I know this question looks like a dupe: I checked and it's not
In talking about NARC, the author of this blog says, "Personally, I like to immediately autorelease anything I NARC-ed, on the same line." This goes completely counter to all the examples I've seen on the Apple site and in books, where autorelease is only used when the object must be returned and cannot be released immediately. In general, the idea is that autorelease is memory intensive and can gum up your program (though it makes code cleaner). From Appress Beginning iPhone 3 Development:
these objects can have a detrimental
effect on your application’s memory
footprint. It is OK to use
autorelease, but try to use it only
when you really need to, not just to
save typing a line or two of code.
I am not asking if autorelease is worse than explicitly calling release (it is), but rather:
In most 'normal' situations on iPhone, just how bad is it to replace a later release with an earlier autorelease (in the same method)? Also, in what situations would it be absolutely prohibitive to do this?
My guess is that, compared to using a garbage collector (as MonoTouch apps do sucessfully), autorelease will hardly make a dent in your memory footprint, and that Vincent's advice it right on, and can make for cleaner code and less accidental memory-leaks.
There's nothing wrong with using autorelease, but when you allocate objects in a loop, you should always call release explicitly.
Using autorelease:
for (int i=0;i<1000;i++) {
NSString *s = [[[NSString alloc] init] autorelease];
}
// at this point, there are 1,000 unreleased string objects in memory
Using release:
for (int i=0;i<1000;i++) {
NSString *s = [[NSString alloc] init];
[s release];
}
// at this point, no string objects are "alive"
As you can see, you have to be really careful when using autorelease in loops.
You should be aware how autorelease works. Each thread in your application normally has a single autorelease pool. Objects can be registered in the pool. At the time they are registered, the pool determines the stackframe they belong to and will automatically pop the from the pool whenever that stackframe is left.
While this may seem costly (and it certainly is compared to direct retain/release), I don't think it even close to the cost a generation mark and sweep garbage collector can have.
Where autorelease really shines is in all situations where exceptions may be raised and there's no try/catch around. Autorelease is definitely preferable to a direct release in such cases.
There are, however, situations where you should avoid autorelease (the same goes for garabge collected environments where you should try to avoid these situations too). Creating temporary, autoreleased objects in a loop which runs a huge number of times is such a scenario, which puts significant stress on a garbage collector or the autorelease pool.
Replacing release with autorelease should be avoided in worker threads that are very simple and can live without the overhead of an autorelease pool. So the guideline is: Whenever you can avoid it, you should, whenever you're unsure autorelease.
Related
Which one of these is better practice?
A) retain and release the object myself later
NSMutableArray* array = [[self getArray] retain];
....
[array release];
B) autorelease from the function returning the object
getArray {
NSMutableArray* returnedArray = [[[NSMutableArray alloc] init] autorelease];
.....
return returnedArray;
}
The simplest rule of thumb when it comes to memory management in Objective-C is that you should release anything that you've explicitly allocated (alloc), copied (copy), newed up (new), or retained (retain).
The release should be done within the scope of the aforementioned actions. If you allocate space for an object that is returned by a method, you should autorelease it before returning it. So, given the two options you've provided, B is the recommended practice.
You could read and follow Apples guidelines on memory management and performance.
Personally I think the reasons for choosing one way over the other:
Using Autorelease pros:
You can't stuff it up, memory will be freed at some point. That I like to think of as "falling into the pit of success".
cons:
Using autorelease a lot may cause you memory problems as lots of objects build up awaiting be released by the autorelease pools.
Using retain/release pros:
More control when your memory is used/freed.
On ios apple recommends that you use release instead of autorelease whenever possible to keep the size of the pool small.
cons:
Like C/C++ malloc/free new/delete you have to be careful to keep them matched up and it is easy to stuff that up, causing memory leaks.
For member variables you have no choice, retain/release is it.
I think, whichever style you choose comes down to the situation your code is in and choosing the best style based on there pro's and con's. I don't think there is any one answer to this.
If you want to return an object you have to use the second approach. In all cases where possible you should use the retain-release approach because this uses less memory.
