I have a method that returns a NSMutableArray:
//implementation of class Students
-(NSMutableArray *)listOfStudents {
NSMutableArray *students = [[NSMutableArray alloc] init];
//add objects to students and other operations here
return students;
}
The problem here is, when and where do I release the object students? If it was an instance variable, I would add a [students release] in the dealloc method, but it's not. The class Students allocated the object, so it owns the new NSMutableArray and it must release it, right?
Since I can't release it before return it, the only option I see here is... return it as:
return [students autorelease];
But it doesn't feel right to use autorelease objects on the iPhone. This method will be called many times... and I would like to release the memory as soon as possible. Also, the autorelease pool is in the main function and it looks like it will take a while to clean the mess.
How would you do it?
I tend to avoid using autorelease wherever I can within my iPhone applications, but there are some cases where it doesn't hurt you that much. If the array that you're generating via this method will be retained for a reasonably long duration, then you won't be sacrificing any performance or memory usage by returning it as an autoreleased result.
However, if you will be using and discarding the returned object quite frequently, you may wish to do something different. I have a naming convention with my methods that if something is prefixed by "generate", like generateStudentsArray, it returns an object that is owned by the caller and must be manually released (like with copy). This helps to avoid using autoreleased objects, but it adds another thing to remember when doing memory management.
Additionally, because memory allocation / deallocation is costly (especially on the iPhone), you may wish to avoid the frequent allocations and deallocations within a method called quite a lot and instead recycle the mutable array by creating it ahead of time and passing it into the method, which only adds to the array's contents.
You're right, autorelease is the standard idiom to use in a situation like this.
UIKit actually creates an autorelease pool at the start of each event cycle and releases it at the end, so your autoreleased objects will get cleared up then; they won't be hanging around forever.
If you had a loop that was calling this method many times within a single event cycle, you might want to create your own autorelease pool inside that loop so that these objects get released at the end of each iteration, rather than building up loads of objects to be released at the end of the current event cycle.
But unless you're doing something like that, or you are encountering another specific out-of-memory situation, UIKit's standard autorelease pools should handle it fine.
Related
Which one of these is better practice?
A) retain and release the object myself later
NSMutableArray* array = [[self getArray] retain];
....
[array release];
B) autorelease from the function returning the object
getArray {
NSMutableArray* returnedArray = [[[NSMutableArray alloc] init] autorelease];
.....
return returnedArray;
}
The simplest rule of thumb when it comes to memory management in Objective-C is that you should release anything that you've explicitly allocated (alloc), copied (copy), newed up (new), or retained (retain).
The release should be done within the scope of the aforementioned actions. If you allocate space for an object that is returned by a method, you should autorelease it before returning it. So, given the two options you've provided, B is the recommended practice.
You could read and follow Apples guidelines on memory management and performance.
Personally I think the reasons for choosing one way over the other:
Using Autorelease pros:
You can't stuff it up, memory will be freed at some point. That I like to think of as "falling into the pit of success".
cons:
Using autorelease a lot may cause you memory problems as lots of objects build up awaiting be released by the autorelease pools.
Using retain/release pros:
More control when your memory is used/freed.
On ios apple recommends that you use release instead of autorelease whenever possible to keep the size of the pool small.
cons:
Like C/C++ malloc/free new/delete you have to be careful to keep them matched up and it is easy to stuff that up, causing memory leaks.
For member variables you have no choice, retain/release is it.
I think, whichever style you choose comes down to the situation your code is in and choosing the best style based on there pro's and con's. I don't think there is any one answer to this.
If you want to return an object you have to use the second approach. In all cases where possible you should use the retain-release approach because this uses less memory.
If you new, alloc init, retain, or copy (NARC) an object, you have to release it.
When the name of a method starts with any of those words, it means it's being created for the caller, who has the responsibility to release the object when he is done with it.
Otherwise the method returned is not owned by the caller, and he has to indicate he wants to keep it calling retain on the object.
If you can choose, don't abuse autorelease when memory is a concern.
Some comments:
The first rule logically results in the second. Example: if the outcome of a function (the returned object) survives the execution of the function, all the function can do is autorelease the object.
