Swift: how add offset to memcpy(…)

问题

How to add offset for array for memcpy(...) invocation?

I have array of String :

var source = ["a","b","c","d"]
var dest = [String](count:n, repeatedValue: "")
memcpy(&dest, source, UInt(2 * sizeof(String))

This copy ["a","b"] to dest. I'ts obvious. How can i copy ["b", "c"] ?

回答1:

Do not use memcpy or other low-level "C" operators on objects. That will not work for many reasons.

Use the slice operator:

var source = ["a","b","c","d"]
var dest = Array(source[1...2])
println("dest: \(dest)")

output:

dest: [b, c]

Unicode is handled correctly:

var source = ["🇪🇸", "😂", "a","b","c","d"]
var dest = Array(source[1...2])
println("dest: \(dest)")

output:

dest: [😂, a]

回答2:

I'm still new to Swift, and use of methods with "Unsafe" in the name still worries me, but I'm fairly sure this is a usable technique for calling memcpy() and specifying an offset for the destination and/or source address. But this only works for byte arrays, i.e., [UInt8]. Definitely not for strings, as explained by @zaph.

public class SystemMisc {

   /// Wrapper for the memcpy() method that allows specification of an offset for the destination
   /// and/or the source addresses.
   ///
   /// This version for when destination is a normal Swift byte array.
   ///
   /// - Parameters:
   ///   - destPointer:    Address for destination byte array, typically Swift [UInt8].
   ///   - destOffset:     Offset to be added to the destination address, may be zero.
   ///   - sourcePointer:  Address for source byte array, typically Swift [UInt8].
   ///   - sourceOffset:   Offset to be added to the source address, may be zero.
   ///   - byteLength:     Number of bytes to be copied.
   public static func memoryCopy(_ destPointer : UnsafeRawPointer, _ destOffset : Int,
                                 _ sourcePointer : UnsafeRawPointer, _ sourceOffset : Int,
                                 _ byteLength : Int) {

      memoryCopy(UnsafeMutableRawPointer(mutating: destPointer), destOffset,
                 sourcePointer, sourceOffset, byteLength)
   }


   /// Wrapper for the memcpy() method that allows specification of an offset for the destination 
   /// and/or the source addresses.
   ///
   /// This version for when destination address is already available as an UnsafeMutableRawPointer, 
   /// for example if caller has used UnsafeMutableRawPointer() to create it or is working with 
   /// unmanaged memory. The destPointer argument may also be a converted pointer, as done by the 
   /// above wrapper method.
   ///
   /// - Parameters:
   ///   - destPointer:    Address for destination byte array, see above notes.
   ///   - destOffset:     Offset to be added to the destination address, may be zero.
   ///   - sourcePointer:  Address for source byte array, typically Swift [UInt8].
   ///   - sourceOffset:   Offset to be added to the source address, may be zero.
   ///   - byteLength:     Number of bytes to be copied.
   public static func memoryCopy(_ destPointer : UnsafeMutableRawPointer, _ destOffset : Int,
                                 _ sourcePointer : UnsafeRawPointer, _ sourceOffset : Int,
                                 _ byteLength : Int) {

      memcpy(destPointer.advanced(by: destOffset),
             sourcePointer.advanced(by: sourceOffset),
             byteLength)
   }
}

And here's some test code:

  // Test the memoryCopy() method, using extra UnsafeMutableRawPointer conversion

  let destArray1 : [UInt8] = [ 0, 1, 2, 3 ]  // Note - doesn't need to be var
  let sourceArray1 : [UInt8] = [ 42, 43, 44, 45 ]

  SystemMisc.memoryCopy(destArray1, 1, sourceArray1, 1, 2)

  assert(destArray1[0] == 0 && destArray1[1] == 43 && destArray1[2] == 44 && destArray1[3] == 3)


  // Test the memoryCopy() method, providing UnsafeMutableRawPointer for destination

  var destArray2 : [UInt8] = [ 0, 1, 2, 3 ]
  let sourceArray2 : [UInt8] = [ 42, 43, 44, 45 ]

  let destArray2Pointer = UnsafeMutableRawPointer(&destArray2)

  SystemMisc.memoryCopy(destArray2Pointer, 1, sourceArray2, 1, 2)

  assert(destArray2[0] == 0 && destArray2[1] == 43 && destArray2[2] == 44 && destArray2[3] == 3)

回答3:

First of all, there's something none of the writers seem to have understood: an array of object (here String instances) do not store the content but a reference to this object. Therefore UTF-8, UTF-16, whatever has nothing to do with it. What the backing array actually contains is pointers (ie addresses == unsigned integers). Aside from that, unless an array in swift is an actual array in memory, you shouldn't use memcpy on it, even more so if it is backed by an NSArray!

Nonetheless, to answer the original question that seems to be working perfectly and makes me think that in this case the Swift Array is a contiguous zone of memory here is what you should do:

source and dest are pointers to contiguous memory zones: the first object being at the base address, the second @+sizeof(type), the nth element at @+(n-1)*sizeof(type).

All you have to do is specify the write offset for dest, in your particular case 0, and the offset in source, in your case 1.

来源：https://stackoverflow.com/questions/25079380/swift-how-add-offset-to-memcpy

标签

ios

arrays

performance

swift

memcpy