Do you means a array of protocol P, rather than a array of element conform with P? Dave Abrahams via swift-evolution <[email protected]>于2016年8月6日 周六13:22写道：
> > on Fri Aug 05 2016, Boris Wang <[email protected]> wrote: > > > Addition: > > > > I think protocol should't has size, or it's size should be zero. > > Because you can't put the 40 bytes data in anywhere. > > You certainly can. If protocols didn't have a size, Array wouldn't be > able to store them. > > > > > > > Boris Wang <[email protected]>于2016年8月6日 周六10:35写道： > > > >> codes in swift REPL: > >> > >> protocol P { > >> var x:Int {get} > >> } > >> MemoryLayout<P>.size > >> //r0 : Int = 40 > >> > >> struct S1 { > >> var v1:Int = 0 > >> } > >> MemoryLayout<S1>.size > >> //r1: Int =8 > >> > >> struct S2: P { > >> var v2: Int > >> var x:Int > >> } > >> MemoryLayout <S2>.size > >> //r2: Int = 16 > >> > >> ** Question: > >> Why we need to known the size of a object that can't be instanced? > >> > >> ** Confuse: > >> MemoryLayout <S2.Type>.size > >> //r3: Int = 0 > >> > >> MemoryLayout <P.Type>.size > >> //r4: Int = 16 > >> > >> > >> > >> Xiaodi Wu via swift-evolution <[email protected]>于2016年8月5日 > >> 周五16:34写道： > >> > >>> On Thu, Aug 4, 2016 at 6:02 PM, Dave Abrahams <[email protected]> > >>> 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 > >>>> > <[email protected]> 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 > >>>> > >>> _______________________________________________ > >>> swift-evolution mailing list > >>> [email protected] > >>> https://lists.swift.org/mailman/listinfo/swift-evolution > >>> > >> > > _______________________________________________ > > swift-evolution mailing list > > [email protected] > > https://lists.swift.org/mailman/listinfo/swift-evolution > > > > -- > -Dave > > _______________________________________________ > swift-evolution mailing list > [email protected] > https://lists.swift.org/mailman/listinfo/swift-evolution >
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