Re: [boost] [outcome] High level summary of review feedback accepted so far

Subject: Re: [boost] [outcome] High level summary of review feedback accepted so far
From: Vicente J. Botet Escriba (vicente.botet_at_[hidden])
Date: 2017-06-03 08:39:58

Le 27/05/2017 à 16:24, Peter Dimov via Boost a écrit :
> Niall Douglas wrote:
>
>> I would assume an expected<T, E1, .., En> could only return a 
>> std::variant<E1, ..., En> from its .error(). I can't think what else 
>> it could do.
>
> As the foremost authority on the nonexistent expected<T, E...>, I can 
> tell you what it does:
>
>    // F shall be in E...
>    template<class F> bool has_error() const;
>    template<class F> F error() const;
>
>    // if sizeof...(E) == 1
>    bool has_error() const;
>    E1 error() const;
>
> It returns the equivalent of variant<E...> not from error(), but from
>
>    unexpected<E...> unexpected() const;
>
> which allows you to
>
>    expected<T, E1, E2, E3> function()
>    {
>        expected<U, E1, E2> e1 = function1();
>        if( !e1 ) return e1.unexpected();
>
>        expected<V, E3> e2 = function2();
>        if( !e2 ) return e2.unexpected();
>
>        return e1.value() + e2.value();
>    }
Yes this is the reason d'être fro the unexpected function.

I see that the implementation you have done don't allows to return it by 
reference, and I was wondering if the variant2::subset function (which 
could for usability purposes be a non-member function) couldn't return 
by reference.

I know that this is something related to changes to the standard. There 
is currently a thread on C++-Core about the whether it is UB to cast a 
type to another with a common layout in order to access the common 
members. It seems that this is valid in C, but this is not anymore valid 
in C++. If at the end this kind of casts were valid as well in C++, I 
believe that we could return subset/unexpected by reference without UB.

Note that a variant with less alternatives than another one has the 
"same shared representation".

Best,
Vicente