If you new, alloc init, retain, or copy (NARC) an object, you have to release it.
When the name of a method starts with any of those words, it means it's being created for the caller, who has the responsibility to release the object when he is done with it.
Otherwise the method returned is not owned by the caller, and he has to indicate he wants to keep it calling retain on the object.
If you can choose, don't abuse autorelease when memory is a concern.
Some comments:
The first rule logically results in the second. Example: if the outcome of a function (the returned object) survives the execution of the function, all the function can do is autorelease the object.
Unless there is a memory/performance concern, the overhead of automatic memory management is better than the memory leak chances plus the increased developing time.
Try to retain/release symmetrically. Example: if you retained on init, then release on dealloc. Same for viewDidLoad/viewDidUnload.
If you use #property(retain) you have to release in dealloc.
Example:
// created with autoreleased, just call retain if you intend to keep it
NSString *orange = [NSString stringWithString:#"orange"];
// created for the caller, you'll have to release it when you are done
NSObject *apple = [NSString initWithString:#"apple"];
Sometimes I see developers use:
ClassA *obj = [[ClassA alloc]...]autorelease];
Why 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
I have been looking through the questions asked on StackOverflow, but there are so many about memory management in Objective-C that I couldn't find the answer I was looking for.
The question is if it is ok (and recommnded) to call autorelease before adding a newly created object to a collection (like NSMutableArray)? Or should I release it explicitly after adding it. (I know NSMutableArray willl retain the object)
This illustrates my question:
Scenario A (autorelease):
- (void) add {
// array is an instance of NSMutableArray
MyClass *obj = [[[MyClass alloc] init] autorelease];
[array addObject:obj];
}
Scenario B (explicit release):
- (void) add {
// array is an instance of NSMutableArray
MyClass *obj = [[MyClass alloc] init];
[array addObject:obj];
[obj release];
}
I assume both are correct, but I am not sure, and I sure don't know what the preffered way is.
Can the Objective-C gurus shed some light on this?
IMHO, which way is 'right' is a matter of preference. I don't disagree with the responders who advocate not using autorelease, but my preference is to use autorelease unless there is an overwhelmingly compelling reason not to. I'll list my reasons and you can decide whether or not their appropriate to your style of programming.
As Chuck pointed out, there is a semi-urban legend that there's some kind of overhead to using autorelease pools. This could not be further from the truth, and this comes from countless hours spent using Shark.app to squeeze the last bit of performance out of code. Trying to optimize for this is deep in to "premature optimization" territory. If, and only if, Shark.app gives you hard data that this might be a problem should you even consider looking in to it.
As others pointed out, an autoreleased object is "released at some later point". This means they linger around, taking up memory, until that "later point" rolls around. For "most" cases, this is at the bottom of an event processing pass before the run loop sleeps until the next event (timer, user clicking something, etc).
Occasionally, though, you will need to get rid of those temporary objects sooner, rather than later. For example, you need to process a huge, multi-megabyte file, or tens of thousands of rows from a database. When this happens, you'll need to place a NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init]; at a well chosen point, followed by a [pool release]; at the bottom. This almost always happens in some kind of "loop batch processing", so it's usually at the start and bottom of some critical loop. Again, this should be evidence based, not hunch based. Instrument.app's ObjectAlloc is what you use to find these trouble spots.
The main reason why I prefer autorelease to release, though, is that it is much easier to write leak-free programs. In short, if you choose to go the release route, you need to guarantee that release is eventually sent to obj, under all circumstances. While this seems like it might be simple, it is actually surprisingly hard to do in practice. Take your example, for instance:
// array is an instance of NSMutableArray
MyClass *obj = [[MyClass alloc] init];
[array addObject:obj];
// Assume a few more lines of work....
[obj release];
Now imagine that for some reason, something, somewhere, subtly violates your assumption that array is mutable, maybe as the result of using some method to process the results, and the returned array containing the processed results was created as a NSArray. When you send addObject: to that immutable NSArray, an exception will be thrown, and you will never send obj its release message. Or maybe something goes wrong somewhere between when obj was allocd and the required call to release, like you check some condition and return() immediately by mistake because it slipped your mind that that call to release later on must take place.
