For example, string was returned from NSDictionary, which in turn read it from file system.
So, we don't know whether string was created through [NSString stringWith...] or [[NSString alloc] initWith...] or in any other way.
How do you generally know whether you are responsible for releasing returned object, if API doesn't mention it?
PS I've seen similar questions here, but mostly they deal with constants #"" or explicitly created strings.
No, you should only release instances that you've alloc'd, retain'd, copy'd or new'd.
See the Cocoa Memory Management Rules. If you can only afford to read a single part of the documentation, this is the one :) And there’s also the static analyzer (Build → Build and Analyze) that’s pretty good with the memory-management rules, it should be able to catch what you miss.
The Objective-C rule of thumb: if your code performs an alloc/copy/retain to initialize an object, then you need to release it as you are the owner. If you do not perform an alloc/copy/retain to initialize an object (such as using a method to get an instance), then you are not the owner and should not release the object.
In this event, you did not create the string, so you are not responsible for releasing it. You do not need to worry how the string was initialized when it was created and added to the NSDictionary (unless you are creating and adding it to the dictionary yourself). With most collection objects, they take ownership of the objects that are added to them and so they are responsible for releasing the objects in their care.
Members of a dictionary are accessed via -objectForKey:, so you are not responsible for releasing them. The dictionary was the only object in this example that you directly created. If you did so via an alloc, copy, or new method (e.g. [[NSDictionary alloc] initWithContentsOfFile:…]) then you must release it. If it came from somewhere else including one of the [NSDictionary dictionary…] factories, you own no references to it and do not need to release it.
Related
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 trying to learn/understand what happens and why when working with or creating various objects. (Hopefully to LEARN from the docs.)
I'm reading "Programming in Objective-C 2.0" (2nd edition, by Steven Kochan). On page 408, in the first paragraph is a discussion of retain counts:
Note that its reference count then goes to 2. The addObject: method does this automatically; if you check your documentation for the addObject: method, you will see this fact described there.
So I read the addObject: docs:
Inserts a given object at the end of the array.
There, the description is missing, while other items, like arrayByAddingObject:, state it:
Returns a new array that is a copy of the receiving array with a given object added to the end.
Where in the reference does it indicate that addObject: increases the retain count? Given the presence of ARC, I should still understand what these methods are doing to avoid bugs and issues. What does ARC bring to this? (Going to read that again...)
Great question, I'm glad to see someone actually reading the docs and trying to understand them!
Since you are looking for how to research answers using Apple's documentation more so than the actual answer itself, here is how I found the answer:
First I look at the class reference for addObject: which is a method of NSMutableArray and there is no mention of memory management.
Then I look at the Overview section at the top... Hmmm, still no luck.
Since the behavior might be inherited from a parent class, I look at the Inherits from section at the top of the class reference and see that NSArray is the most immediate parent. Let's check there:
Under the Overview There is one small section about retain's:
Special Considerations
In most cases your custom NSArray class should conform to Cocoa’s
object-ownership conventions. Thus you must send retain to each object
that you add to your collection and release to each object that you
remove from the collection. Of course, if the reason for subclassing
NSArray is to implement object-retention behavior different from the
norm (for example, a non-retaining array), then you can ignore this
requirement.
Okay, I'm still not happy... Where next? The parent class of NSArray is NSObject and I know that it won't be covered there in this case (from experience) so I won't bother checking that. (If the parent was another class or something that might be covered by NSObject, I would keep moving up the tree until I found something.)
The Companion Guides usually contains a lot of good information for these types of classes. Let's try the first one, Collections Programming Topics.
The first section (after Overview) is Accessing Indexes and Easily Enumerating Elements: Arrays. Sounds promising! Click on Relevant Chapters: “Arrays: Ordered Collections”
There it is under Array Fundamentals along with a link to even more information:
And when you add an object to an NSMutableArray object, the object
isn’t copied, (unless you pass YES as the argument to
initWithArray:copyItems:). Rather, an object is added directly to an
array. In a managed memory environment, an object receives a retain
message when it’s added; in a garbage collected environment, it is
strongly referenced. When an array is deallocated in a managed memory
environment, each element is sent a release message. For more
information on copying and memory management, see “Copying
Collections.”