Unless there is a memory/performance concern, the overhead of automatic memory management is better than the memory leak chances plus the increased developing time.
Try to retain/release symmetrically. Example: if you retained on init, then release on dealloc. Same for viewDidLoad/viewDidUnload.
If you use #property(retain) you have to release in dealloc.
Example:
// created with autoreleased, just call retain if you intend to keep it
NSString *orange = [NSString stringWithString:#"orange"];
// created for the caller, you'll have to release it when you are done
NSObject *apple = [NSString initWithString:#"apple"];
I'm looking at someone's code and I know the general rule is if you have alloc/init, you need to release that memory. He uses a lot of NSMutableArrays, alloc/inits them, but does not release them. Can I simply send the autorelease message to the array that gets created if I do not see any other release/autorelease message getting sent to that array? I basically don't want to get his code to crash and stop working either :P.
With NSMutableArrays, when you send the message addObject and the object in that array increases its retain account, if that array gets released, but the object never gets sent a release or removeObject from the array, is that also a memory leak? Thanks.
You need to either -release or -autorelease anything you -retain, +alloc, -copy, +allocWithZone: or -copyWithZone:. (And, if you retain something twice you also need to release it twice.)
When an NSMutableArray (or NSArray, NSSet, or NSDictionary and mutable subclasses) object is dealloc'd (retain count reaches zero), it releases anything it contains. When you add an object to an NSMutableArray, the array retains the object (it does not copy it like some people claim).
I highly recommend the Memory Management Programming Guide to both you and the someone you referred to in the question.
I hope this answer helps you and someone. Good luck. :)
Also, enable the Clang Static Analyser in the build settings. This will tell you at compile time when a leak is going to happen (and much, much more). In fact, it's the first thing I always do when I start a new project. The analyzer never lied to me.
NSArray and NSMutableArray release all of their objects when they are destroyed, so if the array is managed properly with retain and release (or autorelease) the objects within will not be leaked. But if there are arrays that are never released, they will leak, along with everything inside them.
Without seeing the code, it's hard to advocate for just adding autoreleases everywhere, but for arrays that are used only in the context of a single function (and not assigned to ivars or static variables), the answer is yes, they should be autoreleased. Or more idiomatically, create them with methods like +arrayWithCapacity, which returns an object that has already been added to the autorelease pool.
It is general practice to release all the objects you initialize. But in your case, releasing the array should release all objects as well.
But it all depends on how you are using your objects !!!
From reading the memory management docs in the SDK, I gathered that factory methods (static constructor methods) would typically be retaining the object for me and adding it to the autorelease pool?
This would mean I have no need to retain & release an object as long as the pool does not get released before I expect? (Which should be at the end of the app, for the default autorelease pool in main()? )
This question: Cocoa Touch Question. Should [NSMutableArray array] be retained? seems to agree with this.
However when I have used the NSMutableArray arrayWithCapacity: method, I've found I have to retain the array or all heck breaks loose.
I'm sure this is all just total n00b-ness, or a bizarre bug elsewhere in the code, but if someone could explain how exactly I may have misunderstood, I'd be much obliged.
Thanks!
Update: Thanks for the answers. I think I've got it now!
From reading the memory management docs in the SDK, I gathered that factory methods (static constructor methods) would typically be retaining the object for me and adding it to the autorelease pool?
Yes, you don't have to worry about releasing objects returned from any method, except the alloc, init, new, and copy methods.
This would mean I have no need to retain & release an object as long as the pool does not get released before I expect?
Yes.
(Which should be at the end of the app, for the default autorelease pool in main()? )
No. You can only count on the object being around until you return from whichever method or function you're in. Typically, autorelease pools are flushed when control returns back to your run loop.
If you want an object instance to survive beyond the current method you must take ownership of it, by calling 'retain'. You are then also responsible for 'releasing' the instance when you no longer need it.
In your case, if you want your NSMutableArray to stick around, you need to retain it. Better yet, use [[NSMutableArray alloc] initWithCapacity: ];
See Practical Memory Management
Usually auto-release pools are drained at the end of the current event loop. A pretty good rule of thumb is that unless you're returning an autoreleased object, if you want that object to remain outside of the current method, you should retain it.
