In Swift 3, how do I get UnsafeRawPointer from Data?

Question

According to the documentation of Data in Swift 3, there is an initializer that I can use to create a Data from UnsafeRawPointer. What I need actually is the opposite. I have a Data, and I want to create a UnsafeRawPointer that points to the bytes of the Data. Here's what I'm doing right now:

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.

I verified that this code works in Xcode Playground. The code even works without the line number 3. I'm not sure what is the difference with or without the line. Anyway, my question is, am I doing right for what I want? Is there a simpler way to do it?

Answer 1

withUnsafeBytes() gives you a (typed) pointer to the bytes, this can be converted to a raw pointer:

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

The pointer is only valid during the lifetime of the call to the closure.

Alternatively, you can bridge to NSData and access the raw bytes:

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

Now the pointer is valid in the same scope where nsData is valid.

As of Swift 5 it is

let data = <Data from somewhere>
data.withUnsafeBytes { rawBufferPointer in
    let rawPtr = rawBufferPointer.baseAddress!
    // ... use `rawPtr` ...
}

because the closure argument is now a UnsafeRawBufferPointer.