Pass by reference and objc_unretainedPointer from iOS5 - ios

Question

I'm attempting to use the SFHF keychain classes (from here) with an IOS 5 project. I've successfully converted most of the class over to abide by the new ARC rules.
I'm having some trouble with one small section of the code as follows
OSStatus status = SecItemCopyMatching((CFDictionaryRef) objc_unretainedPointer(attributeQuery), (CFTypeRef *) objc_unretainedPointer(&attributeResult)
This gives the following syntax issue:
warning: Semantic Issue: Incompatible pointer types passing 'NSDictionary *__strong *' to parameter of type 'id'
I'm rather new to iOS development and this has me pretty much stumped right now. Any help is greatly appreciated.

Answer 1

This is the declaration of the API:
OSStatus SecItemCopyMatching (
CFDictionaryRef query,
CFTypeRef *result
);
The result is a pass-by-reference return value.
Declare a local variable of type CFTypeRef, call the function and pass the address of said local as per the API, then do any ARC specific shenanigans after the function call.
Yes -- the error is correct. You aren't passing a CFTypeRef, you are passing a CFTypeRef* and objc_unretainedPointer() has no clue what to do with that.
Do something like:
CFTypeRef localResult
SecItemCopyMatching(query, &localResult);
if (... no error ...) {
result = objc_retainedObject(localResult);
}

Answer 2

Had trouble with this call, this is the code I got to work:
NSMutableDictionary *queryDictionary = [[NSMutableDictionary alloc] init];
// Set some properties.
[queryDictionary setObject:[key dataUsingEncoding:NSUTF8StringEncoding] forKey:(__bridge id)kSecAttrGeneric];
[queryDictionary setObject:(id) kCFBooleanTrue forKey:(__bridge id)kSecReturnAttributes];
[queryDictionary setObject:(__bridge id) kSecMatchLimitOne forKey:(__bridge id)kSecMatchLimit];
[queryDictionary setObject:(id) kCFBooleanTrue forKey:(__bridge id)kSecReturnData];
[queryDictionary setObject:(__bridge id) kSecClassGenericPassword forKey:(__bridge id)kSecClass];
CFTypeRef attributes;
OSStatus keychainError = SecItemCopyMatching((__bridge CFDictionaryRef)(queryDictionary), &attributes);
if (keychainError == errSecSuccess)
{
NSDictionary *returnedDictionary = (__bridge_transfer NSDictionary *)attributes;
NSData *rawData = [returnedDictionary objectForKey:(__bridge id)kSecValueData];
return [[NSString alloc] initWithBytes:[rawData bytes] length:[rawData length] encoding:NSUTF8StringEncoding];
}