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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).
I have an error that I can't understand, that is happening while I want to release all objects in an NSMutableDictionary.
It's happening for a custom object called body and the output is :
-[__NSTaggedDate body]: unrecognized selector sent to instance 0xffffffffffffffff
I found very poor informations about it on the Internet.
That's a private class of Apple. Errors like this usually occur when you mess up your memory management.
Why are you trying to release all objects in a dictionary? When you add an object to a dictionary (or an array), the dictionary will retain it (take ownership). And when you remove the object from the dictionary it will be released, you don't have to do that.
Did you already consider using ARC? It makes memory management a lot easier. You don't have to worry about retaining and releasing objects anymore.
It's an internal undocumented cocoa class. But you are not concerned with it as it's not really what's happening, it's a red herring that is probably happening for reasons that are complex to explain and irrelevant here.
Look at the reported address: 0xffffffffffffffff. That's a value that makes no sense. You should have got a segmentation fault, if it was not for that red herring.
You are for some reason sending the message body to an invalid pointer (maybe some corrupted data somewhere?).
Don't know this class, but it is probably a private class (my bet would be that it is a internal representation for NSDate objects that use the "tagged pointers" trick, but I'm just guessing).
Anyway your crash is happening not on an object called body, but when calling a method called body. And the crash is probably due to bad memory managment in your code that generates memory corruption
You should activate Zombies when running your app in debug to help you track over-released objects
You normally don't have to retain and release objects of an NSDictionary yourself, as container classes like NSArray and NSDictionary retain the objects they hold, and release them when the object is removed from them. So I don't see why you "want to release all objects in an NSMutableDictionary" : you only need to call removeAllObjects on that NSDictionary and you're done, no need to call release on the objects by yourself (neither do you need to call retain on the objects when adding them in the dictionary)
Whenever you try to set one data type to another data type like "if you directly assign an date component to text to an UIlabel" in this case I ll occurs
[toDateLabel setText:[tempArr lastObject]]; // Cause
[toDateLabel setText:[NSString stringWithFormat:#"%#",[tempArr lastObject]]]; // Solution
I'm still trying to understand this piece of code that I found in a project I'm working on where the guy that created it left the company before I could ask.
This is the code:
-(void)releaseMySelf{
for (int i=myRetainCount; i>1; i--) {
[self release];
}
[self autorelease];
}
As far as I know, in Objective-C memory management model, the first rule is that the object that allocates another object, is also responsible to release it in the future. That's the reason I don't understand the meaning of this code. Is there is any meaning?
The author is trying to work around not understand memory management. He assumes that an object has a retain count that is increased by each retain and so tries to decrease it by calling that number of releases. Probably he has not implemented the "is also responsible to release it in the future." part of your understanding.
However see many answers here e.g. here and here and here.
Read Apple's memory management concepts.
The first link includes a quote from Apple
The retainCount method does not account for any pending autorelease
messages sent to the receiver.
Important: This method is typically of no value in debugging memory
management issues. Because any number of framework objects may have
retained an object in order to hold references to it, while at the
same time autorelease pools may be holding any number of deferred
releases on an object, it is very unlikely that you can get useful
information from this method. To understand the fundamental rules of
memory management that you must abide by, read “Memory Management
Rules”. To diagnose memory management problems, use a suitable tool:
The LLVM/Clang Static analyzer can typically find memory management
problems even before you run your program. The Object Alloc instrument
in the Instruments application (see Instruments User Guide) can track
object allocation and destruction. Shark (see Shark User Guide) also
profiles memory allocations (amongst numerous other aspects of your
program).
Since all answers seem to misread myRetainCount as [self retainCount], let me offer a reason why this code could have been written: It could be that this code is somehow spawning threads or otherwise having clients register with it, and that myRetainCount is effectively the number of those clients, kept separately from the actual OS retain count. However, each of the clients might get its own ObjC-style retain as well.
So this function might be called in a case where a request is aborted, and could just dispose of all the clients at once, and afterwards perform all the releases. It's not a good design, but if that's how the code works, (and you didn't leave out an int myRetainCount = [self retainCount], or overrides of retain/release) at least it's not necessarily buggy.
It is, however, very likely a bad distribution of responsibilities or a kludgey and hackneyed attempt at avoiding retain circles without really improving anything.
