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.
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 >> >
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