You have just leaked an object. And probably signed yourself up to several days of trying to find out where and why it is your leaking it. From experience, you will spend many hours looking at that code above, convinced that it could not possibly be the source of the leak because you very clearly send obj a release. Then, after several days, you will experience what can only be described as a religious epiphany as you are enlightened to the cause of the problem.
Consider the autorelease case:
// array is an instance of NSMutableArray
MyClass *obj = [[[MyClass alloc] init] autorelease];
[array addObject:obj];
// Assume a few more lines of work....
Now, it no longer matters what happens because it's virtually impossible to leak obj accidentally, even under extremely unusual or exceptional corner cases.
Both are correct and will work as you're expecting them to.
I personally prefer to use the latter method, but only because I like to be explicit about when objects get released. By autoreleasing the object, all we're doing is saying "this object will get released at some arbitrary point in the future." That means you can put the autoreleased object into the array, destroy the array, and the object might (probably) still exist.
With the latter method, the object would get destroyed immediately with the array (providing that nothing else has come along and retained it in the meantime). If I'm in a memory-constrained environment (say, the iPhone) where I need to be careful about how much memory I'm using, I'll use the latter method just so I don't have so many objects lingering in an NSAutoreleasePool somewhere. If memory usage isn't a big concern for you (and it usually isn't for me, either), then either method is totally acceptable.
They are both correct but B may be preferred because it has no overhead at all. Autorelease causes the autorelease pool to take charge of the object. This has a very slight overhead which, of course, gets multiplied by the number of objects involved.
So with one object A and B are more or less the same but definitely don't use A in scenarios with lots of objects to add to the array.
In different situations autoreleasing may delay and accumulate the freeing of many objects at the end of the thread. This may be sub-optimal. Take care that anyway autoreleasing happens a lot without explicit intervention. For example many getters are implemented this way:
return [[myObject retain] autorelease];
so whenever you call the getter you add an object to the autorelease pool.
You can send the autorelease message at any point, because it isn't acted on until the application's message loop repeats (i.e. until all your methods have finished executing in response to user input).
http://macdevcenter.com/pub/a/mac/2001/07/27/cocoa.html?page=last&x-showcontent=text
You have alloc'ed the object, then it's your job to release it at some point. Both code snippets work merely the same, correct way, with the autorelease way being the potentionally slower counterpart.
Personally speaking, I prefer the autorelease way, since it's just easier to type and almost never is a bottleneck.
They're both OK. Some people will tell you to avoid autorelease because of "overhead" or some such thing, but the truth is, there is practically no overhead. Go ahead and benchmark it and try to find the "overhead." The only reason you'd avoid it is in a memory-starved situation like on the iPhone. On OS X, you have practically unlimited memory, so it isn't going to make much of a difference. Just use whichever is most convenient for you.
I prefer A (autoreleasing) for brevity and "safety", as johne calls it. It simplifies my code, and I've never run into problems with it.
That is, until today: I had a problem with autoreleasing a block before adding it to an array. See my stackoverflow question:
[myArray addObject:[[objcBlock copy] autorelease]] crashes on dealloc'ing the array (Update: Turns out the problem was elsewhere in my code, but still, there was a subtle difference in behavior with autorelease…)
I have a method that returns a NSMutableArray:
//implementation of class Students
-(NSMutableArray *)listOfStudents {
NSMutableArray *students = [[NSMutableArray alloc] init];
//add objects to students and other operations here
return students;
}
The problem here is, when and where do I release the object students? If it was an instance variable, I would add a [students release] in the dealloc method, but it's not. The class Students allocated the object, so it owns the new NSMutableArray and it must release it, right?
Since I can't release it before return it, the only option I see here is... return it as:
return [students autorelease];
But it doesn't feel right to use autorelease objects on the iPhone. This method will be called many times... and I would like to release the memory as soon as possible. Also, the autorelease pool is in the main function and it looks like it will take a while to clean the mess.
How would you do it?