This is a dirty hack to force a memory release: if the rest of your program is written correctly, you never need to do anything like this. Normally, your retains and releases are in balance, so you never need to look at the retain count. What this piece of code says is "I don't know who retained me and forgot to release, I just want my memory to get released; I don't care that the others references would be dangling from now on". This is not going to compile with ARC (oddly enough, switching to ARC may just fix the error the author was trying to work around).
The meaning of the code is to force the object to deallocate right now, no matter what the future consequences may be. (And there will be consequences!)
The code is fatally flawed because it doesn't account for the fact that someone else actually "owns" that object. In other words, something "alloced" that object, and any number of other things may have "retained" that object (maybe a data structure like NSArray, maybe an autorelease pool, maybe some code on the stackframe that just does a "retain"); all those things share ownership in this object. If the object commits suicide (which is what releaseMySelf does), these "owners" suddenly point to bad memory, and this will lead to unexpected behavior.
Hopefully code written like this will just crash. Perhaps the original author avoided these crashes by leaking memory elsewhere.
How would you write a unit test—using OCUnit, for instance—to ensure that objects are being released/retained properly in Cocoa/Objective-C?
A naïve way to do this would be to check the value of retainCount, but of course you should never use retainCount. Can you simply check whether an object's reference is assigned a value of nil to indicate that it has been released? Also, what guarantees do you have about the timing at which objects are actually deallocated?
I'm hoping for a concise solution of only a few lines of code, as I will probably use this extensively. There may actually be two answers: one that uses the autorelease pool, and another that does not.
To clarify, I'm not looking for a way to comprehensively test every object that I create. It's impossible to unit test any behavior comprehensively, let alone memory management. At the very least, though, it would be nice to check the behavior of released objects for regression testing (and ensure that the same memory-related bug doesn't happen twice).
About the Answers
I accepted BJ Homer's answer because I found it to be the easiest, most concise way of accomplishing what I had in mind, given the caveat that the weak pointers provided with Automatic Reference Counting aren't available in production versions of XCode (prior to 4.2?) as of July 23rd, 2011. I was also impressed to learn that
ARC can be enabled on a per-file basis; it does not require that your
entire project use it. You could compile your unit tests with ARC and
leave your main project on manual retain-release, and this test would
still work.
That being said, for a far more detailed exploration of the potential issues involved with unit testing memory management in Objective-C, I highly recommend Peter Hosey's in-depth response.
Can you simply check whether an object's reference is assigned a value of nil to indicate that it has been released?
No, because sending a release message to an object and assigning nil to a variable are two different and unrelated things.
The closest you can get is that assigning anything to a strong/retaining or copying property, which translates to an accessor message, causes the previous value of the property to be released (which is done by the setter). Even so, watching the value of the property—using KVO, say—does not mean you will know when the object is released; most especially, when the owning object is deallocated, you will not get a notification when it sends release directly to the owned object. You will also get a warning message in your console (because the owning object died while you were observing it), and you do not want noisy warning messages from a unit test. Plus, you would have to specifically observe every property of every object to pull this off—miss one, and you may be missing a bug.
A release message to an object has no effect on any variables that point to that object. Neither does deallocation.
This changes slightly under ARC: Weak-referencing variables will be automatically assigned nil when the referenced object goes away. That doesn't help you much, though, because strongly-referencing variables, by definition, will not: If there's a strong reference to the object, the object won't (well, shouldn't) go away, because the strong reference will (should) keep it alive. An object dying before it should is one of the problems you're looking for, not something you'll want to use as a tool.
You could theoretically create a weak reference to every object you create, but you would have to refer to every object specifically, creating a variable for it manually in your code. As you can imagine, a tremendous pain and certain to miss objects.
Also, what guarantees do you have about the timing at which objects are actually released?
An object is released by sending it a release message, so the object is released when it receives that message.
Perhaps you meant “deallocated”. Releasing merely brings it closer to that point; an object can be released many times and still have a long life ahead of it if each release merely balanced out a previous retain.
An object is deallocated when it is released for the last time. This happens immediately. The infamous retainCount doesn't even go down to 0, as many a clever person who tried to write while ([obj retainCount] > 0) [obj release]; has found out.
There may actually be two answers: one that uses the autorelease pool, and another that does not.
A solution that uses the autorelease pool only works for objects that are autoreleased; by definition, objects not autoreleased do not go into the pool. It is entirely valid, and occasionally desirable, to never autorelease certain objects (particularly those you create many thousands of). Moreover, you can't look into the pool to see what's in it and what's not, or attempt to poke each object to see if it's dead.
