When is manual memory management necessary on the iPhone?

Sometimes I see developers use:

ClassA *obj = [[ClassA alloc]...]autorelease];

Wh开发者_JS百科y does this sometimes autorelease the objects and sometimes not? Or is this an error?

Autorelease just means "this will be released at a later date". If you autorelease something three times, it will be released three times, later.

Instead of guessing, read through and understand the Memory Management Programming Guide before you do anything else. It will save you a ton of time and frustration.

You also asked, in the comments to another answer, why people use autorelease to begin with. Autorelease is necessary for a common case where you have to create a new object and return it. By the other rules of memory management, you would need to release the object at some point or it would be leaked, but if you release it before you return it, it will go away immediately.

There are two ways that you could deal with this: a) to have these methods return a new object which the caller is expected to release when it is done with the object, and b) to make sure that the object is released as soon as the caller is done with it.

The way Cocoa and Cocoa Touch handle this situation by convention is option b): to use autorelease pools, because you won't have to keep track of ownership except for the objects you explicitly create. Some people use autorelease for nearly everything instead of release, and this is a bit more inefficient than release, but more importantly it hides any useful distinctions you might make between release and autorelease when you're trying to read, navigate and understand the code.

As I said earlier, please read the Memory Management Programming Guide and all these questions will be answered. Better yet, if you just follow the simple rules listed in there, you won't have to think deep thoughts about every combination; there'll be a general rule to follow for your situation and the rule will work.

There's no error with autorelease - it just sets the object to be automatically released at a later time, which you don't necessarily have control over.

I try to always explicitly release my objects, unless I don't have a choice such as when returning a newly alloced/init-ed object from a method.

Actually when you create an object by using the following statement,

ClassA *obj = [[ClassA alloc]init ...];

Then you need to release that object, other wise it will be remain in that app and not useful by any other objects. Then the memory will be wasted.

So we have to release the object by [obj release];

in case some time we can't release at specific time. So we will put autorelease. If we do like that then the NSAutoReleasePool handle the release operation.

We can use any of the above.

Reagrds,

Satya.

No, it's not an error at all. Autorelease means that the object will be released at the end of the current Autorelease pool.

Look at your main.m class file (Every Cocoa/Cocoa Touch project gets it). You will see it has a method that looks a bit like this:

NSAutoreleasePool *pool = [[NSAutoreleasePool alloc]init];
UIApplicationMain(.....);
[pool release];

UIApplicationMain starts your app's run loop. When you reach the end of it, (The application is closed) anything in the autorelease pool is dumped.

Now, if you impliement multithreading and you create your own Autorelease pools within the pool, you'll release the object when that pool is released.

The documentation on the method is here: http://developer.apple.com/library/mac/documentation/Cocoa/Reference/Foundation/Protocols/NSObject_Protocol/Reference/NSObject.html#//apple_ref/occ/intfm/NSObject/autorelease