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

Question

I know how to copy memory from an array to an UnsafeMutableRawPointer starting at index 0 by using:

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

where bytes is an array of floats.

However, I would like to copy from my array to a mutable raw pointer starting from an index that might not be zero.

Eg.:

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

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

So, if the pointee was [0, 0, 0, 0, 0], it will become [0, 0, 1, 2, 3].

How can I achieve this in Swift 4?

`(mutableRawPointer + byteOffset).copyMemory(...)`? Though I would strongly advise using a typed pointer if you know that the underlying memory is bound to a given type. — Hamish, Oct 14 '18 at 14:15
It is actually a pointer given by the `contents` func from a MTLBuffer. — BroccoliFinancials, Oct 14 '18 at 15:15

score 3 · Accepted Answer · answered Oct 14 '18 at 14:57

You can write something like this:

//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)
}

Testing code:

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)

Output:

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

But, I recommend you to consider what Hamish says in the comment.

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

1 Answers1