How would you write a unit test—using OCUnit, for instance—to ensure that objects are being released/retained properly in Cocoa/Objective-C?
The best you could do is to set NSZombieEnabled to YES in setUp and restore its previous value in tearDown. This will catch over-releases/under-retains, but not leaks of any kind.
Even if you could write a unit test that thoroughly tests memory management, it would still be imperfect because it can only test the testable code—model objects and maybe certain controllers. You could still have leaks and crashes in your application caused by view code, nib-borne references and certain options (“Release When Closed” comes to mind), and so on.
There's no out-of-application test you can write that will ensure that your application is memory-bug-free.
That said, a test like you're imagining, if it were self-contained and automatic, would be pretty cool, even if it couldn't test everything. So I hope that I'm wrong and there is a way.
If you can use the newly-introduced Automatic Reference Counting (not yet available in production versions of Xcode, but documented here), then you could use weak pointers to test whether anything was over-retained.
- (void)testMemory {
__weak id testingPointer = nil;
id someObject = // some object with a 'foo' property
#autoreleasepool {
// Point the weak pointer to the thing we expect to be dealloc'd
// when we're done.
id theFoo = [someObject theFoo];
testingPointer = theFoo;
[someObject setTheFoo:somethingElse];
// At this point, we still have a reference to 'theFoo',
// so 'testingPointer' is still valid. We need to nil it out.
STAssertNotNil(testingPointer, #"This will never happen, since we're still holding it.")
theFoo = nil;
}
// Now the last strong reference to 'theFoo' should be gone, so 'testingPointer' will revert to nil
STAssertNil(testingPointer, #"Something didn't release %# when it should have", testingPointer);
}
Note that this works under ARC because of this change to the language semantics:
A retainable object pointer is either a null pointer or a pointer to a valid object.
Thus, the act of setting a pointer to nil is guaranteed to release the object it points to, and there's no way (under ARC) to release an object without removing a pointer to it.
One thing to note is that ARC can be enabled on a per-file basis; it does not require that your entire project use it. You could compile your unit tests with ARC and leave your main project on manual retain-release, and this test would still work.
The above does not detect over-releasing, but that's fairly easy to catch with NSZombieEnabled anyway.
If ARC is simply not an option, you may be able to do something similar with Mike Ash's MAZeroingWeakRef. I haven't used it much, but it seems to provide similar functionality to __weak pointers in a backwards-compatible way.
this is possibly not what you're looking for, but as a thought experiment I wondered if this might do something close to what you want: what if you created a mechanism to track the retain/release behavior for particular objects you wanted to test. Work it something like this:
create an override of NSObject dealloc
create a CFMutableSetRef and set up a custom retain/release functions to do nothing
make a unit test routine like registerForRRTracking: (id) object
make a unit test routine like clearRRTrackingReportingLeaks: (BOOL) report that will report any object in the set at that point in time.
call [tracker clearRRTrackignReportingLeaks: NO]; at the start of your unit test
call the register method in your unit test for every object you want to track and it'll be removed automatically on dealloc.
At the end of your test call the [tracker clearRRTrackingReportingLeaks: YES]; and it'll list all the objects that were not disposed of properly.
you could override NSObject alloc as well and just track everything but I imagine your set would get overly large (!!!).
Even better would be to put the CFMutableSetRef in a separate process and thus not have it impact your program runtime memory footprint overly much. Adds the complexity and runtime hit of inter-process communication though. Could use a private heap ( or zone - do those still exist?) to isolate it to a lesser degree.
Plenty of Objective-C classes return objects. Statements like [[instanceOfNSWhatever objectForKey:aKey] stringValue], for instance, appear all over my (and hopefully everyone else's code).
How am I supposed to memory manage these "intermediate" objects?
Were they just created or did they always exist?
Can I retain them, and if I release the object that created them, will they be released as well?
Are they autoreleased?
What if I run [instanceOfNSWhatever stringValue] a million times in a loop? Can I dispose of all those NSStrings as needed?
I'm still learning ObjC, and while I've been good at balancing my retain counts overall, I'm definitely lacking some understanding about how these methods work. Can anyone fill me in?
You've probably already read this section of Apple's docs about memory management, but I'll point you to the section about the Object Ownership Policy just in case. You are only responsible for managing the memory of objects you "own". To quote the docs:
You own any object you create.
