（iPhone）基本内存管理问题

Question

I have used following 2 patterns to create a view.

@property (retain, nonatomic) SomeView* someView;

...

// First pattern
self.someView = [[SomeView alloc] initWithFrame frame];

// Second pattern
SomeView* aSomeView = [[SomeView alloc] initWithFrame];
self.someView = aSomeView;
[aSomeView release];

Now, looking back at this code, the first pattern's method should be changed to

self.someView = [[[SomeView alloc] initWithFrame frame] autorelease];

shouldn't it?

I feel dumb :(

Answer 1

Look at it like this:

[[SomeView alloc] initWithFrame: frame];

The above line creates an object and gives it a retain count of 1.

self.someView = [[SomeView alloc] initWithFrame: frame];

This line leaves it with a retain count of two, because the someView property is declared with retain:

@property (**retain**, nonatomic) SomeView* someView;

So, doing it this way leaves your someView property pointing to an object with retain count of 2. You can do it this way if you add an autorelease call to it:

self.someView = [[[SomeView alloc] initWithFrame: frame] autorelease];

Your second pattern is better, if you ask me. You create an object with a retain count of one. You assign it to a retaining property (now it has a retain count of 2) and then you release the original variable, leaving the object again with a retain count of 1. It's three lines where you might want only one, but it makes sense in the right context. Additionally, it's usually best to avoid using autorelease outside of an alloc or copy method since its usually an indication you don't fully understand memory management in Obj-C.

And as a commenter said in the comments to the question, don't feel dumb. None of this is intuitive at first. Nobody picks up a guitar and plays like Hendrix their first time.

Answer 2

Yes, you are right. autorelease means "release a bit later".

Answer 3

Yes, I think you should change that. With self.someView = you are calling the setter which increases the retain count.

Answer 4

Now, looking back at this code, 1's method should be changed to self.someView = [[[SomeView alloc] initWithFrame frame] autorelease]; shouldn't it?

correct

a)

SomeView * view = [[SomeView alloc] initWithFrame:frame];
self.someView = view;
[view release], view = nil;

b)

self.someView = [[[SomeView alloc] initWithFrame:frame] autorelease];

many people prefer b, simply because it is less to type.

i prefer an approach similar to a because:

• defects (such as over-releasing) are often exposed near the call site, rather than when the pool is destroyed (this often means you have to load up Instruments in Zombie mode to locate the callsite)
• it performs better and minimizes memory usage (in general, but not much in this specific case)
• you have more opportunity to check for invalid states and results
• you have a chance to init/configure the view/object for its usage before adding it to self, which is usually preferred