[swift-evolution] MemoryLayout for a value

[Boris Wang]

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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