You "create" an object using a method whose name begins with “alloc” or “new” or contains “copy” (for example, alloc, newObject, or mutableCopy).
If you own an object, you are responsible for relinquishing ownership when you have finished with it. [ie: release]
If you do not own an object, you must not release it.
The "Simple Examples" section of those docs provide good elaboration, but to put the points above in the context of your specific questions:
How am I supposed to memory manage these "intermediate" objects?
The good news is: you don't. Ignore the the memory management aspect of the "intermediate" objects in your example.
Were they just created or did they always exist?
They may have always existed, or they may have just been created. The beauty of objective-c is that you, as a consumer of those objects, don't have to care.
Can I retain them, and if I release the object that created them, will they be released as well?
You don't need to retain them if you're just passing them on to some other function, or using them as intermediate values yourself in your own calculations within the function. Say, for example, that you're returning the stringValue from your example function to someone else. There's no point in retaining it just to return it.
If you DO happen to retain it, then yes, you are responsible for issuing a corresponding release message as some point. You might, for example, retain the stringValue from your example if you want to hold on to that value as a property in your own instance. Objective-C uses reference counting. If you need that object to stick around for a long time, you must retain it so that someone else's release message doesn't cause it to vanish if the retain count falls to 0.
Are they autoreleased?
Depends. Let's say you ask for a string from instanceOfNSWhatever. If instanceOfNSWhatever has to create that string just special for you (in order to service your request), but doesn't otherwise care about that string, then yes... instanceOfNSWhatever probably put that string into the autorelease pool. If the string was already a property of instanceOfNSWhatever and it was just sending it to you in response to your request, then no, it probably wasn't autoreleased.
Again, the beauty is: you don't know and don't need to care. Since instanceOfNSWhatever created the string, it is responsible for managing it. You can ignore the memory management unless you add to the string by sending it a retain message.
What if I run [instanceOfNSWhatever stringValue] a million times in a loop? Can I dispose of all those NSStrings as needed?
No need. Again... stringValue isn't yours to manage because you didn't create it. As a technical note, if instanceOfNSWhatever really did have to create 1 million copies of stringValue to service your 1 million calls, it probably put them all in an autorelease pool, which would be drained at the end of the current cocoa event loop. Fortunately, unless you send each of those stringValue objects a retain message, you can gleefully ignore the memory management question here.
You basically manage all your memory according to the Memory Management Programming Guide for Cocoa. In short, however, you basically only need to worry about objects that you "own". You own an object if you create it (in Cocoa, you create an object by specifically allocating it using alloc or copying it using copy or one of their derivatives). If you own an object, you are responsible for releasing it when you are finished with it.
Any other object is, therefore, not owned by you. If you need to use such an object for any extended period (for example, outside the scope in which you received it), you need to specifically take ownership of the object by either sending it a retain message or copying it.
To answer your last question, if you are creating a lot of temporary objects in a loop or some other way, you can create your own autorelease pools. See the documentation for NSAutoreleasePool for more information about using them. Please note, however, that you should really only do this after you've profiled your application and found that it is using too much memory and would benefit from this kind of optimization.
Finally, if you are creating and releasing a lot of heavy objects and don't want to rely on autorelease pools, you can specifically allocate and initialize them and then make sure to release them on your own as soon as you're finished with them. Most objects that have convenience creators have similar initializers for creating the object specifically.
When working on the iPhone/iPod Touch, the autorelease objects are released when your application exits. This may be what you don't want. Especially when working with images or large chunks of data. To insure large pools of memory that are tagged autorelease get released sooner, create local autorelease pools. Like this:
NSAutoreleasePool *localPool = [[NSAutoreleasePool alloc] init];
-- do something that creates large autorelease memory blocks --
[localPool drain];
If you don't do this, you will find your application exiting unexpectedly.
I'll tell you a simple rules I wish I'd known when I first started Objective-C :)
If an method contains the words "alloc" or "copy" then you must [release] the object when finished.
If a method does not contain these words, you must [retain] it for it to remain valid outside of your function.
If you call [retain] you must later call [release] when finished.
This covers practically all you need to know about the basics of memory management.
ObjC makes heavy use of what are known as "auto release" pools. Objects returned from non alloc/copy functions are placed into these pools and automatically freed after your function exists.
That is why you do not need to release the results of something like [obj stringValue].