[swift-evolution] [Review] SE-0101: Rename sizeof and related functions to comply with API Guidelines

[Dave Abrahams]

on Tue Jul 12 2016, Jordan Rose <jordan_rose-AT-apple.com> wrote:

>> On Jun 30, 2016, at 12:30, Dave Abrahams via swift-evolution <swift-evolution at swift.org> wrote:
>> 
>> 
>> on Wed Jun 29 2016, Erica Sadun <erica-AT-ericasadun.com> wrote:
>> 
>
>>>> On Jun 29, 2016, at 3:59 PM, Xiaodi Wu via swift-evolution <swift-evolution at swift.org> wrote:
>>>> 
>>>> On Wed, Jun 29, 2016 at 4:50 PM, David Sweeris <davesweeris at mac.com <mailto:davesweeris at mac.com>> wrote:
>>>> That’s the “as proposed” usage for getting the size of a value (from
>>>> https://gist.github.com/erica/57a64163870486468180b8bab8a6294e
>>> 
>>>> <https://gist.github.com/erica/57a64163870486468180b8bab8a6294e>)
>>>> // Types
>>>> MemoryLayout<Int>.size // 8 
>>>> MemoryLayout<Int>.arraySpacing // 8
>>>> MemoryLayout<Int>.alignment // 8
>>>> 
>>>> // Value
>>>> let x: UInt8 = 5
>>>> MemoryLayout(x).dynamicType.size // 1
>>>> MemoryLayout("hello").dynamicType.arraySpacing // 24
>>>> MemoryLayout(29.2).dynamicType.alignment // 8
>>>> 
>>>> 
>>>> At least, I thought that was the latest version of the proposal. Maybe I’ve gotten confused.
>>>> 
>>>> There must be a typo in these examples. `MemoryLayout(x.dynamicType).size` perhaps?
>>> 
>>> I have listened. I have updated.
>>> 
>>> https://gist.github.com/erica/57a64163870486468180b8bab8a6294e
>>> 
>>> // Types
>>> MemoryLayout<Int>.size // 8
>>> MemoryLayout<Int>.arraySpacing // 8
>>> MemoryLayout<Int>.alignment // 8
>>> 
>>> // Value
>>> let x: UInt8 = 5
>>> MemoryLayout.of(x).size // 1
>>> MemoryLayout.of(1).size // 8
>>> MemoryLayout.of("hello").arraySpacing // 24
>>> MemoryLayout.of(29.2).alignment // 8
>> 
>> I am still very skeptical that anyone needs the “Value” version, and as
>> long as we're resyntaxing I am inclined to take it away and see how many
>> people complain.  You can still always write it yourself.
>
> Sorry to only come across this now. The proposed implementation does not work.
>
> public static func of(_ candidate : @autoclosure () -> T) -> MemoryLayout<T>.Type {
>   return MemoryLayout.init(candidate).dynamicType
> }
>
> let value: Any = 2 // dynamicType is Int
> let layout = MemoryLayout(value).dynamicType // inlined from above
> let arrayType = [value].dynamicType
>
> ‘layout’ here is still 'MemoryLayout<Any>', not 'MemoryLayout<Int>',
> for the same reason that ‘arrayType’ is ‘Array<Any>’ rather than
> ‘Array<Int>’.

That is the right answer.  If you want to store an array of things with
the same type as “value” (== Any) you need that MemoryLayout.

> If we want to support sizeofValue et al, we’d need to make these
> instance properties rather than static properties, and may then want
> to give up on the struct being generic.

That wouldn't change anything.  Nothing can possibly unwrap the Any and
find out the size of the thing.  The current sizeofValue() doesn't work
that way either:

  print(sizeofValue(1 as Int8 as Any))

-- 
Dave