I am getting a bit confused. I am creating an app with storyboard, and running it on iPad 1. the application uses a lot of memory, so reached the 120mb, crashes. accordingly to what I have understood to remove this problem you need to release, dealloc... the point is that with ARC this should be automatic. In fact if I add for e.g.: [label1 release]; it gives me an error. But this ARC automatic release and dealloc does not seem to work! Is this because there are different ways to release with ARC??
You don't need to manually retain/release/autorelease with ARC. However if you have active references to a lot of unused objects they will still remain in memory. Profile your app with Instruments and it will show you how many objects you're creating of each class and how much memory they're consuming.
With ARC you still need to think about memory usage you just don't need to worry as much about memory leaks.
NSObject *bigMemObj = [[BigMemClass alloc] init];
//This creates the object in memory. In both arc and manual counting the retain count is 1
//Do stuff
//Prior to ARC you would have had to call [bigMemObj release]; before setting the variable to nil
bigMemObj = nil
//With ARC you don't have to do anything. The compiler inserts the release at compile time
Also read the documentation on declaring iVars __strong vs __weak.
Without looking at your code it's hard to identify what is consuming all the memory but hopefully that should help you determine where to start looking.
You should implement #autoreleasePool{} inside each method. In essence, each method will look like the following:
-(void)methodName{
#autoreleasePool{
//do method stuff
}
}
This will ensure that, upon exiting the autoreleasePool, memory is properly released.
I can't vote this back up, otherwise I would. I think Alessandro is asking about ARC vs using release and dealloc, not about what he's loading!
So, Alessandro, your understanding is correct that with ARC you don't release or dealloc. Therefore, those won't work if you're using ARC. Also, there is no alternative to release/dealloc, since ARC doesn't use it.
My suggestion would be to look at what you're using in the app that is taking up all this memory. Do you have a large number of pictures, for example, that are very large? Keep any graphics as small as possible, matching the resolution of the iPad. Especially the iPad 1, which doesn't have the "retina display".
You can use Autorelease pools in ARC. Here is some documentation on when to use them:
NSAutoreleasePool Class Reference
Advanced Memory Management Programming Guide: About Memory Management
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).
Coming from the world of managed memory, wondering what would be the proper way to clean up objects when using ARC.
For example: if declaring an instance variable in C#, .NET will allow the GC to pick it up once it leaves scope (method/loop body, etc)
What's the proper way to clean-up in Objective-C? Just set the reference/pointer to nil or call dealloc or will ARC detect that no external references are pointing to the instance once execution leaves scope and do the job for you?
ARC means "Automatic Reference Counting" and is just a way to let the compiler add the calls to retain/release/autorelease for you. It's not the same as GC but in most cases, you can consider that objects lifetime is automatically managed for you, like in GC.
If you want more information, you should read LLVM document on ARC
Last note: never call dealloc yourself. dealloc is the object's finalizer which is called once the ObjC runtime determines that the object reference count has reached 0. This method is only meant to be overriden by subclasses. In ARC mode, you generally don't need to do that, except if your object references non-object ivars that need to be finalized once the object itself is finalized.
will ARC detect that no external references are pointing to the
instance once execution leaves scope and do the job for you
Basically, yes, that's exactly what ARC will do. You don't need to clean up objects when you're using ARC; in fact, you can't (it stops you from trying to perform manual memory management).
You might want to consult the relevant discussion in my book:
http://www.apeth.com/iOSBook/ch12.html#_memory_management
It explains what's really happening behind the scenes (how memory is actually managed) and then goes on to describe how ARC shields you from most of it.
Note that (as I explain in the URL referenced above) it mostly isn't done by anything like garbage collection: it's done by inserting invisible explicit memory management throughout your code.
Well, in the past, iOS programmers were responsible for telling the system when they were done using an object that they allocated by sending the object a release message. That was done in accordance with a memory management system known as manual reference counting. As of Xcode 4.2, programmers no longer have to worry about this and can rely on the system to take care of releasing memory as necessary. This is done through a mechanism known as Automatic Reference Counting, or ARC for short. ARC is enabled by default when you compile new applications using Xcode 4.2 or later.
