swift - 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 float 数组。

但是，我想从我的数组复制到一个从可能不为零的索引开始的可变原始指针。

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 中实现这一点？

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", $0)}.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。