Swift：如何向memcpy添加偏移量（…）
如何为memcpy（…）调用添加数组的偏移量？
我有String数组：
var source = ["a","b","c","d"] var dest = [String](count:n, repeatedValue: "") memcpy(&dest, source, UInt(2 * sizeof(String))
这个副本[“a”，“b”]到dest。 很明显。 我如何复制[“b”，“c”]？
不要在对象上使用memcpy或其他低级“C”运算符。 这不会有很多原因。
使用切片运算符：
var source = ["a","b","c","d"] var dest = Array(source[1...2]) println("dest: \(dest)")
输出：
dest：[b，c]
Unicode处理正确：
var source = ["🇪🇸", "😂", "a","b","c","d"] var dest = Array(source[1...2]) println("dest: \(dest)")
输出：
dest：[😂，a]
我还是Swift的新手，在名称中使用“Unsafe”的方法仍然让我担心，但我很确定这是一种可用的技术，用于调用memcpy（）并指定目标和/或源的偏移量地址。 但这仅适用于字节数组，即[UInt8]。 绝对不是字符串，正如@zaph所解释的那样。
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) } }
这是一些测试代码：
// 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)
首先，有些东西似乎都没有理解：一个对象数组（这里是String实例）不存储内容，而是存储对象的引用。 因此UTF-8，UTF-16，无论与它无关。 支持数组实际包含的是指针（即地址==无符号整数）。 除此之外，除非swift中的数组是内存中的实际数组，否则不应该在其上使用memcpy，如果它由NSArray支持则更是如此！
尽管如此，要回答原本似乎完美运行的问题，让我觉得在这种情况下，Swift数组是一个连续的内存区域，这是你应该做的：
source和dest是连续内存区域的指针：第一个对象位于基址，第二个是@ + sizeof（类型），第n个元素位于@ +（n-1）* sizeof（类型）。
您需要做的就是在特定情况下指定dest的写入偏移量0，在源代码中指定偏移量，在您的情况下为1。