如何在 Swift 3 中将 UInt16 转换为 UInt8?

问题描述

我想将 UInt16 转换为 UInt8 数组，但收到以下错误消息:

I want to convert UInt16 to UInt8 array, but am getting the following error message:

'init' 不可用:使用 'withMemoryRebound(to:capacity:_)'暂时将内存视为另一种布局兼容的类型.

'init' is unavailable: use 'withMemoryRebound(to:capacity:_)' to temporarily view memory as another layout-compatible type.

代码:

    let statusByte: UInt8 = UInt8(status)
    let lenghtByte: UInt16 = UInt16(passwordBytes.count)

    var bigEndian = lenghtByte.bigEndian

    let bytePtr = withUnsafePointer(to: &bigEndian) {
        UnsafeBufferPointer<UInt8>(start: UnsafePointer($0), count: MemoryLayout.size(ofValue: bigEndian))
    }

推荐答案

如错误信息所示，你必须使用 withMemoryRebound()将指向 UInt16 的指针重新解释为指向 UInt8 的指针:

As the error message indicates, you have to use withMemoryRebound() to reinterpret the pointer to UInt16 as a pointer to UInt8:

let bytes = withUnsafePointer(to: &bigEndian) {
    $0.withMemoryRebound(to: UInt8.self, capacity: MemoryLayout.size(ofValue: bigEndian)) {
        Array(UnsafeBufferPointer(start: $0, count: MemoryLayout.size(ofValue: bigEndian)))
    }
}

闭包是用指针($0)调用的，这些指针只有效在闭包的整个生命周期内，不得传递给外部供以后使用.这就是创建 Array 并将其用作返回值的原因.

The closures are invoked with pointers ($0) which are only valid for the lifetime of the closure and must not be passed to the outside for later use. That's why an Array is created and used as return value.

但是有一个更简单的解决方案:

There is a simpler solution however:

let bytes = withUnsafeBytes(of: &bigEndian) { Array($0) }

解释: withUnsafeBytes 调用带有 UnsafeRawBufferPointer 的闭包到 bigEndian 变量的存储.由于 UnsafeRawBufferPointer 是 UInt8 的 Sequence，因此是一个数组可以使用 Array($0) 来创建.

Explanation: withUnsafeBytes invokes the closure with a UnsafeRawBufferPointer to the storage of the bigEndian variable. Since UnsafeRawBufferPointer is a Sequence of UInt8, an array can be created from that with Array($0).

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