Question

I have created a little test project to try to resolve a problem I am having in my main project. I’ve noticed that when retrieving an object from a container the reference count dosen’t increment.

I am confused why this is not the case?

For example this code will not increase the reference count of the hereDoggy object:

//Retrieve the dog, why does this not increment the reference count?
Dog* hereDoggy = [cont1 objectAtIndex:0];

Below is the full example:

-(void)doZombieProblem
{
    NSMutableArray* cont1 = [NSMutableArray array];
    NSMutableArray* cont2 = [NSMutableArray array];
    NSMutableArray* cont3 = nil;


    //Create the dog pointer
    Dog* doggy = [[Dog alloc] initWithName:@"Bernard"];

    //Add to container1
    [cont1 addObject:doggy];

    //Release the dog pointer
    [doggy release];

    while ([cont1 count] > 0)
    {
        //Retrieve the dog, why does this not increment the reference count?
        Dog* hereDoggy = [cont1 objectAtIndex:0];

       //Add it to cont2
        [cont2 addObject:hereDoggy];

        //Remove it from cont1.
        [cont1 removeObjectAtIndex:0];

        //No need to release as we haven't increased the reference count.
        //[hereDoggy release];
    }

    //I should be able to retrieve the dog here from cont2.
    Dog* bernard = [cont2 objectAtIndex:0];

    //No need to release as we haven't increased the reference count.
    //[bernard release];
}

Answer 1

It’s your responsibility as the user of the object to manage it’s retain count. This is because only you, the consumer, know when you are done with it. That’s why just calling [cont1 objectAtIndex:0] doesn’t increment it. NSArray has no clue what you have planned with the object it returns.

Think of retain count to indicate the number of things owning something. When it’s 0, no one owns it, so let it be garbage collected. If it’s 1, then only 1 thing needs it/owns it (and on up).

When you call [cont1 addObject:doggy] NSMutableArray will absolutely increment the retain count on it (behind the scenes), just like when you call [cont1 removeObjectAtIndex:0] NSMutableArray will decrement the retain count on it.

Now, if you need hereDoggy for any period of time, just call retain on it yourself, and then release where appropriate.