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).
How can I check if object isn't deallocated on Objective-C?
The following standard condition checks only if object is initialized:
NSObject *objectVariable = nil;
...
if (objectVariable) {...}
You can't check after-the-fact whether an object is already deallocated or not, because it is invalid to do anything with an object that is deallocated (once an object is deallocated, any pointer that used to point to it is now an invalid pointer, and dereferencing it is undefined behavior). And since you don't know whether it is deallocated or not, and it may be deallocated, you cannot meaningfully examine it in any way. And a deallocated object may well "look like" a valid object if you try to examine it (it is undefined behavior, so any behavior is possible).
If this is an object of your own class, the obvious thing would be to print something or perform some other indicator action in the class's -dealloc method. That will tell you when it is deallocated.
Even if this is an object that is not of your own class, you may be able to either 1) add -dealloc as a category method, if the class doesn't override -dealloc, or 2) swizzle the -dealloc method, if the class does override it. Then you can print something in your version.
You can also profile it in Instruments; the Allocations instrument can tell you how many objects of a given class have been allocated, and how many are alive. If you suspect that you are losing track of objects or there is a retain cycle, the Leaks instrument may be able to tell you that.
I'm in agreement with the comments, If you're doing memory management right, there should be no need for such a check. Nor am I aware of such a check, if the address gets populated with a new object, the check would pass but could still crash your app.
My suggestions are to:
Read up on manual memory management rules, pay special attention to how blocks affect memory management, alloc/init methods, when to use assign, etc. Memory management rules should become second nature to you. Start with this Apple guide.
Run static analysis on your app and fix any memory errors. Fix all the errors really, these are bugs in your app. (CMD+Shift+B or Product->Analyze in the menu)
Reproduce the crash in Instruments using zombies. Read the retain/release report to find out where the object may have been over-released. (CMD+I or Product->Profile. Select Zombies in the window that appears)
Consider converting to ARC. Converting to ARC doesn't get you completely off the hook for understanding ObjC memory management, but it will take a lot of the burden off of you.
thanks for viewing this post, it'd be great if you guys can help me out. I've been doing some objective-c and learned about the objective-c way of memory management, like making sure to call release whenever I own the object, when to call autorelease, etc. I also do not want to use ARC or the newly introduced GC because I like to manage my own memory, I plan to advance later on into iOS development, and I know it's a good practice to manage my own memory. But there's still one small detail that I seem to have hit a brick wall in. It has to do with sending objects the -retain message. I learned that sending the -retain message increments the reference count by 1. But would this be an appropriate time to send -retain? :
- (void) setName : (NSString* ) theName
{
// name is an instance variable of type NSString
[theName retain]; // Must release this
name = [theName copy]; // Must release this in dealloc
[theName release]; // decrement the reference count because of retain
}
Should I call retain here so that I own the argument temporarily and ensure it doesnt'
get released somehow before I get to use it?
Any help would be appreciated! Thanks!
No. You the object supplied as an argument to the method will generally be around until your method returns. You don't need the retain messages there. You copy the string here to keep it around after the method returns.
This is documented in Apple's Documentation on this page in the "Avoid Causing Deallocation of Objects You’re Using" Section. Specifically:
Cocoa’s ownership policy specifies that received objects should
typically remain valid throughout the scope of the calling method. It
should also be possible to return a received object from the current
scope without fear of it being released. It should not matter to your
application that the getter method of an object returns a cached
instance variable or a computed value. What matters is that the object
remains valid for the time you need it.
As an aside you really should consider using ARC. Its not good practise to manage your own memory. No matter how good one can be at managing their own memory the LLVM compiler is still better. Managing your own memory will lead to hard to troubleshoot issues caused only by yourself. It is an extra level of cognitive load that you really don't have to deal with and, when you finally let manual memory management go, you will breathe a sigh of relief at all the mental overhead you didn't even know was there.
Let's suppose I create a few objects and I add them to an array.
House *myCrib = [House house];
House *johnHome = [House house];
House *lisaHome = [House house];
House *whiteHouse = [House house];
NSArray *houses = [NSArray arrayWithObjects: myCrib, johnHome, lisaHome, whiteHouse, nil];
Normally, all House objects have a retain count of two, but they're being autoreleased once. After a while, I decide to release myCrib, even if I'm not the owner — I never retained or initialized.
[myCrib release];
The retain count should drop to zero and my object should be deallocated. My question now is: will this illegal action cause my app to work erroneously or even crash, or will NSArray simply delete my object from its list with bad consequences.
I'm looking for a way to maintain a list of objects, but I want the list to maintain itself. When some object disappears, I want the reference to it to disappear from my array gracefully and automatically. I'm thinking of subclassing or wrapping NSArray.
Thank you.
My question now is: will this illegal
action cause my app to work
erroneously or even crash, or will
NSArray simply delete my object from
its list with bad consequences.
Your array now has an invalid object pointer. There's no way to tell that the pointer is invalid just by looking at it, and the array isn't notified that the object has been deallocated. The problem isn't with the array, after all, the problem is with the code that improperly releases the object. So yes, the application will likely crash or otherwise behave incorrectly due to that bad pointer, and no, NSArray won't detect and deal with the problem for you.
