在Swift 3中,如何从Data获取UnsafeRawPointer？

Question

根据Swift 3中Data的文档,我可以使用一个初始化程序从UnsafeRawPointer创建数据.实际上我需要的是相反的.我有一个数据,我想创建一个指向数据字节的UnsafeRawPointer.这就是我现在正在做的事情:

1. let data = <from some where>
2. let unsafePointer = UnsafeMutablePointer<UInt8>.allocate(capacity: data.count)
3. unsafePointer.initialize(to: 0, count: data.count) // is this necessary?
4. data.copyBytes(to: unsafePointer, count: data.count)
5. let unsafeRawPointer = unsafePointer.deinitialize() // this is of the type UnsafeMutalbleRawPointer, and I can use it where UnsafeRawPointer is needed.

我确认此代码适用于Xcode Playground.代码甚至可以在没有行号3的情况下工作.我不确定有没有线路的区别.无论如何,我的问题是,我做得对吗？有更简单的方法吗？

Answer 1

withUnsafeBytes() 给你一个(类型)指向字节的指针,这可以转换为原始指针:

let data = <Data from somewhere>
data.withUnsafeBytes { (u8Ptr: UnsafePointer<UInt8>) in
    let rawPtr = UnsafeRawPointer(u8Ptr)
    // ... use `rawPtr` ...
}

指针仅在对闭包调用的生命周期内有效.

或者,您可以桥接NSData并访问原始字节:

let nsData = data as NSData
let rawPtr = nsData.bytes

现在指针在有效的相同范围内nsData有效.

Answer 2

查看最新的参考.

我们找不到UnsafeRawPointer从a 检索的方法或属性Data.

因此,替代方法: func withUnsafeBytes((UnsafePointer) - > ResultType)

你可以写这样的东西:

let data: Data = <initialize it as you like>

data.withUnsafeBytes {(uint8Ptr: UnsafePointer<UInt8>) in
    let rawPtr = UnsafeRawPointer(uint8Ptr)
    //`rawPtr` (and `uint8Ptr`) is guaranteed to be valid in this closure
    //...
    //You have no need manage `rawPtr`.
}

(哦,这与Martin R的答案的前半部分相同.)

但是如果你想保持你的UnsafeRawPointer有效期比封闭期更长,你需要复制以下内容Data:

例如:

let uint8Ptr = UnsafeMutablePointer<UInt8>.allocate(capacity: data.count)
uint8Ptr.initialize(from: data) //<-copying the data
//You need to keep `uint8Ptr` and `data.count` for future management
let uint8PtrCount = data.count
//You can convert it to `UnsafeRawPointer`
let rawPtr = UnsafeRawPointer(uint8Ptr)
//Use `rawPtr` while `uint8Ptr` is valid
//...
//Deinitialize and deallocate the region
uint8Ptr.deinitialize(count: uint8PtrCount)
uint8Ptr.deallocate(capacity: uint8PtrCount)

(您可以获得UnsafeMutableRawPointer返回值deinitialize(count:),但该区域处于未初始化状态,因此您不应访问该区域.)