I tend to avoid using autorelease wherever I can within my iPhone applications, but there are some cases where it doesn't hurt you that much. If the array that you're generating via this method will be retained for a reasonably long duration, then you won't be sacrificing any performance or memory usage by returning it as an autoreleased result.
However, if you will be using and discarding the returned object quite frequently, you may wish to do something different. I have a naming convention with my methods that if something is prefixed by "generate", like generateStudentsArray, it returns an object that is owned by the caller and must be manually released (like with copy). This helps to avoid using autoreleased objects, but it adds another thing to remember when doing memory management.
Additionally, because memory allocation / deallocation is costly (especially on the iPhone), you may wish to avoid the frequent allocations and deallocations within a method called quite a lot and instead recycle the mutable array by creating it ahead of time and passing it into the method, which only adds to the array's contents.
You're right, autorelease is the standard idiom to use in a situation like this.
UIKit actually creates an autorelease pool at the start of each event cycle and releases it at the end, so your autoreleased objects will get cleared up then; they won't be hanging around forever.
If you had a loop that was calling this method many times within a single event cycle, you might want to create your own autorelease pool inside that loop so that these objects get released at the end of each iteration, rather than building up loads of objects to be released at the end of the current event cycle.
But unless you're doing something like that, or you are encountering another specific out-of-memory situation, UIKit's standard autorelease pools should handle it fine.
I'm just beginning to have a look at Objective-C and Cocoa with a view to playing with the iPhone SDK. I'm reasonably comfortable with C's malloc and free concept, but Cocoa's references counting scheme has me rather confused. I'm told it's very elegant once you understand it, but I'm just not over the hump yet.
How do release, retain and autorelease work and what are the conventions about their use?
(Or failing that, what did you read which helped you get it?)
Let's start with retain and release; autorelease is really just a special case once you understand the basic concepts.
In Cocoa, each object keeps track of how many times it is being referenced (specifically, the NSObject base class implements this). By calling retain on an object, you are telling it that you want to up its reference count by one. By calling release, you tell the object you are letting go of it, and its reference count is decremented. If, after calling release, the reference count is now zero, then that object's memory is freed by the system.
The basic way this differs from malloc and free is that any given object doesn't need to worry about other parts of the system crashing because you've freed memory they were using. Assuming everyone is playing along and retaining/releasing according to the rules, when one piece of code retains and then releases the object, any other piece of code also referencing the object will be unaffected.
What can sometimes be confusing is knowing the circumstances under which you should call retain and release. My general rule of thumb is that if I want to hang on to an object for some length of time (if it's a member variable in a class, for instance), then I need to make sure the object's reference count knows about me. As described above, an object's reference count is incremented by calling retain. By convention, it is also incremented (set to 1, really) when the object is created with an "init" method. In either of these cases, it is my responsibility to call release on the object when I'm done with it. If I don't, there will be a memory leak.
Example of object creation:
NSString* s = [[NSString alloc] init]; // Ref count is 1
[s retain]; // Ref count is 2 - silly
// to do this after init
[s release]; // Ref count is back to 1
[s release]; // Ref count is 0, object is freed
Now for autorelease. Autorelease is used as a convenient (and sometimes necessary) way to tell the system to free this object up after a little while. From a plumbing perspective, when autorelease is called, the current thread's NSAutoreleasePool is alerted of the call. The NSAutoreleasePool now knows that once it gets an opportunity (after the current iteration of the event loop), it can call release on the object. From our perspective as programmers, it takes care of calling release for us, so we don't have to (and in fact, we shouldn't).
What's important to note is that (again, by convention) all object creation class methods return an autoreleased object. For example, in the following example, the variable "s" has a reference count of 1, but after the event loop completes, it will be destroyed.
NSString* s = [NSString stringWithString:#"Hello World"];
If you want to hang onto that string, you'd need to call retain explicitly, and then explicitly release it when you're done.
Consider the following (very contrived) bit of code, and you'll see a situation where autorelease is required:
- (NSString*)createHelloWorldString
{
NSString* s = [[NSString alloc] initWithString:#"Hello World"];
// Now what? We want to return s, but we've upped its reference count.