You can also disable ARC, in your Xcode interface, go to your main project (not main.h) your actual Xcode project, and select it, you should see a window in Xcode that displays the settings for your project, there will be one that says 'Objective-C Automatic Reference Counting' and it will be set to 'Yes', deactivate it (to 'No') and you shouldn't worry about the ARC, if you come from the world of data management and memory as you said, but keep in mind that it would be easier to you to keep updated to the iOS new features system, that are easier to the programmer to program, it just makes our life easier.
And now, the 'proper way to clean-up in Xcode' with ARC is with 'alloc' and 'init'.
With ARC in Xcode you do not need to worry for 'cleaning' that's the job of Xcode now, you just need to:
1) Create a variable.
2) Allocate.
3) Initialize.
That's it.
An example here:
int main (int argc, char * argv[])
{
#autoreleasepool {
Variable *myVariable;
// Create an instance of a Variable and initialize it
myVariable = [Variable alloc];
myVariable = [myVariable init];
// Set variable to 4/20
[myVariable setNumerator: 4];
[myVariable setDenominator: 20];
// Display the variable using the print method
NSLog (#"The value of myVariable is:");
[myVariable print];
}
return 0;
}
Just allocate and then initialize, yo do not need to do any thing else.
Keep in mind getters and setters.
I'm pretty new to Objective-C, as you may gather, and until recently, I hadn't really understood the need for all this AutoRelease malarky. I think that's mostly because I've started Objective-C with ARC, and haven't had any exposure to doing retains and release.
Anyway, my understanding now is that pre-ARC, if you created an object and needed to return a pointer to it as the returning object of the method/function, you would need to autorelease it, because you are unable to do the "[obj release]" after doing "return obj;"
Worrying about retains and releases isn't an issue with ARC. Does this mean that in our own code, there is really point in creating our own autoreleased objects? Ie, doing [[[Class alloc] init] autorelease]? From what I've gathered, we should still setup autorelease pools, but only because other frameworks or libraries may still return autoreleased objects, but we no longer need to explicitly create autoreleased objects ourselves - is this a fair understanding?
Thanks,
Nick
When using ARC, you do not want to do any memory management yourself. Specifically you will not be calling release and auto release because it does it all for you. In fact, the compiler should probably complain if you try to manage memory yourself.
Instead of [[[Class alloc] init] autorelease]; you'll just call [[Class alloc] init];
I recommend reading this blog post for some really good background on ARC and memory management in general.
Well, your understanding is quite correct. With ARC we do not release or autorelease any more. Just have to make sure that we assign nil (or some other reasonable value) to any reference to objects, which we do not need any more. In the worst case we could still constantly consume additional memory but the memory cannot leak any ore.
And yes, we still maintain autorelease pools for the sake of using framework libraries (linked ones) that may not use ARC.
To answer your question between the lines about the purpose of autorelease. This applies to non-ARC project only, of course.
In the good old days Objective-C did not offer any reference counting but its retain counting. Any allocated memory of objects, that are not retained (or have a retain count of 0) is considered free and may soon be claimed and used by other objects.
This means that every object needs to be retained after its allocation, assuming that you want to keep it around. When the object is not used any more then you need to release it. This comes with two risks. Well, alloc does retain it once automatically.
1) You may forget to release an object that is unused. In the worst case you may even loose all references to an object that stays in memory for ever since.
2) You may still refer to an object hat has been released already and then try accessing it which will most likely end in an BAD_EXC exception.
All this can be quite annoying. In order to get rid of some of these obligations for objects that don't stay around very long, the autorelease was invented. For temporary objects only you alloc it (release-count = 1) and autorelease it. That means that the object will be automatically released (retain count reduced by 1) within the next autorelease circle. But the object remains allocated for your method while it is being executed. Typically the reference variable would be a local one.
Sample:
-(void) myMethod{
AClass *someObject = [[[AClass alloc] init] autorelease];
// use the object
// probably hand it to another object if that takes ownership, i.e. add it ot an Array using addObject:
// don't care any more
}
And that not required any more when using ARC.
I'm following the Stanford iOS development lectures and I have a Calculator brain class which has been alloc init in a controller but I haven't released it in the dealloc.
