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

Subject: Re: [boost] [outcome] Possible extensions/changes to std::experimental::expected
From: Peter Dimov (lists_at_[hidden])
Date: 2017-05-26 11:12:22

Peter Dimov wrote:
> Andrzej Krzemienski wrote:
>
> > 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?
>
> By not providing a default constructor, I suppose. Otherwise not, there's 
> return value elision so if you do
>
> result<T> function()
> {
>     result<T> r;
>
>     // do things
>
>     return r;
> }
>
> nothing is ever called on 'r' on your side if you forget to initialize it.

Actually, there is a way, by providing two types, one default-constructible 
with a singular empty state, one without. In the code above, you will still 
declare the function to return result<>, which can never be empty and has no 
default constructor, but declare `r` to be of type result_option, or 
optional_result, or optional<result>, or whatever. Then the `return r` line 
will convert and check.

(Or you could use `result<result<T>> r` and return `r.value()`, which is a 
funny exercise in ambiguity.)