Re: [boost] [outcome] expected, etc. why are these assignable

Subject: Re: [boost] [outcome] expected, etc. why are these assignable
From: Robert Ramey (ramey_at_[hidden])
Date: 2017-06-02 16:55:25

On 6/2/17 9:22 AM, Richard Hodges via Boost wrote:
> Forgive me chiming in late, but what is the semantic difference between:
> 
> enum class no_difference
> {
>      _
> }
> 
> expected<std::path, no_difference, std::error_code, std::exception_ptr>
> 
> and this:
> 
> using path_difference = std::variant<std::path, no_difference>;
> using error = std::variant<std::error_code, std::exception_ptr>;
> 
> expected<path_difference, error> return_first_diff(...);
> 
> Doesn't the latter form satisfy all concerns in that:
> 1. It conforms to a desirable 2-state 'value or error' KISS paradigm.
> 2. it contains all the information we'll ever need.
> 3. success code paths are now co-located, as are failure code paths. There
> is no need for N code-paths to cater for N states. Isn't this more
> maintainable?

Right.  By leaving the second parameter E as a template parameter, it 
can contain as much state or as little state as one wants.  Your 
formulation might be considered more heavy weight - but hey, that's up 
to the user - not us.

Robert Ramey