Question

Without any preamble I want to show you problem I have in my program, I commented out steps and my thoughts for that steps. (I didn’t include @interface part for shortness, it has same method with the same signature as in @implementation)

@implementation Dummy

- (int)testing:(NSError *__strong *)error 
{
    *error = [[NSError alloc] initWithDomain:@"hello" code:42 userInfo:nil]; 
    // 3. retain count = 1

    // 4. because of ARC 'error' object was released for this time 
    // (assembly output is my proof) object is deallocated
    // retain count = 0

    return 0;
}

@end

int main()
{
    NSError *e = nil; // 1. retain count = 0 (obviously)
    Dummy *dummy = [[Dummy alloc] init];

    [dummy testing:&e]; // 2. passing reference to an error object

    // 'e' for this time has to be just a trash, or nil maybe, 
    // but next log gives me correct output: 
    NSLog(@"%@ %li", [e domain], [e code]); // 'hello 42'

    return 0;
}

How does an error object exist after it death? I understand that using NSError *__autoreleasing * will be right way to go, and situation will be trivial in that case, but how compiler reasoning for this code, where is my mistake in judgements?

It’s a bit an artificial question, but I can’t throw out this situation from my head, I think I’m loosing something.

Here is part of disassembly for -[Dummy testing:]

callq  0x100000e8c <dyld_stub_objc_msgSend>
mov    -0x18(%rbp),%rcx
mov    (%rcx),%rdx
mov    %rax,(%rcx)
mov    %rdx,%rdi
callq  0x100000e92 <dyld_stub_objc_release>
mov    -0x24(%rbp),%eax
add    $0x40,%rsp
pop    %rbp
retq   

If I understood correctly, there is only one object in this method, and it’s clearly released, not autoreleased or something else.

Answer 1

I suspect your confused about what’s getting released. I just checked the assembly output, and there is a call to objc_release(), though I’m not familiar enough with x86 assembly to actually trace precisely what’s going on. However, I do know that the code here is expected to emit something of the equivalent of:

NSError *temp = [[NSError alloc] initWithDomain:@"hello" code:42 userInfo:nil];
[*error release];
*error = [temp retain];
[temp release];

and of course the optimizer will shrink that to

NSError *temp = ...
[*error release];
*error = temp;

So I think you’re seeing the call to objc_release() and thinking that your newly-allocated error is being released. It’s not. The previous value of *error is being released before the newly-allocated error is placed in that location.