如何为 memcpy(...) 调用添加数组偏移量？
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"] ？
【问题讨论】：
标签： ios arrays performance swift memcpy
我还是 Swift 的新手，使用名称中带有“Unsafe”的方法仍然让我担心，但我相当确定这是一种用于调用 memcpy() 并为目标指定偏移量的可用技术和/或源地址。但这仅适用于字节数组，即 [UInt8]。正如@zaph 所解释的，绝对不适用于字符串。
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)
}
}
下面是一些测试代码：
// 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)
【讨论】：
首先，作者似乎都没有理解：对象数组（这里是 String 实例）不存储内容，而是对该对象的引用。因此 UTF-8、UTF-16 都与它无关。后备数组实际上包含的是指针（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，要回答似乎运行良好的原始问题并让我认为在这种情况下 Swift 数组是一个连续的内存区域，您应该这样做：
source 和 dest 是指向连续内存区域的指针：第一个对象位于基地址，第二个 @+sizeof(type)，第 n 个元素位于 @+(n-1)*sizeof(type)。
您所要做的就是指定 dest 的写入偏移量，在您的特定情况下为 0，在源中的偏移量在您的情况下为 1。
【讨论】：
Array 不能保证是一个连续的内存区域，另一方面 ContiguousArray 是
不要对对象使用 memcpy 或其他低级“C”运算符。由于很多原因，这行不通。
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]
【讨论】：