[swift-evolution] MemoryLayout for a value

[Boris Wang]

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 <kona.ming at gmail.com>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 <swift-evolution at swift.org>于2016年8月5日
> 周五16:34写道：
>
>> 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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>> https://lists.swift.org/mailman/listinfo/swift-evolution
>>
>
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