Warning: Initialization of 'UnsafeBufferPointer<T>' results in a dangling buffer pointer

Question

After update to Swift 5.2 / Xcode 11.4 got a warning to following code:

extension Data {      init<T>(from value: T) {         var value = value         let pointer = UnsafeBufferPointer(start: &value, count: 1)         self.init(buffer: pointer)     }      func to<T>(type: T.Type) -> T {         return self.withUnsafeBytes { $0.load(as: T.self) }     } } 

On line let pointer = UnsafeBufferPointer(start: &value, count: 1) I got

Initialization of 'UnsafeBufferPointer' results in a dangling buffer pointer

I can use @silenceWarning but it's dirty solution. Maybe I need to store pointer somewhere and clean it in the future?

Answer 1

I also met these annoying warnings.

var str = "aaaaabbbbbccccc" var num1 = 1 var num2 = 22  var data = Data() // Initialization of 'UnsafeBufferPointer<String>' results in a dangling buffer pointer data.append(UnsafeBufferPointer(start: &str, count: 1))  // Initialization of 'UnsafeBufferPointer<Int>' results in a dangling buffer pointer data.append(UnsafeBufferPointer(start: &num1, count: 1)) // Initialization of 'UnsafeBufferPointer<Int>' results in a dangling buffer pointer  data.append(UnsafeBufferPointer(start: &num2, count: 1))  

Considering @greg's answer, I put the Data.append into withUnsafePointer's closure, and it does not show warnings anymore.

withUnsafePointer(to: &str) { data.append(UnsafeBufferPointer(start: $0, count: 1)) } // ok withUnsafePointer(to: &num1) { data.append(UnsafeBufferPointer(start: $0, count: 1)) } // ok withUnsafePointer(to: &num2) { data.append(UnsafeBufferPointer(start: $0, count: 1)) } // ok 

Here is the extension

extension Data {     init<T>(value: T) {         self = withUnsafePointer(to: value) { (ptr: UnsafePointer<T>) -> Data in             return Data(buffer: UnsafeBufferPointer(start: ptr, count: 1))         }     }      mutating func append<T>(value: T) {         withUnsafePointer(to: value) { (ptr: UnsafePointer<T>) in             append(UnsafeBufferPointer(start: ptr, count: 1))         }     } }