UnsafeMutablePointer到Array元素

Question

var a = [1,2,3]
let ptr1 = UnsafeMutablePointer<Int>(&a[0]) //works fine

let index = 0
let ptr2 = UnsafeMutablePointer<Int>(&a[index]) //compiler throws error

error:无法为类型UnsafeMutablePointer<Int>为参数列表的类型调用初始值设定项(inout Int)

为什么后者不编译？这里有什么我想念的吗？

---

我想做一个下面的片段.

class Holder {
    var numbers: [Int] = [1,2,3,4]
    var modifier: Modifier

    init(index: Int) {
       self.modifier = Modifier(UnsafeMutablePointer(&self.numbers) + index)
    }

}

class Modifer {
  var ptr: UnsafeMutablePointer<Int>

  init(_ ptr: UnsafeMutablePointer<Int>) {
     self.ptr = ptr
  }

  func change(to: Int) {
     self.ptr.pointee = to
    // expected the change to be reflected in numbers array
    // but as Rob Napier said it became invalid and throws EXC_BAD_ACCESS
  }
}

如何达到上述预期效果？

Answer 1

请注意,这些都不是创建UnsafeMutablePointer的有效方法.Swift可以在最后一次引用后立即释放a,所以当你使用这些指针时,它们可能无效.你想要的工具是a.withUnsafeMutableBufferPointer.

也就是说,这里的正确语法是:

let ptr2 = UnsafeMutablePointer(&a) + index

查看更新后的代码,没有办法在Array上有意义.我想你假设Arrays是引用类型.他们是价值类型.没有办法改变一块numbers.对它的任何更改都会用完全不同的数组替换整个数组.Swift有一些聪明的写时复制技巧可以提高效率,实际上它可能实际上并没有取代整个数组,但你应该编程就像它一样.您应该考虑以下行:

array[1] = 2

相当于:

array = <a new array that is identical, but element 1 has been replaced by 2>

这意味着Array在非常受控制的情况下(例如在withUnsafeMutableBufferPointer块内),指针变得毫无意义.

你想要的是更像这样的东西:

class Holder {
    var numbers: [Int] = [1,2,3,4]
    private(set) var modifier: ((Int) -> ())! // ! is an artifact of capturing self in init

    init(index: Int) {
        self.modifier = { [weak self] in self?.numbers[index] = $0 }
    }
}

let holder = Holder(index: 2)
holder.numbers  // [1, 2, 3, 4]
holder.modifier(0)
holder.numbers  // [1, 2, 0, 4]