The book must be referring to out of date documentation because you are correct it doesn't mention anything about the retain count. It does in fact retain the object though. The way you need to think of it is not in terms of retain counts (which are useless) but rather ownership. Especially so when using ARC.
When you add an object to an NSMutableArray, it is taking ownership of that object (in ARC terminology it has a strong reference to it).
"What does ARC bring to this?"
ARC does nothing different. All ARC does (besides some optimization) is add the same release, retain, and autorelease statements that you would add yourself without using ARC. All you need to care about is that once you add an object to the array, it will live at least as long as the array.
And the arrayByAddingObject: method creates a new NSArray (or NSMutableArray) containing the object you're passing, and keeps a strong reference to the passed object. The actual array object that it creates has no references yet unless you assign it to either an ivar, property, or local variable. What you assign it to determines it's lifespan.
Basically even without ARC, it's best to think of object life-cycles in terms of ownership, ARC just formalizes that. So because of that, when using the frameworks, it doesn't matter when retains happen or don't happen, you are only responsible for your objects until you pass ownership to another object and you can trust that the framework will keep the object alive as long as it needs it.
Now of course you have to intuit what constitutes ownership. For instance delegate properties are often assign, or in ARC unsafe_unretained or weak, to prevent circular retains cycles (where two objects each retain each other), though are sometimes retained/strong so you need to look into those on a case by case basis.
And also in cases like key value observing and NSNotification observing the object you are observing does not retain the observer.
But those are really exceptions to the rule. Generally you can assume a strong reference.
Regarding this sentence above: "The actual array object that it creates has no references yet unless you assign it to either an ivar, property, or local variable. What you assign it to determines it's lifespan." I'll try to explain:
When you run this piece of code: [someArray arrayByAddingObject:someObject]; you've instantiated a new NSArray or NSMutableArray object (depending on which object type someArray is) but you haven't actually assigned it to any reference. That means that if you're using ARC, it may be immediately released afterwards, or if not using ARC, it will be released when it's autoreleasepool is drained (probably on the next iteration of that thread's runloop).
Now if instead you did this: NSArray *someOtherArray = [someArray arrayByAddingObject:someObject]; you now have a reference to the newly created array, called someOtherArray. In this case, this is a local variable who's scope is only within whichever set of { } it resides (so it could be inside an if statement, a loop, or a method. Now if you do nothing else with it, it will die sometime after it's scope ends (it isn't guaranteed to die right away, but that isn't important, you just can't assume it lives longer).
Now if in your class you have an iVar (instance variable) declared in the header like NSArray *someOtherArray; (which is strong by default in ARC) and you run someOtherArray = [someArray arrayByAddingObject:someObject]; somewhere in your class, the object will live until you either remove the reference (someOtherArray = nil), you overwrite the reference (someOtherArray = someThirdArray), or the class is deallocated. If you were not using ARC, you would have to make sure to retain that to achieve the same effect (someOtherArray = [[someArray arrayByAddingObject:someObject] retain]; which is essentially what ARC is doing behind the scenes).
Or you may have a property declared instead like #property (nonatomic, strong) NSArray *someOtherArray in which self.someOtherArray = [someArray arrayByAddingObject:someObject]; would achieve the same effect but would use the proprety accessor (setSomeOtherArray:) or you could still use someOtherArray = [someArray arrayByAddingObject:someObject]; to set the iVar directly (assuming you #synthesized it).
Or assuming non-ARC, you might have declared the property like #property (nonatomic, retain) NSArray *someOtherArray in which self.someOtherArray = [someArray arrayByAddingObject:someObject]; would behave exactly as ARC would, but when setting the iVar directly you would still need to add that retain manually.
I hope that clears things up a bit, please let me know if there's anything I glossed over or left out.
As you mentioned in your comment, the key here is intuitively knowing when an object would be considered owned by another one or not. Luckily, the Cocoa frameworks follow a pretty strict set of conventions that allow you to make safe assumptions:
When setting an NSString property of a framework object (say the text property of a UILabel for example) it is always copied (if anyone knows of a counter-example, please comment or edit). So you don't have to worry about your string once you pass it. Strings are copied to prevent a mutable string from being changed after it's passed.
