解読したい、Swiftコード、unSafe族が良く判らん！<- ✳️コメント有り✳️

unsolved.swift

import Foundation
enum Vehicle {  
    case Car(windows: Int, wheels: Int)
    case Ship(windows: Int, funnels: Int, anchors: Int)
    case Plane(windows: Int, wheels: Int, wings: Int, engines: Int)
}


var aPlane: Vehicle = .Plane(windows: 1, wheels: 9, wings: 8, engines: 4)
var voidPtr: UnsafePointer<Vehicle> = unsafeBitCast(&aPlane,                    
                                                 UnsafePointer<Vehicle>.self)

// Int8 takes 1 byte (8 bits) memory space.
//let bytePointer = withUnsafePointer(&aPlane) { UnsafePointer<Int8>($0) }
let bytePointer = withUnsafePointer(&aPlane) { ptr in
unsafeBitCast(ptr, UnsafePointer<Int8>.self)
//UnsafePointer<Int8>(ptr)
}
let bytePointer1 = withUnsafePointer(&aPlane) { $0 }
print(bytePointer.dynamicType) // UnsafePointer<Int8>
print(bytePointer1.dynamicType) //UnsafePointer<Vehicle>

var offsetString: String = ""  
var contentString: String = ""

for offset in 0..<sizeof(Vehicle) {  
    offsetString += String(format: "%02d ", offset)
    contentString += String(format: "%02d ", bytePointer[offset])
}

print("offset:\t" + offsetString)  
print("content:\t" + contentString)  
// With unsafeBitCast

//extension Vehicle {  
//  var windows: Int {
//    let casted = unsafeBitCast(aPlane, (Int, Int, Int, Int, Int8).self)
//    return casted.0
//  }
//}
// With withUnsafePointer

extension Vehicle {  
  var windows: Int {
    var mutableSelf = self
    let ptr = withUnsafePointer(&mutableSelf) {
      UnsafePointer<(Int, Int, Int, Int, Int8)>($0)
    }
    print(ptr)
    print(ptr.memory)
    print(ptr.memory.0)
    //return ptr[0].0
    return ptr.memory.0
  }                                                                             
}
print(aPlane.windows)

✳️unsafeBitCast関数
func unsafeBitCast<T, U>(x: T, _: U.Type) -> U

インスタンスをビット列と見なして型解釈し直して、インスタンスを返す。
インスタンスがアドレスであっても良い。その場合はアドレスが返る。

✳️withUnsafePointer関数
Useful for calling Objective-C APIs、ポインタを引数に取るAPIを呼ぶのに便利、例えばVoid型のポインタとか。withUnsafeMutablePointers型も仲間⁉️
必ず、アドレスを引数にとる⁉️アドレスの型変換ができる。void* -> char*とか⁉️

✳️UnsafePointer型（構造体）、アドレスを表す
memoryメソッドで値を返す。（アドレスにある値）
UnsafeMutablePointer型も仲間⁉️

⭕️withUnsafePointer
Invokes body with an UnsafePointer to arg and returns the result.

argへのポインタを受取るclosure(body)を呼び出しresultを返す。▶️って事は、ポインタを受け取るって事だよねぇ。戻り値に関しては、なんでも良いのか？アドレスでもインスタンスでも？▶️その通り
enum Vehicle {
case Car(windows: Int, wheels: Int)
case Ship(windows: Int, funnels: Int, anchors: Int)
case Plane(windows: Int, wheels: Int, wings: Int, engines: Int)
}

var aPlane: Vehicle = .Plane(windows: 1, wheels: 9, wings: 8, engines: 4)
//print(aPlane[windows])
let bytePointer1 = withUnsafePointer(&aPlane) { $0.memory }
print(bytePointer1) //Plane(1, 9, 8, 4, 2)

let bytePointer2 = withUnsafePointer(&aPlane) { $0 }
print(bytePointer2) //UnsafePointer