Reference counting in Cocoa

Question

I assume according to "Cocoa design patterns" book i'm reading that the retain function is implemented using something like this :

   - (int)retainCount
// Returns the receiver's current reference count
{
int result = 1; // receiver not in table, its count is 1
void *tableValue = NSMapGet(
[[self class] _myRefCountMapTable], self);
if(NULL != tableValue )
{ // if receiver is in table, its count is the value stored
result = (int)tableValue;
}
return result;
}

- (id)retain
// Increases the receiver's reference count
{
// store the increased value in the table
NSMapInsert([[self class] _myRefCountMapTable], self,
(void *)([self retainCount] + 1));
return self;
}

As the example imply every reference object has the same self member. How does that happen ? maybe I don't understand the meaning of self - I though it's like "this" in C++.

If I just use assignment operator (A=B) Does it copy the pointer(self) and that's it ? I though it would use "copywithzone" and it's relatives and the "self" members won't be equal. Moreover, I though copywithzone is like copy constructor in c++.

I guess i'm confusing between the 2 worlds.

Answer 1

As the example imply every reference object …

There is no such thing as a “reference object”. I suspect that's not what you meant, so please clarify.

has the same self member.

Objects do not have members (instances have instance variables, which are similar in concept but not the same in implementation).

self is not a “member”, nor is it an instance variable. Note that classes have self as well. self is a special hidden argument to the message, containing the object that is the receiver of the message.

And no, self does not refer to every object at once. If you send the same message to two different objects, even of the same class, the self argument will contain a different pointer in each message.

maybe I don't understand the meaning of self - I though it's like "this" in C++.

As I understand “this”, yes. self is the object that received the message—in your examples, the object that something is retaining or asking the retain count of.

If I just use assignment operator (A=B) Does it copy the pointer(self) and that's it ?

The pointer copied will only be self if B is self. That is, if you say A = self, then it will copy the self pointer to A. If you say B = self and then you say A = B, same thing, since B and self contain the same pointer. If you had not said B = self, then B is probably some other value, so that other value is what will be copied to A. And that's assuming A and B are pointer variables.

It will copy the value (pointer) you tell it to copy. Nothing else.

I though it would use "copywithzone" and it's relatives and the "self" members won't be equal.

No. The object is only sent a copyWithZone: message (do not omit colons—they are significant) when something sends it a copyWithZone: message. The most common way is to send it a copy message, as that will send a copyWithZone: message in turn.

Furthermore, even a “copy” does not always copy the object. Immutable objects can implement copyWithZone: to return [self retain] instead.

However, plain assignment never copies the object. It only copies the pointer.

Moreover, I though copywithzone is like copy constructor in c++.

Roughly. I don't know enough C++ to say how much like it it is.

Answer 2

I remember having heard that you shouldn't make any assumptions on the retainCount. :-)

self is indeed very similar to this.

Assignment just copies the pointer, and it's the same in C++.

NSObject *objA =[[NSObject alloc] init];
NSObject *objB = objA;

objA and objB reference the same object.

Not that your code example uses [self class], so they'd use one table per class for all instances of that class.