Re: [boost] [outcome] To variant, or not to variant?

On 31/05/2017 22:31, Vicente J. Botet Escriba wrote:

Does none_t mean success or failure?

Either; that's up to the method in question. It also wasn't really the focus of the question so you can pretend it doesn't exist if that makes you happier.

For what I see it means failure as it is not the result of value().

That was a consequence of trying to keep it returning by reference as some people seem to prefer. If it returned by value then it could return an optional<T>, which might be better.

In addition it default construct to none_t.

That was intentional.

...

private: optional<T> m_value; error_code m_error; exception_ptr m_exception; };

I guess you want a variant here. You don't want to store all of them, isn't it?

No, that was the entire point: to store all of them, and not as a variant.

I could understand

varaint>

Nested variants are utterly pointless. In an ideal world variant would automatically collapse nested sub-variants to reduce storage, although that does unfortunately complicate return-by-reference semantics.

This corresponds to the status_value [1] model where you store the reason why you don't have a value or why you have it and an optional<value>. As described by L. Crowl in [2], there are use cases for status_value, expected and exceptions.

pair>

Where we have a value of error_code to mean success.

That's closer to my second suggestion, yes, although I think using pair<> is a disservice. I also don't think error_code and exception_ptr are necessarily exclusive.