So, when you create your NSMutableArray as an autoreleased object, once your method ends, all bets are off, and that autorelease pool could drain at any time. Retain it, and then release it in your dealloc.
Checkout this diagram below explaining the Application lifecycle. Once the application has launched, your application will be handling various events which could be user generated such as a touch, or a network activity, or one of many events. Once the event has been handled, control returns to the Event Loop that you see below.
This event loop maintains the top level autorelease pool. In pseudocode, this event loop would look something like:
while(wait for event) {
1) create autorelease pool
2) handle event
3) release pool
}
So, whenever a event has been handled, control goes back to the main event loop, and this outer pool will be released. You should retain all objects that were created using convenience methods and will be needed even after the event handling finishes. With the extra retain count, you take ownership of the object and are responsible for releasing it when no longer needed.
iPhone Application http://developer.apple.com/iphone/library/documentation/iPhone/Conceptual/iPhoneOSProgrammingGuide/Art/app_life_cycle.jpg
As soon as you add your object to the array, there is an implicit call to retain for this object, which means once added to the array you can release it from the source, and when you'll call the remove method from the array there will also be an implicit call to release.
Currently I am jumping into the ice cold water called "memory management in iPhone OS".
Here's one rule i've learned:
Every time I see an alloc in my method, I will release that corresponding variable at the bottom of the method.
Every time I create an #property(...) in my header file which says copy or retain, I put a release message on that variable into the dealloc method.
Every time I have an IBOutlet, I do the same thing. Only exception: If the IBOutlet has something like #property(... assign), or in other words: If it has the assign keyword at all. Then I don't care about releasing it in the dealloc method.
I feel that there are many more good rules to know! Just write down what you have. Let's scrape them all together. Links to great descriptions are welcome, too.
Actually, any time you initialize an object and the method name includes "init" you are responsible for releasing it. If you create an object using a Class method that does not include the word "init" then you don't.
For example:
NSString *person = [NSString stringWithFormat:"My name is %#", name];
does not need a release. But:
Person *person = [[Person alloc] init];
needs a release (as you stated in your question). Likewise:
Person *person = [[Person alloc] initWithName:#"Matt"]];
also needs a release.
This is a convention, not a rule of the language, but you will find that it is true for all Apple-supplied APIs.
The rules I use
Release all objects you create using a method whose name begins "alloc" or "new" or contains "copy".
Release all objects you retain.
Do not release objects created using a +className convenience constructor. (The class creates it and is responsible for releasing it.)
Do not release objects you receive in other ways E.g.
mySprockets = [widget sprockets];
If you store an object you receive in an instance variable, retain it or copy it. (Unless it's a weak reference - just a pointer to another object, usually to avoid cyclical references.)
Received objects are valid within the method they are received in (generally) and are also valid if passed back to the invoker.
Some good links:
http://www.gehacktes.net/2009/02/iphone-programming-part-2-objective-c-memory-management/
http://mauvilasoftware.com/iphone_software_development/2008/01/iphone-memory-management-a-bri.html
Memory management can seem daunting when you're seeing segfaults spring from every seeming innocent line of code, but it's actually pretty easy once you get the hang of it. Spend a little time reading this page and then Apple's documentation, and you should be writing bug-free code in no time.
I tend to create only autoreleased objects, either by using a class method or by autoreleasing it immediately after creation, unless I can state a reason not to. For example:
I am assigning it to a member variable because I intend to hold onto it for a while.
I am only creating it to pass it on immediately to another method, and I send it a release message right after that method call.
For performance reasons, I need to free that memory before the nearest NSAutoreleasePool will be released, such as creating a large number of objects inside a loop or the objects are holding onto a large amount of data (e.g., images).
That way, I am less likely to leak objects. By default, I create them autoreleased, and when I make the deliberate decision not to autorelease them, I am immediately faced with the question of where they will be released.
For object properties, rather than releasing them in my dealloc method, I like to assign nil to them. That way, retained or copied properties are sent a release, while assigned properties are simply overwritten, and I don't have to update my dealloc method if I change the property to/from retained.
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).