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