When setting any other property other than delegate, it's (almost?) always retained (or strong reference in ARC)
When setting delegate properties, it's (almost?) always an assign (or weak reference) to prevent circular retain cycles. (For instance, object a has a property b that is strong referenced and b has a strong referenced delegate property. You set a as the delegate for b. Now a and b are both strongly referencing each other, and neither object will ever reach a retain count of 0 and will never reach it's dealloc method to dealloc the other object. NSURLConnection is a counter-example that does strongly reference it's delegate, because it's delegate is set via a method -- see that convention below -- and it's convention to nil out or release an NSURLConnection after it completes rather than in dealloc, which will remove the circular retain)
When adding to an array or dictionary, it's always retained (or strong reference).
When calling a method and passing block(s), they are always copied to move them from the stack (where they are initially created for performance purposes) into the heap.
Methods that take in object parameters and don't return a result immediately are (always? I can't think of any that don't) either copying or retaining (strong referencing) the parameters that you pass to ensure that the method can do what it needs to with them. For instance, NSURLConnection even retains it's delegate because it's passed in via a method, whereas when setting the delegate property of other objects will not retain, as that is the convention.
It's suggested that you follow these same conventions in your own classes as well for consistency.
Also, don't forget that the headers of all classes are available to you, so you can easily see whether a property is retain or assign (or strong or weak). You can't check what methods do with their parameters, but there's no need because of the convention that parameters are owned by the receiver.
In general, you should look in the "most global" spot for information about anything in the Cocoa APIs. Since memory management is pervasive across the system APIs and the APIs are consistent in their implementation of the Cocoa memory management policy, you simply need to read and understand the Cocoa memory management guide.
Once understood, you can safely assume that all system APIs implement to that memory management policy unless explicitly documented otherwise.
Thus, for NSMutableArray's addObject: method, it would have to retain the object added to the array or else it would be in violation of that standard policy.
You'll see this throughout the documentation. This prevents every method's documentation from being a page or more long and it makes it obvious when the rare method or class implements something that is, for whatever reason (sometimes not so good), an exception to the rule.
In the "Basic Memory Management Rules" section of the memory management guide:
You can take ownership of an object using retain.
A received object is normally guaranteed to remain valid within the
method it was received in, and that method may also safely return the
object to its invoker. You use retain in two situations: (1) In the
implementation of an accessor method or an init method, to take
ownership of an object you want to store as a property value; and (2)
To prevent an object from being invalidated as a side-effect of some
other operation (as explained in “Avoid Causing Deallocation of
Objects You’re Using”).
(2) is the key; an NS{Mutable}Array must retain any added object(s) exactly because it needs to prevent the added object(s) from being invalidated due to some side-effect. To not do so would be divergent from the above rule and, thus, would be explicitly documented.
I am using a shared instance of a singleton class in a function, do I need to do a retain on the object? A few examples I have seen do this: AVAudioSession *session = [[ AVAudioSession sharedInstance] retain]; while a few simply do: AVAudioSession *session = [ AVAudioSession sharedInstance];. I am sure there is a rule somewhere which should tell me what to do but I just don't know it.
Simply treat it as if it were any other object. Sometimes a singleton class will override memory management methods to ensure that it doesn't get deallocated, but there is no way to know whether or not a shared object has this. With shared objects and singletons which don't, if the first gets deallocated a new one will be created as needed. Therefore, if you need to ensure that a pointer is valid in a future method, retain the shared object when you get it. If you don't need to keep it, you don't need to do anything as the sharedInstance method will autorelease it.
You're right, there are rules that tell you what to do. They're the same rules you use everywhere else in Cocoa Touch. Clients of a singleton shouldn't care that the object they're using is a singleton, and they definitely shouldn't rely on the singleton's single-ness to avoid the usual memory management conventions.
Yes, if you know that an object is a singleton and you know that it won't be deallocated, retaining it might seem redundant. On the other hand, retaining the singleton when you acquire it and releasing it when you're done better documents the programmer's intent and avoids the need to explain to anyone (including the future you) why it's okay not to retain this particular object. Most importantly, it prevents your code from depending on an implementation detail of some other part of your code.
you do'nt need to retain the sharedInstances because it's implemented as the singleton class.
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].