Re: [boost] Outcome v2

Subject: Re: [boost] Outcome v2
From: Gavin Lambert (gavinl_at_[hidden])
Date: 2017-07-12 00:06:30

Mere moments ago, quoth I:
> If you're going to maintain strict no-value-plus-error semantics then 
> union/variant storage makes sense, as otherwise you're wasting memory. 
> I'm not sure why this would increase complexity.

Although of course unions can be problematic for T with non-trivial copy 
constructors (and for exception_ptr, which also has a non-trivial copy 
constructor).

A proper variant implementation should take care of that for you, though 
this didn't help you in Outcome v1 since you were rolling your own 
variant implementation.  Perhaps this is what you were referring to?

If you're using struct { T; EC } storage, what do you do with 
non-default-constructible T when constructed with an error code only? 
Leaving it unconstructed seems like the only reasonable solution, but 
this then either requires mucking about with constructors and 
destructors in annoying fashions or using std::aligned_storage rather 
than embedding T directly (which still requires mucking about with 
constructors and destructors).

Using a variant just seems like the simpler way to go for this case.