[swift-evolution] MemoryLayout for a value

[Boris Wang]

excuse me, does this means there no sizeof(), like C language?

In C:

Struct S {}

Let x= S()

size of(S) == size of(x)


Xiaodi Wu via swift-evolution <swift-evolution at swift.org>于2016年8月4日
周四15:32写道：

> On Thu, Aug 4, 2016 at 2:29 AM, Karl <razielim at gmail.com> wrote:
>
>>
>> On 4 Aug 2016, at 06:27, Dave Abrahams via swift-evolution <
>> swift-evolution at swift.org> wrote:
>>
>>
>> on Wed Aug 03 2016, Xiaodi Wu <xiaodi.wu-AT-gmail.com
>> <http://xiaodi.wu-at-gmail.com/>> wrote:
>>
>> Why not just MemoryLayout.init(of instance: T), and drop the autoclosure
>> magic altogether?
>>
>>
>> My proposal *does* drop the autoclosure magic.  `type(of: x)` is already
>> in
>> the standard library (replacing `x.dynamicType`). The reasons not to have:
>>
>>   MemoryLayout(of: x)
>>
>> where x is an arbitrary instance, is that it reads and pronounces the
>> same as
>>
>>   MemoryLayout<X>
>>
>> but has different meaning, and even a different type (which results in
>> additional API complexity—the forwarding vars I showed in the [Aside]
>> box from my previous post).  Imagine explaining the difference between
>> these two in that world:
>>
>>   MemoryLayout<Int>
>>   MemoryLayout(of: Int.self)
>>
>> The first is a type representing the layout of Int.  The second is an
>> instance of that type representing the layout of Int's metatype.
>>
>>
>> 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.
>
>> The classic sizeofValue evaluated its argument, and in Foundation several
>> uses of it actually relied on that side effect. While autoclosures are
>> quite clever, in general I think the user expectation is that given
>> `a(b(c))` both a and b are invoked, side effects and all.
>>
>> Note that both type(of:) as it's currently implemented and the old
>> dynamicType evaluate its argument/receiver.
>>
>>
>> I didn't realize that, but it's fine.  I'm attached to the use of
>> `type(of:)` in this idiom, not to having an autoclosure involved.
>>
>> No one afaik has ever thought that behavior to be anomalous (though I
>> bet we're about to hear some arguments to that effect now).
>>
>> On Wed, Aug 3, 2016 at 15:46 Dave Abrahams via swift-evolution <
>> swift-evolution at swift.org> wrote:
>>
>>
>> Having seen the effects in the standard library and in other
>> code, I'm concerned that we may have made a mistake in removing
>> `sizeofValue` et al without providing a replacement.  In the standard
>> library, we ended up adding an underscored API that allows
>>
>>  MemoryLayout._ofInstance(someExpression).size
>>
>> Where someExpression is an autoclosure, and thus not evaluated.  I
>> wanted to bring up the possibility of introducing a replacement as a
>> bufix.
>>
>> I propose that the way to express the above should be:
>>
>>  MemoryLayout.of(type(of: someExpression)).size
>>
>> implementable as:
>>
>>  extension MemoryLayout {
>>    @_transparent
>>    public
>>    static func of(_: T.Type) -> MemoryLayout<T>.Type {
>>      return MemoryLayout<T>.self
>>    }
>>  }
>>
>> I think this API would solve the concerns I had about confusability that
>> led me to advocate dropping the ability to ask for the size of a value.
>> The only way to use it is to pass a type and these two expressions have
>> equivalent meaning:
>>
>>    MemoryLayout<Int>
>>    MemoryLayout.of(Int.self)
>>
>> It also has the benefit of isolating the autoclosure magic to type(of:).
>>
>> ,----[ Aside ]
>> | A slightly cleaner use site is possible with a larger API change:
>> |
>> |   MemoryLayout(type(of: someExpression)).size
>> |
>> | Which would involve changing MemoryLayout from an `enum` to
>> | a `struct` and adding the following:
>> |
>> |   extension MemoryLayout {
>> |     public init(_: T.Type) {}
>> |
>> |     public var size: Int { return MemoryLayout.size }
>> |     public var stride: Int { return MemoryLayout.stride }
>> |     public var alignment: Int { return MemoryLayout.alignment }
>> |   }
>> |
>> | However I am concerned that dropping ".of" at the use site is worth the
>> | added API complexity.
>> `----
>>
>> Thoughts?
>> --
>> -Dave
>>
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>>
>>
>> --
>> -Dave
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