Why does ARC retain method arguments?

Question

When compiling with ARC, method arguments often appear to be retained at the beginning of the method and released at the end. This retain/release pair seems superfluous, and

Answer 1

See this reply from the Objc-language mailing list:

When the compiler doesn't know anything about the memory management behavior of a function or method (and this happens a lot), then the compiler must assume:

1) That the function or method might completely rearrange or replace the entire object graph of the application (it probably won't, but it could). 2) That the caller might be manual reference counted code, and therefore the lifetime of passed in parameters is not realistically knowable.

Given #1 and #2; and given that ARC must never allow an object to be prematurely deallocated, then these two assumptions force the compiler to retain passed in objects more often than not.

I think that the main problem is that your method’s body might lead to the arguments being released, so that ARC has to act defensively and retain them:

- (void) processItems
{
    [self setItems:[NSArray arrayWithObject:[NSNumber numberWithInt:0]]];
    [self doSomethingSillyWith:[items lastObject]];
}

- (void) doSomethingSillyWith: (id) foo
{
    [self setItems:nil];
    NSLog(@"%@", foo); // if ARC did not retain foo, you could be in trouble
}

That might also be the reason that you don’t see the extra retain when there’s just a single call in your method.