Question

I am still learning …, so for the following property

@property (copy) NSNumber *foo;

What the copy really do? make a copy of the (value of) foo and put it to new place? Just like a copy constructor?

And also to clarify, the following is basically doing a AddRef, right?

@property (retain) NSNumber *foo;

Answer 1

What the copy really do?

In general, NSNumber is immutable — I’d expect that copy in those cases would be implemented using retain:

- (id)copyWithZone:(NSZone*)zone {
  return [self retain];
}

And also to clarify, the following is basically doing a AddRef, right?

Well, synthesizing it will add the reference counting boilerplate – the actual operation is more complex and takes the generalized form:

- (void)setFoo:(NSNumber *)arg {
  NSNumber * prev = foo;
  foo = [arg retain]; // << or foo = [arg copy]; if you have specified 'copy'
  [prev release];
}