What happens when a object is assigned to another object?

Question

For example:

myVC.bunnies = self.myBunnies;

or

[self getBunniesWithCompletion:^(NSArray *bunnies) {
     self.myBunnies = bunnies;
}];

Is the object copied, or it's just its reference that is copied, and if it gets destroyed everything will break?

The property is declared like so:

@property (strong, nonatomic) NSArray *myBunnies;

As an aside - using `get` in a method name usually indicates that a result is being returned by reference in one of the passed in parameters. It's only a convention, but convention matters. — Abizern, Dec 09 '13 at 13:39

score 2 · Answer 1 · answered Dec 09 '13 at 13:35

2

When you do this assignment, a reference is copied, and a reference count is incremented. All of this is done implicitly through ARC, because you declared your property strong.

If you would like a copy to be made, change the declaration to

@property (copy, nonatomic) NSArray *myBunnies

This would be more expensive, but the array inside your object would be insulated from the array passed into the setter.

answered Dec 09 '13 at 13:35

Sergey Kalinichenko

714,442
84
1,110
1,523

So the reference won't get destroyed until the reference count is 0? – Devfly Dec 09 '13 at 13:36
@Devfly Correct, the object will not get destroyed until the reference count goes down to zero. – Sergey Kalinichenko Dec 09 '13 at 13:37

score 1 · Accepted Answer · answered Dec 09 '13 at 13:39

It totally depends on the way you declared the property:

@property(strong) : the object reference count is increased, meaning that the object self keeps a reference to the affected object so that this object is not released (until the reference has been released by setting it to nil)
@propery(weak) : the object reference is simply assigned but the reference count is not increased, meaning that self does not retain a reference to it
@property(copy) : the object is copied (using <NSCopying>'s copy method) and thus a new instance is stored, independant from the first one

I strongly recommand you to read the Advanced Memory Managment Programming Guide in the Apple Doc. It is not totally up-to-date as some parts of the doc still describe the way it works before ARC was the standard, but it's still always interesting to read to understand those mecanisms.

What happens when a object is assigned to another object?

2 Answers2