在没有复制或分配的 Swift 数组类型之间进行转换

Question

我想像访问 Int8 数组一样访问现有的 UInt64 数组。关键要求是效率——我不想复制或重新分配数据，只想直接访问。我不想要副作用（例如，我希望能够在执行此代码块后继续使用 uint64Array，正在阅读有关具有未定义副作用的重新绑定。）

我尝试使用 Swift 4.2:

var uint64Array = [UInt64](repeating: 0, count: 100)

uint64Array.withUnsafeMutableBufferPointer() {
    uint64Pointer in
    uint64Pointer.withMemoryRebound(to: Int8.self) {   // <- Error occurs here.
        int8Pointer in
        int8Pointer[0] = 1
        int8Pointer[1] = 2
        int8Pointer[2] = 3
        int8Pointer[3] = 4
    }
}

但是我在运行时在以下行中遇到致命错误：

    uint64Pointer.withMemoryRebound(to: Int8.self) {

这是正确的方法吗？如果是这样，为什么我会收到致命错误？

Answer 1

我认为问题是您不能按照文档中的注释直接绑定到不同的类型：

Only use this method to rebind the buffer’s memory to a type with the same size and stride as the currently bound Element type. To bind a region of memory to a type that is a different size, convert the buffer to a raw buffer and use the bindMemory(to:) method.

如果字节是您想要的，那么最快的路线是：

var uint64Array = [UInt64](repeating: 0, count: 100)
uint64Array.withUnsafeMutableBytes { x in

    x[0] = 1
    x[1] = 2
    x[3] = 3
    x[4] = 4

}

如果您想使用其他类型，可以这样做：

var uint64Array = [UInt64](repeating: 0, count: 100)

uint64Array.withUnsafeMutableBufferPointer() {
    uint64Pointer in

    let x = UnsafeMutableRawBufferPointer(uint64Pointer).bindMemory(to: Int32.self)
    x[0] = 1
    x[1] = 2
    x[3] = 3
    x[4] = 4

}

Answer 2

感谢@brindy 解决了这个问题。这是一个尽可能干净的扩展实现。

扩展名：

extension Array {
    mutating func bindMutableMemoryTo<T,R>(_ type: T.Type, _ closure: (UnsafeMutableBufferPointer<T>) throws -> R) rethrows -> R {
        return try self.withUnsafeMutableBytes() {
            return try closure([=10=].bindMemory(to: type))
        }
    }
}

用法：

   var uint64Array = [UInt64](repeating: 0, count: 100)
   uint64Array.bindMutableMemoryTo(Int8.self) {
        int8Pointer in
        int8Pointer[0] = 1 // LSB of uint64Array[0]
        int8Pointer[1] = 2
        int8Pointer[2] = 3
        int8Pointer[3] = 4 // MSB of uint64Array[0]
    }

在没有复制或分配的 Swift 数组类型之间进行转换

Casting between Swift array types without copy or allocations

arrays

swift

unsafemutablepointer

swift4.2