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).
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.
I am just starting climbing the Objective C learning curve (using Nerd Ranch iOS programming book).
Based on what I have know from other languages about "nesting" multiple executions within one line I assumed that I can alter:
NSString* descriptionString = [[NSString alloc] initWithFormat:#"%#", possesionName]
with a two line version:
NSString* descriptionString = [NSString alloc];
[descriptionString initWithFormat:#"%#", possesionName]
but it seems that the second attempt raises an exception
2012-01-22 18:25:09.753 RandomPossessions[4183:707] *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '*** -length only defined for abstract class. Define -[NSPlaceholderString length]!'
Could someone help me understand what exactly I am doing wrong here? Thanks a lot in advance.
PS. If this is a way Objective C messages work and you have to make alloc and init in one line just let me know - I assumed this is just a set of functions that either can be executed two in one go or one after another.
An important difference between both versions (they are not exactly equal) is that in the first version you use the result of initWithFormat for the variable descriptionString, while you use the result of alloc in the second. If you change your code to
NSString* descriptionString = [NSString alloc];
descriptionString = [descriptionString initWithFormat:#"%#", possesionName]
all should be well again. It is specified that an object returned by alloc shall not be seen as initialized and functional until some init Method has been called and init might return something else.
The alloc method will allocate memory for a new object. But the init method might throw away that memory and return a completely different object. Or it might return nil. This is why you must always do self = [super init] when you override an init method.
NSString is one class that does this kind of thing all the time.
I'm not exactly sure why the exception is happening, but I believe it could be ARC injecting code in between your two lines of code or something similar. Whatever it is, something is trying to act on the allocated object that has never been initialised, and this is a huge problem that can lead to all kinds of issues. Consider yourself lucky it threw an exception, sometimes it wont.
The NSString class might not actually be a real class. It may contain almost no methods and almost no variables. All it has is a bunch of factory methods to create "real" string objects of some other class, and this is done using methods like initWithFormat:. So, by long standing convention alloc/init must always be done in a single statement and there are a handful of places where, usually for performance reasons, something will rely on this convention being used.
Basically, objective-c is a language where you don't need to know exactly what is going on inside an object. You just need to know what messages can be sent to an object, and how it will respond. Anything else is undefined behaviour and even if you learn how it works, it is subject to change without notice. Sometimes the behaviour will change depending on circumstances that are completely illogical, for example you might expect the "copy" method to give you a copy of the object you send it to, and while this is the default behaviour, there are many cases where it will actually just return the same object with slightly different memory management flags. This is because the internal logic of the class knows that returning the same object is much faster and effectively identical to returning an actual copy.
My understanding is copy sent to NSString may return a new object, or it may return itself. It depends on which NSString subclass is actually being used, and there isn't even any documentation for what subclasses exist, let alone how they're implemented. All you need to know, is that copy will return a pointer to an object that is perfectly safe to treat as if it was a copy even though it might not be.
In a "proper" object oriented language like Objective-C, objects are "black boxes" which can intelligently change their internal behaviour at any time for any reason, but their external behaviour always remains the same.
With regard to avoiding nesting... The coding style for Objective-C often does require extensive nesting, or else you'll be writing 10 lines of code when only 1 is really needed. The square brace syntax is particularly suited to nesting without making your code messy.
As a rule of thumb, I turn on Xcode's "Page Guide at column" feature, and set it to 120 characters. If the line of code exceeds that width then I'll think about breaking it into multiple lines. But often it's cleaner to have a really long line than three short lines.
Be pragmatic about it. :)
From Apple's library reference, initWithFormat:
Returns an NSString object initialized by converting given data into Unicode characters using a given encoding.
So you can use these two lines of code:
NSString* descriptionString = [NSString alloc];
descriptionString = [descriptionString initWithFormat:#"%#", possesionName];
For more info please go to:
https://developer.apple.com/library/mac/documentation/Cocoa/Reference/Foundation/Classes/NSString_Class/Reference/NSString.html#//apple_ref/occ/instm/NSString/initWithFormat:
If I release mainPath in following example the program gives an error (because I’m releasing an object with zero counter)
NSString *mainPath = [NSString stringWithFormat:#"%#/Documents/downloadFile.plist",NSHomeDirectory()];
NSLog(#"address is = %#",mainPath);
[mainPath release]; //Program failed here
But the following code works fine.
NSString *aa=#"hiiiii";
[aa release];
Can anyone explain this?
Actually I’m not clear about the pointer concept (give a suitable link to clear it)
Constant NSStrings are a special case. They are allocated statically at compile time and can't be deallocated. You can send release to a constant string as many times as you like, it'll never get deallocated. This is achieved in the current implementation by setting the retain count to INT_MAX which is treated as a special value meaning "don't decrement me on release".
You should read the Cocoa Memory Management Guide or at least the Objective-C Tutorial by Scott Stevenson. (Really. Do it, you’ll save a lot of time in the long run.) The difference is that the first string is autoreleased, you do not own it and should not release it. The second string is special, I think it’s not allocated on the heap at all and the release is essentially a no-op here.
String with format is a convenience method that autoreleases the string so in this case you are likely to be sending a release message to an already deallocated object.
In your second example, you are creating the string statically so retain counts don't apply.
You don't need to release your objects in either of those cases.
As a rule of thumb, if you didn't use init (or initWithFoo:) to create the object, and didn't deliberately use retain to retain the object (plus couple of other rarer cases), you don't need to use release.
I am having trouble understanding why NSLog reports "dog" when the code is run. I understand about retain counts and dealloc e.t.c. What simple thing am i missing ?
NSString *newFoo = #"dog";
[newFoo release];
NSLog(newFoo);
[#"String Literal" release];
is a noop;
NSString *literal = #"String Literal";
[literal release];
is also a noop. This is only the case for string literals; and you should never expect this behaviour anywhere else. (This is to say; even though you tell the object to release, it just doesn't.)
I believe it's because #"dog" is effectively treated as a constant by the compiler. It creates some subclass of NSString (which is a class cluster) which persists for the lifetime of the application.
Just discovered this question for the definitive answer, which is essentially the same as mine.
A String Literal is a special case.. http://thaesofereode.info/clocFAQ/
But in general assuming you are not using garbage collection just stick to the few simple rules..
http://developer.apple.com/mac/library/documentation/Cocoa/Conceptual/MemoryMgmt/MemoryMgmt.html
more pertinently.. retain an object when you need it to stick around - balance it with a release when done. If you didn't create or retain it, you don't need to release it.
Sending -release to an object isn't like freeing the memory, it only signals that you are done with it. It may well still be around or it may not. You have no way of knowing, you don't need to know, and you shouldn't try to find out if it is used elsewhere, if clever apple code has decided to cache it, if it's a singleton, etc.
It may well still be valid but when you have sent release you have effectively said "I'm done" and not "reclaim this memory".
You are de-allocating the chunk of memory the pointer's pointing at, but the data is still there.
Releasing it doesn't automatically zeroes out that part of the memory.
So you can still read the data out just fine but it's just might be snapped (allocated) by something else down the line... but just not yet.
I'm not an ObjC coder by trade but as since its compatible with C that's I'm guessing from my C experience.
Although the object is probably released (not sure how Obj-C handles strings internally, some languages basically cache strings), you have not writted anything new in the memory of newFoo after you released it. The memory where the string was stored keeps its content, but could be overwritten at any time.
newFoo still points to the same adress in memory, but that memory could become anything else at any moment.
The way to really be sure that you don't point to potentially dirt memory is to set newFoo to nil after you released it.