[swift-evolution] MemoryLayout for a value

[Dave Abrahams]

on Thu Aug 04 2016, Anton Zhilin <antonyzhilin-AT-gmail.com> wrote:

> 2016-08-04 10:31 GMT+03:00 Xiaodi Wu via swift-evolution <
> swift-evolution at swift.org>:
>
>> On Thu, Aug 4, 2016 at 2:29 AM, Karl <razielim at gmail.com> wrote:
>>
>>> It’s confusing because metatypes in Swift are pretty confusing in
>>> general: Int.self returns Int.Type which is not the same as `type(of: <some
>>> Int>)` (that would be Int).
>>>
>>> If a novice wants to jump in, they’ll have to know that MemoryLayout(of:
>>> Int.self) would return a MemoryLayout<Int.Type>.
>>>
>>
>> Yes, here, I agree Dave is absolutely right. You and Dave have convinced
>> me that neither `MemoryLayout(of: x)` nor `MemoryLayout.of(x)`, where x is
>> an instance, would be appropriate.
>>
>
> Two weeks ago Adrian and I suggested adding dynamic 'size', 'stride',
> 'alignment' to Mirror, which definition would look like:
>
> public struct Mirror {
>     internal metatype_: Any.Type
>
>     public init<T>(_: T.Type)
>
>     public var size: Int { get }
>     public var stride: Int { get }
>     public var align: Int { get }
>
>     // ...
> }
>
> There are some problems with it right now, but I do believe that reflection
> API is where dynamic `size`, `stride`, `alignment` belong.

I see your point, but that would unfortunately be an unacceptably
expensive way to get that information.  Constructing a mirror is a
nontrivial bit of work.

-- 
-Dave