UnsafeMutablePointer in Swift 3

Question

I'm attempting to call SecItemCopyMatching in my keychain utility class in order to get data out of the keychain, yet I'm running into a problem with getting the result argument, UnsafeMutablePointer<CFTypeRef?>.

The original statement (in Swift 2, before migrating to Swift 3) was

// query is a dictionary of [String : AnyObject]

var result: Data?
let status = withUnsafeMutablePointer(to: &result) {
    SecItemCopyMatching(query as CFDictionary, UnsafeMutablePointer($0))
}

But in Swift 3, you are now required to call .withMemoryRebound in order to view memory. Based on what Xcode tells you to do, I tried this

var result: Data?
let status = withUnsafeMutablePointer(to: &result){
    $0.withMemoryRebound(to: Data.self, capacity: 1){
        SecItemCopyMatching(query as CFDictionary, UnsafePointer($0))
    }
}

Yet doing this, I get an error

Cannot convert value of type 'UnsafePointer<_>' to expected argument type 'UnsafeMutablePointer<CFTypeRef?>?'

So, I tried using CFTypeRef instead of Data

var result: CFTypeRef?
let status = withUnsafeMutablePointer(to: &result){
    $0.withMemoryRebound(to: CFTypeRef.self, capacity: 1){
        SecItemCopyMatching(query as CFDictionary, UnsafePointer($0))
    }
}

Replacing UnsafePointer($0) with simply $0 results in the same error message.

How can I get an UnsafeMutablePointer<CFTypeRef?> for getting data from keychain?

Answer 1

In your case, you do not need to use withMemoryRebound or withUnsafeMutablePointer(to:).

Instead, you can just use

var result: AnyObject?
let status = SecItemCopyMatching(query as CFDictionary, &result)

if status == noErr, let data = result as? Data {
    //use data...
}

Generally, when you need to pass an UnsafeMutablePointer<T> to a function, declare a variable of type T and pass it as an inout argument &variable. In your case, T is CFTypeRef?, and in Swift 3, CFTypeRef is just a typealias of AnyObject.

---

Even in Swift 2.2, you did not need to use withUnsafeMutablePointer

var result: AnyObject?  
let status = SecItemCopyMatching(query, &result)  

Answer 2

The error message is somewhat misleading - the actual problem is that result has to be an AnyObject? - withMemoryRebound doesn't need to be used.

var result: AnyObject?
let status = withUnsafeMutablePointer(to: &result){
    SecItemCopyMatching(query as CFDictionary, UnsafeMutablePointer($0))
}

Works as expected and gets the correct result from keychain - it just needs to be casted to Data. In fact, withUnsafeMutablePointer doesn't even have to be used.

So, my new code is

var query: [String : AnyObject] = [:]
//set up the query

var result: AnyObject?
let status = SecItemCopyMatching(query as CFDictionary, &result)

var data: Data?
if status == noErr{
    data = result as? Data
}