[swift-evolution] MemoryLayout for a value

[Xiaodi Wu]

On Thu, Aug 4, 2016 at 6:02 PM, Dave Abrahams <dabrahams at apple.com> wrote:

>
> on Thu Aug 04 2016, Dmitri Gribenko <gribozavr-AT-gmail.com> wrote:
>
> > On Wed, Aug 3, 2016 at 7:28 PM, Xiaodi Wu via swift-evolution
> > <swift-evolution at swift.org> wrote:
> >> Could I suggest an alternative? It's conservative in that it mimics the
> >> relationships we had before the proposal was implemented and also
> maintains
> >> the simplicity of the caseless enum:
> >>
> >> ```
> >> extension MemoryLayout {
> >>   static func size(ofValue _: T) -> Int { return MemoryLayout.size }
> >>   // etc.
> >> }
> >> ```
> >
> > I like this API.  I think given all the alternatives that we explored,
> > it is better than those.  I also think that it nicely avoids the
> > following issue with the proposed MemoryLayout.of(type(of:
> > someExpression)).size syntax.
> >
> > Imagine that you have a value whose static type differs from the
> > dynamic type.  For example, a protocol existential:
> >
> > protocol P {}
> > extension Int : P {}
> > var x: P = 10
> >
> > The question is, what does MemoryLayout.of(type(of: x)).size compute,
> > size of the existential box, or the size of an Int instance?  The
> > semantics of 'type(of:)' are "return the dynamic type", so the
> > straightforward conclusion is that MemoryLayout.of(type(of: x)).size
> > returns the size of the dynamic type instance, of Int.
> >
> > What actually happens is that 'type(of: x)' returns a dynamic value of
> > 'Int.self', statically typed as 'P.Type'.  So P gets deduced for the
> > generic parameter of MemoryLayout, and MemoryLayout.of(type(of:
> > x)).size returns the size of the protocol box.
> >
> > I think due to this complex interaction, using type(of:) might lead to
> > confusing code, and thus I like Xiaodi's approach better.
> >
> > Dmitri
>
> Okay, I'm convinced; that's what we should do.
>

Proposal and stdlib PRs have both been created.


> --
> -Dave
>
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