[swift-evolution] MemoryLayout for a value

[Dave Abrahams]

on Thu Aug 04 2016, David Sweeris <davesweeris-AT-mac.com> wrote:

>> On Aug 4, 2016, at 13:31, Dave Abrahams via swift-evolution
>> <swift-evolution at swift.org> wrote:
>> 
>> 
>>> 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.
>
> Two quick questions:
> 1) Can the expensive bits of mirrors be done lazily?
> 2) IIRC, the reflection API is supposed to be reworked for Swift
> 4. With that in mind, are mirrors likely to remain too be expensive to
> construct for this approach?

I don't know.  IMO none of these questions can be answered in time to
solve the cited problem for Swift 3.

-- 
-Dave