That's one of the options. Another is to make

>     expected<double, unscoped_error> test()
>     {
>         return unscoped_other_error;
>     }

work and return an error, instead of a double (exact match to E).

I know that there are people who insist on explicit everywhere, but implicit 
does have its appeal.

Of course in pathological examples such as expected<variant<int, float>, 
int, float>, implicit conversions will rarely do what one wants. But I'd 
argue that these (IMO atypical) uses should not carry much weight.

expected<double, int> is a more likely case, and there's no way to make 
implicit conversions always do the right thing for it. I'm not sure how much 
we ought to focus on it.

Perhaps the heuristic could be, if the argument can be interpreted as both a 
value and an error, the conversion is ambiguous, regardless of which of the 
two is a better match. This will make the above return ambiguous, which is 
probably as it should be.