Eu estava lendo um pouco sobre o ARC e vi o seguinte:

@interface Address : NSObject {
    @public
    NSString *city;
}
@end

@implementation Address
- (Address*) init: (NSString*) c {
    city = c;

    return self;
}

- (void) dealloc {
    NSLog(@"Destroying address: %@", city);
}
@end

@interface Customer : NSObject {
    NSString *name;
    Address *addr;
}
@end

@implementation Customer
- (Customer*) init: (NSString*) n withAddress: (Address*) a {
    //Note 1: Automatic retain on assignment
    name = n;
    addr = a;

    return self;
}

- (void) dealloc {
    NSLog(@"Destroying: %@", name);
    //Note 2: Automatic release of member variables
}

@end

Customer* objectReturnTest() {
    NSString * n = [[NSString alloc] initWithString: @"Billy Bob"];
    Address * a = [[Address alloc] init: @"New York City"];      

    Customer *c = [[Customer alloc] init: n withAddress: a];

    //Note 3: ARC will put the returned object in autorelease pool.
    return c;
}

A couple of basic things to note here. As "Note 1"  says, when an object is assigned to a variable, a call to retain is made automatically. This increments the reference count. As "Note 2" says, when an object is destroyed, all member variable objects are released for you. You no longer have to do that from the dealloc method.

Finally, when a method returns a newly created object, ARC will put the returned object in an autorelease pool. This is stated in "Note 3".

Now, let’s use the code.

int main (int argc, const char * argv[])
{
    NSString * n = [[NSString alloc] initWithString: @"Johnny Walker"];
    Address * a = [[Address alloc] init: @"Miami"];
    Customer *c = [[Customer alloc] init: n withAddress: a];

    NSLog(@"Before force release");
    c = nil; //Force a release
    NSLog(@"After force release");

    @autoreleasepool {

        Customer *c2 = objectReturnTest();

    }
    NSLog(@"After autorelease pool block.");

    return 0;
}

The log output from this code will be:

Before force release

Destroying: Johnny Walker

After force release

Destroying: Billy Bob

Destroying address: New York City

After autorelease pool block.

Destroying address: Miami

A couple of things to note here. See how force release works. We set a variable to nil. ARC immediately releases the reference count. This causes the Customer object "Johnny Walker" to get destroyed. But, the member Address object "Miami" doesn’t get destroyed. This object gets destroyed at the very end of the main method. This is an extremely odd and non-intuitive behavior. Technically, this is not a memory leak, but, in reality member variables can pile up and take up a lot of memory. This is just as bad as memory leak.

The object return test works as expected. Customer "Billy Bob" is put in auto release pool. At the end of the @autoreleasepool block, the pool is drained and the object is released.

Ver esta parte;

int main (int argc, const char * argv[])
{
    NSString * n = [[NSString alloc] initWithString: @"Johnny Walker"];
    Address * a = [[Address alloc] init: @"Miami"];
    Customer *c = [[Customer alloc] init: n withAddress: a];

    NSLog(@"Before force release");
    c = nil; //Force a release
    NSLog(@"After force release");

    @autoreleasepool {

        Customer *c2 = objectReturnTest();

    }
    NSLog(@"After autorelease pool block.");

    return 0;
}

Quando ele faz c = nil; c e n não deveriam ser destruídos? No entanto, diz que a saída é apenas que n é destruído. Alguém pode explicar o porqu

E ele diz que o resultado é tão ruim quanto um vazamento de memória, então como você o corrige?

E uma última pergunta, quando você deve usar @autoreleaasepool?