[swift-evolution] [swift-evolution-announce] [Review #2] SE-0101: Reconfiguring sizeof and related functions into a unified MemoryLayout struct

[John McCall]

> On Jul 18, 2016, at 8:57 AM, Erica Sadun via swift-evolution <swift-evolution at swift.org> wrote:
>> On Jul 13, 2016, at 5:39 PM, Dave Abrahams via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>> 
>> 
>> on Wed Jul 13 2016, Brent Royal-Gordon <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>> I think grouping these into a type is a sensible approach, but I don't
>>> like that it allows the creation of meaningless MemoryLayout
>>> instances. The simplest fix would be to make `MemoryLayout` an empty
>>> enum instead of an empty struct. This would convey that no
>>> MemoryLayout instances do or can exist.
>> 
>> +1.
> 
> +1 as well.

If MemoryLayout were not a generic type, its instances would not necessarily be meaningless; it could be used to lay out memory in a size + alignment-safe way.

You would need to restructure most of the API you've been talking about, though.

John.

> 
> /// Accesses the memory layout of `T` through its
> /// `size`, `stride`, and `alignment` properties
> public enum MemoryLayout {
>     /// Returns the contiguous memory footprint of `T`.
>     ///
>     /// Does not include any dynamically-allocated or "remote"
>     /// storage. In particular, `MemoryLayout.size`, when
>     /// `T` is a class type, is the same regardless of how many
>     /// stored properties `T` has.
>     public static var size: Int { return _sizeof(T.self) }
>     
>     /// For instances of `T` in an `Array`, returns the number of
>     /// bytes from the start of one instance to the start of the
>     /// next. This is the same as the number of bytes moved when an
>     /// `UnsafePointer` is incremented. `T` may have a lower minimal
>     /// alignment that trades runtime performance for space
>     /// efficiency. The result is always positive.
>     public static var stride: Int { return _strideof(T.self) }
>     
>     /// Returns the default memory alignment of `T`.
>     public static var alignment: Int { return _alignof(T.self) }
> }
> 
> 
> -- E
> 
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