Why does Xcode 4.2 use @autoreleasepool in main.m instead of NSAutoreleasePool?

问题

I've noticed that there is a different way in Xcode 4.2 to start the main function:

int main(int argc, char *argv[])
{
    @autoreleasepool {
        return UIApplicationMain(argc, argv, nil,
                                 NSStringFromClass([PlistAppDelegate class]));
    }
}

and

int main(int argc, char *argv[]) {
    NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];
    int retVal = UIApplicationMain(argc, argv, nil, nil);
    [pool release];
    return retVal;
}

Does anybody know the difference between those two?

回答1:

The first one is using ARC, which is implemented in iOS5 and above to handle memory management for you.

On the second one, you're managing your own memory and creating an autorelease pool to handle every autorelease that happens inside your main function.

So after reading a bit on what's new on Obj-C with iOS5 it appears that the:

@autoreleasepool {
    //some code
}

works the same as

NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];
// some code
[pool release];

with the difference that the last one would throw an error on ARC.

EDIT:

The first one is using ARC or not.

来源：https://stackoverflow.com/questions/8714875/why-does-xcode-4-2-use-autoreleasepool-in-main-m-instead-of-nsautoreleasepool