I'm looking for a way to maintain a
list of objects, but I want the list
to maintain itself. When some object
disappears, I want the reference to it
to disappear from my array gracefully
and automatically.
If the objects in the list are all instances of a common class, you could define your own memory management methods that both retain/release the object and add/remove it from the list, or broadcast appropriate notifications in case there can be multiple lists. I suppose you could even override -retain and -release for this purpose, but I'd think long and hard about that before doing it, and document it well if you do; it's not the sort of thing that other developers would expect.
Another option might be Core Data. If you delete a managed object from the object graph, it'll disappear from any relationships. Strictly speaking, a to-many relationship is a set, not a list, but the difference may not be a concern for your purposes.
Update: I just noticed that you didn't tag your question ios. If you're working under MacOS X, you should definitely take a look at NSPointerArray. If you use garbage collection, NSPointerArray can be configured to use weak references and to replace references to collected objects with null references. This is exactly what you seem to be looking for.
You should not release myCrib if you are not the owner. To do so is a violation of the memory management guidelines and will make your code extremely difficult to maintain. I cannot stress enough that you absolutely should never do this under any sort of circumstance. You're asking for crashes; the array has declared ownership of the object, and you must not subvert that ownership in any way.
So the answer here is: your code is absolutely wrong and you should fix it. If you can't fix it, you should trash it and start over and keep rewriting it until you've come up with another way to achieve the same effect without subverting object ownership. I guarantee that it's possible.
If what you want is a weak-referencing array, then there are a couple ways you can do this (this was just asked a couple of days ago):
NSPointerArray - weakly references its pointers. When you use garbage collection, they're autozeroing (ie, the pointers get removed when the object is deallocated). Unfortunately, this is not available on iOS.
CFMutableArrayRef - you can specify a custom retain and release callback, or just not specify one at all. If you leave them out, the array will simply not retain the objects it contains. However, this does not automatically remove the pointer when the object is deallocated.
DDAutozeroingArray - an NSMutableArray subclass I wrote the other day to provide a weakly-referencing and auto-zeroing array that works on both Mac OS and iOS. However, I strongly encourage you to use this only as a last resort; There are probably much better ways of doing what you're looking for. https://github.com/davedelong/Demos
I'm looking for a way to maintain a
list of objects, but I want the list
to maintain itself. When some object
disappears, I want the reference to it
to disappear from my array gracefully
and automatically. I'm thinking of
subclassing or wrapping NSArray.
If I have understood right, what you want is an array of weak references. Then, you might be interested in reading this post.
You're asking for a crash here. Your NSArray will still have a reference to the object that now no longer exists -- and who knows what it will be pointing to after a while?
Subclassing NSArray might not be the answer either. It's a class cluster which, in short, means that it's harder to subclass than you might hope.
Not entirely sure how you'd implement this. Something like the element sending a notification when they're about to be deallocated which the array would then pick up. You'd need to be careful that you didn't leak or over-release your objects.
I created a wrapper class — in my code it's called a controller — which maintains the (mutable) array for me. I initialize the controller class in my view controllers — the place where I need them — instead of using an array directly.
No invalid code for me. :-p
I think there is something I'm missing about memory management. Here's the code:
NSString *string = #"foo";
[string release];
NSLog(#"%#", string);
I expect to get a memory error with that code, but instead the code is ran without errors. I noticed this as I was following a book and a scrollView was released before setting properties and adding a subView (but after being inserted in the main view).
My question is, when are objects really deallocated? Would this e considered good coding style?
This works because your string variable is pointing to a constant string that has been compiled into your application. retaining it does nothing and releaseing it does nothing as well. It exists in static memory, and it will only be destroyed when your program is unloaded from memory.
If you alloc, retain or copy an object, it is your responsibility to release it. Everything else is dealt with by the system and will be flushed with the auto release pool.
There are way too many memory management questions on SO already, have a quick look around to get yourself acquainted https://stackoverflow.com/search?q=memory+management+iphone
[edit]
Probably the most important part of your question that you need to understand is in your second last paragraph:
I was following a book and a scrollView was released before setting properties and adding a subView (but after being inserted in the main view).
I haven't seen this code but it is likely that you added the scrollView to your UIView instance. In these cases, the receiving view always retain its subview(s) so you are free to release it.
Once the UIView instance is released it wil also send a release message to all its subviews, which includes the scrollView.
Makes sense?
Sending a message to a deallocated object is undefined behavior. You might get a memory error, you might end up sending a message to another object, you might end up sending an message to a phantom version of the former object — it all depends on implementation details and details of the environment at runtime that are mostly out of your control. In this specific case, you're running into the implementation detail that NSString constants are immortal.
As for specifically when an object will be deallocated, that's also an implementation detail. If you don't own an object and don't have a reasonable guarantee that it will still be around (e.g. you just got it by doing [someArrayYouOwn objectAtIndex:0]), you should not be dealing with it.
The best policy: Just don't send messages to objects you no longer own. Follow the memory management rules.