Question

The following code is taken from “The iOS 5 Developer’s Cookbook” used to illustrate how to write a string to a file. It makes use of __autoreleasing without any explanation. Why is it necessary?

NSError __autoreleasing error;
...
if (![myString writeToFile:path atomically:YES error:&error)
{
    NSLog(.... error.localizedFailureReason  ...);
    return;
}

Why not just declare the error on the stack without the use of __autoreleasing?

—— EDIT —–

Additional question: why is the author declaring NSError and not NSError*?

Answer 1

It’s a hint for the automatic reference counting (ARC) system.

The error object will be allocated somewhere in NSString’s code so declaring it as __autoreleasing in your code lets ARC know what the storage characteristics are. That is, when error is set, it will be an autoreleased object.