// The caller shouldn't be responsible for releasing it, since we're the
// ones that created it. If we call release, however, the reference
// count will hit zero and bad memory will be returned to the caller.
// The answer is to call autorelease before returning the string. By
// explicitly calling autorelease, we pass the responsibility for
// releasing the string on to the thread's NSAutoreleasePool, which will
// happen at some later time. The consequence is that the returned string
// will still be valid for the caller of this function.
return [s autorelease];
}
I realize all of this is a bit confusing - at some point, though, it will click. Here are a few references to get you going:
Apple's introduction to memory management.
Cocoa Programming for Mac OS X (4th Edition), by Aaron Hillegas - a very well written book with lots of great examples. It reads like a tutorial.
If you're truly diving in, you could head to Big Nerd Ranch. This is a training facility run by Aaron Hillegas - the author of the book mentioned above. I attended the Intro to Cocoa course there several years ago, and it was a great way to learn.
If you understand the process of retain/release then there are two golden rules that are "duh" obvious to established Cocoa programmers, but unfortunately are rarely spelled out this clearly for newcomers.
If a function which returns an object has alloc, create or copy in its name then the object is yours. You must call [object release] when you are finished with it. Or CFRelease(object), if it's a Core-Foundation object.
If it does NOT have one of these words in its name then the object belongs to someone else. You must call [object retain] if you wish to keep the object after the end of your function.
You would be well served to also follow this convention in functions you create yourself.
(Nitpickers: Yes, there are unfortunately a few API calls that are exceptions to these rules but they are rare).
If you're writing code for the desktop and you can target Mac OS X 10.5, you should at least look into using Objective-C garbage collection. It really will simplify most of your development — that's why Apple put all the effort into creating it in the first place, and making it perform well.
As for the memory management rules when not using GC:
If you create a new object using +alloc/+allocWithZone:, +new, -copy or -mutableCopy or if you -retain an object, you are taking ownership of it and must ensure it is sent -release.
If you receive an object in any other way, you are not the owner of it and should not ensure it is sent -release.
If you want to make sure an object is sent -release you can either send that yourself, or you can send the object -autorelease and the current autorelease pool will send it -release (once per received -autorelease) when the pool is drained.
Typically -autorelease is used as a way of ensuring that objects live for the length of the current event, but are cleaned up afterwards, as there is an autorelease pool that surrounds Cocoa's event processing. In Cocoa, it is far more common to return objects to a caller that are autoreleased than it is to return objets that the caller itself needs to release.
Objective-C uses Reference Counting, which means each Object has a reference count. When an object is created, it has a reference count of "1". Simply speaking, when an object is referred to (ie, stored somewhere), it gets "retained" which means its reference count is increased by one. When an object is no longer needed, it is "released" which means its reference count is decreased by one.
When an object's reference count is 0, the object is freed. This is basic reference counting.
For some languages, references are automatically increased and decreased, but objective-c is not one of those languages. Thus the programmer is responsible for retaining and releasing.
A typical way to write a method is:
id myVar = [someObject someMessage];
.... do something ....;
[myVar release];
return someValue;
The problem of needing to remember to release any acquired resources inside of code is both tedious and error-prone. Objective-C introduces another concept aimed at making this much easier: Autorelease Pools. Autorelease pools are special objects that are installed on each thread. They are a fairly simple class, if you look up NSAutoreleasePool.
When an object gets an "autorelease" message sent to it, the object will look for any autorelease pools sitting on the stack for this current thread. It will add the object to the list as an object to send a "release" message to at some point in the future, which is generally when the pool itself is released.
Taking the code above, you can rewrite it to be shorter and easier to read by saying:
id myVar = [[someObject someMessage] autorelease];
... do something ...;
return someValue;
Because the object is autoreleased, we no longer need to explicitly call "release" on it. This is because we know some autorelease pool will do it for us later.
Hopefully this helps. The Wikipedia article is pretty good about reference counting. More information about autorelease pools can be found here. Also note that if you are building for Mac OS X 10.5 and later, you can tell Xcode to build with garbage collection enabled, allowing you to completely ignore retain/release/autorelease.
