Q:

Storing data to NSUserDefaults

In my iPhone app, I have a class called Contact which consists of an ABRecordRef to serve as a reference to a particular contact.
I need to store groups of these contacts in NSUserDefaults, but things aren't working out so well since Contact is a custom class. 
Any ideas of what to do in this case?

A:

You cannot use NSUserDefaults for a custom class. From the documentation:

The NSUserDefaults class provides convenience methods for accessing
  common types such as floats, doubles, integers, Booleans, and URLs. A
  default object must be a property list, that is, an instance of (or
  for collections a combination of instances of): NSData, NSString,
  NSNumber, NSDate, NSArray, or NSDictionary. If you want to store any
  other type of object, you should typically archive it to create an
  instance of NSData.

Try using NSData. For example, to load custom objects into an array, you can do
NSUserDefaults *currentDefaults = [NSUserDefaults standardUserDefaults];
NSData *dataRepresentingSavedArray = [currentDefaults objectForKey:@"savedArray"];
if (dataRepresentingSavedArray != nil)
{
        NSArray *oldSavedArray = [NSKeyedUnarchiver unarchiveObjectWithData:dataRepresentingSavedArray];
        if (oldSavedArray != nil)
                objectArray = [[NSMutableArray alloc] initWithArray:oldSavedArray];
        else
                objectArray = [[NSMutableArray alloc] init];
}

To archive the data, use:
[[NSUserDefaults standardUserDefaults] setObject:[NSKeyedArchiver archivedDataWithRootObject:objectArray] forKey:@"savedArray"];

This will all work so long as your custom object complies with the NSCoding protocol:
- (void)encodeWithCoder:(NSCoder *)coder;
{
    [coder encodeObject:label forKey:@"label"];
    [coder encodeInteger:numberID forKey:@"numberID"];
}

- (id)initWithCoder:(NSCoder *)coder;
{
    self = [[CustomObject alloc] init];
    if (self != nil)
    {
        label = [coder decodeObjectForKey:@"label"];
        numberID = [coder decodeIntegerForKey:@"numberID"];
    }   
    return self;
}

ABRecord is an opaque C type, so it's not an object in the sense of Objective-C. That means you can not extend it, you can not add a category on it, you can not message it. The only thing you can do is call functions described in ABRecord Reference with the ABRecord as a parameter.
You could do two things to be able to keep the information referenced by the ABRecord around:

Get the ABRecords id by ABRecordGetRecordID(). The ABRecordID is defined as int32_t so you can cast it to an NSInteger and store it wherever you like. You can later get the record back from ABAddressBookGetPersonWithRecordID() or ABAddressBookGetGroupWithRecordID(). However, the record could be changed or even deleted by the user or another app meanwhile.
Copy all values inside the record to a standard NSObject subclass and use NSCoding as discussed above to store it. You will then, of course, not benefit from changes or additions to the record the user could have made.

