如何从 Swift 中的索引开始将内存复制到 UnsafeMutableRawPointer？

Question

我知道如何使用以下方法将内存从数组复制到从索引 0 开始的 UnsafeMutableRawPointer：

mutableRawPointer.copyMemory(from: bytes, byteCount: bytes.count * MemoryLayout<Float>.stride)

其中 bytes 是一个浮点数数组。

但是，我想从我的数组复制到一个可变的原始指针，该指针从可能不为零的索引开始。

例如：

let array: [Float] = [1, 2, 3]

copyMemoryStartingAtIndex(to: myPointer, from: array, startIndexAtPointer: 2)

因此，如果指针对象是 [0, 0, 0, 0, 0]，它将变为 [0, 0, 1, 2, 3]。

如何在 Swift 4 中实现此目的？

Answer 1

你可以这样写：

//Caution: when T is not a `primitive` type, this code may cause severe memory issue
func copyMemoryStartingAtIndex<T>(to umrp: UnsafeMutableRawPointer, from arr: [T], startIndexAtPointer toIndex: Int) {
    let byteOffset = MemoryLayout<T>.stride * toIndex
    let byteCount = MemoryLayout<T>.stride * arr.count
    umrp.advanced(by: byteOffset).copyMemory(from: arr, byteCount: byteCount)
}

测试代码：

let size = MemoryLayout<Float>.stride * 5
let myPointer = UnsafeMutableRawPointer.allocate(byteCount: size, alignment: MemoryLayout<Float>.alignment)
defer {myPointer.deallocate()}

let uint8ptr = myPointer.initializeMemory(as: UInt8.self, repeating: 0, count: size)
defer {uint8ptr.deinitialize(count: size)}

func dump(_ urp: UnsafeRawPointer, _ size: Int) {
    let urbp = UnsafeRawBufferPointer(start: urp, count: size)
    print(urbp.map{String(format: "%02X", [=11=])}.joined(separator: " "))
}

let array: [Float] = [1, 2, 3]

dump(myPointer, size)
copyMemoryStartingAtIndex(to: myPointer, from: array, startIndexAtPointer: 2)
dump(myPointer, size)

输出：

00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 80 3F 00 00 00 40 00 00 40 40

但是，我建议您考虑 Hamish 在评论中所说的话。

如何从 Swift 中的索引开始将内存复制到 UnsafeMutableRawPointer？

How to copy memory to an UnsafeMutableRawPointer starting from an index in Swift?

pointers

unsafe-pointers

swift

unsafemutablepointer