Joshua (#6591) - The Garbage collection stuff in Mac OS X 10.5 seems pretty cool, but isn't available for the iPhone (or if you want your app to run on pre-10.5 versions of Mac OS X).
Also, if you're writing a library or something that might be reused, using the GC mode locks anyone using the code into also using the GC mode, so as I understand it, anyone trying to write widely reusable code tends to go for managing memory manually.
As ever, when people start trying to re-word the reference material they almost invariably get something wrong or provide an incomplete description.
Apple provides a complete description of Cocoa's memory management system in Memory Management Programming Guide for Cocoa, at the end of which there is a brief but accurate summary of the Memory Management Rules.
I'll not add to the specific of retain/release other than you might want to think about dropping $50 and getting the Hillegass book, but I would strongly suggest getting into using the Instruments tools very early in the development of your application (even your first one!). To do so, Run->Start with performance tools. I'd start with Leaks which is just one of many of the instruments available but will help to show you when you've forgot to release. It's quit daunting how much information you'll be presented with. But check out this tutorial to get up and going fast:
COCOA TUTORIAL: FIXING MEMORY LEAKS WITH INSTRUMENTS
Actually trying to force leaks might be a better way of, in turn, learning how to prevent them! Good luck ;)
Matt Dillard wrote:
return [[s autorelease] release];
Autorelease does not retain the object. Autorelease simply puts it in queue to be released later. You do not want to have a release statement there.
My usual collection of Cocoa memory management articles:
cocoa memory management
There's a free screencast available from the iDeveloperTV Network
Memory Management in Objective-C
NilObject's answer is a good start. Here's some supplemental info pertaining to manual memory management (required on the iPhone).
If you personally alloc/init an object, it comes with a reference count of 1. You are responsible for cleaning up after it when it's no longer needed, either by calling [foo release] or [foo autorelease]. release cleans it up right away, whereas autorelease adds the object to the autorelease pool, which will automatically release it at a later time.
autorelease is primarily for when you have a method that needs to return the object in question (so you can't manually release it, else you'll be returning a nil object) but you don't want to hold on to it, either.
If you acquire an object where you did not call alloc/init to get it -- for example:
foo = [NSString stringWithString:#"hello"];
but you want to hang on to this object, you need to call [foo retain]. Otherwise, it's possible it will get autoreleased and you'll be holding on to a nil reference (as it would in the above stringWithString example). When you no longer need it, call [foo release].
The answers above give clear restatements of what the documentation says; the problem most new people run into is the undocumented cases. For example:
Autorelease: docs say it will trigger a release "at some point in the future." WHEN?! Basically, you can count on the object being around until you exit your code back into the system event loop. The system MAY release the object any time after the current event cycle. (I think Matt said that, earlier.)
Static strings: NSString *foo = #"bar"; -- do you have to retain or release that? No. How about
-(void)getBar {
return #"bar";
}
...
NSString *foo = [self getBar]; // still no need to retain or release
The Creation Rule: If you created it, you own it, and are expected to release it.
In general, the way new Cocoa programmers get messed up is by not understanding which routines return an object with a retainCount > 0.
Here is a snippet from Very Simple Rules For Memory Management In Cocoa:
Retention Count rules
Within a given block, the use of -copy, -alloc and -retain should equal the use of -release and -autorelease.
Objects created using convenience constructors (e.g. NSString's stringWithString) are considered autoreleased.
Implement a -dealloc method to release the instancevariables you own
The 1st bullet says: if you called alloc (or new fooCopy), you need to call release on that object.
The 2nd bullet says: if you use a convenience constructor and you need the object to hang around (as with an image to be drawn later), you need to retain (and then later release) it.
The 3rd should be self-explanatory.
Lots of good information on cocoadev too:
MemoryManagement
RulesOfThumb
As several people mentioned already, Apple's Intro to Memory Management is by far the best place to start.
One useful link I haven't seen mentioned yet is Practical Memory Management. You'll find it in the middle of Apple's docs if you read through them, but it's worth direct linking. It's a brilliant executive summary of the memory management rules with examples and common mistakes (basically what other answers here are trying to explain, but not as well).