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.