- (CalculatorBrain *)brain
{
if (!brain)
brain = [[CalculatorBrain alloc] init];
return brain;
}
I ran from XCode -> Run with Performance Tool and the app started and no leaks appeared, I then clicked the home button in the iOS Simulator and nothing, I then double clicked the home button and closed the app and still nothing.
I also did Build & Analyse and it didnt spot anything
Could you let me know why its not picking it up?
It appears as if there is no detectable leak. Look at this line:
brain = [[CalculatorBrain alloc] init];
As long as brain points to an object, that object won't be considered a "memory leak". If at some point you do this,
brain = nil;
Then the leak will register. Deallocating the container object will also achieve this, but are you sure it's being deallocated? (It won't be deallocated when your program exits, for example.)
The problem: Leak detectors cannot detect all memory leaks -- this is a mathematically proven fact. Most detectors only detect unreachable objects, and many leak detectors are especially susceptible to false negatives -- in C it is hard to tell the difference at runtime between a pointer and an integer.
Edit: It sounds like your application only ever creates one instance of the controller, which only creates one instance of CalculatorBrain. If you think about what a memory leak really is, you can define it as unused memory that your program does not release back to the operating system.
While the program is running, CalculatorBrain is always in use and therefore it is not a leak.
When the program exits, the operating system automatically reclaims all memory used by your process, therefore there cannot be any memory leaks after a program exits.
If you want to create a leak to see what it looks like, you could create a new CalculatorBrain multiple times while your program is running, but forget to release the unused versions. In this scenario, as your program runs, more and more instances of CalculatorBrain would accumulate. On iOS and other embedded systems, this will generally crash your program. On a modern 64 bit computer it will gradually fill up the available swap space until you run out of swap, address space, or some other resource -- causing the program to crash or making the system very unresponsive.
Standard practice is to not care about deallocating objects which are supposed to exist for the entire program's run.
The analyzer cannot find all memory leaks. As far as it is concerned, storing the instance into the ivar doesn't leak it from that method, and then in dealloc it doesn't realize that the ivar should be released. XCode 4 may have improved in this respect, I don't recall (I still use XCode 3 myself).
As for the performance tool, remember that an object won't be considered leaked until nothing holds a reference to it anymore. So even though your controller doesn't deallocate the brain, the brain won't be considered leaked until the controller is deallocated (or receives a brain transplant). Also, note that on *nix-like systems, memory allocations are automatically cleaned up on process exit. So it isn't really a leak if you allocate memory for objects that should exist for the lifetime of your process (e.g. the app delegate and anything it permanently holds on to) and rely on this behavior to free it on process exit.
Well... it's true that leaks can't detect all memory leaks, but let's say that you are doing this:
myIvarBrain=[self brain];
If you are giving it to an iVar (released in the dealloc of your class, and without accessors), actually there is no leak at all. The returned RC is one and it will be one since the deallocation of your class. If you don't release it in the dealloc, you should wait a dealloc of your class to see a memory leak.
Does make sense?
I'm thinking of writing an app for the iphone. But I'm confused in regards to the objective-c language. Am I correct in saying that I have to do my own memory management? Is Objective-C a managed language such as C# and/or Java?
Thank you!
"Managed" is a marketing term that has no technical meaning. Objective C for the iPhone is not garbage collected, but uses retain counting memory management. So in some sense, you have to do your own memory management, but it's limited to knowing the Cocoa retain counting methodology.
Edit: My comments on "managed" refer to a previous post version. The comment is left here because I'm tired of seeing the word.
There is no garbage collection feature. However, any object you copy, retain, alloc etc. has its retain count bumped up by one, and you are responsible for calling a releaseor an autorelease - you own an instance of that object. If you don't call [<YOUROBJECT> release]; or [<YOUROBJECT> autorelease];, the object remains in the iPhone's/iPod Touch's/iPad's memory, but the pointer no longer remains, and a memory leak occurs, as that bit of memory can no longer be accessed. Autoreleasing adds the object to the autorelease pool, and means that it will become released at some point in the future. Always use a standard release where possible, as autorelease means that it will still stay in the device's memory for a while, you can never be sure when it will be released, and it is a slightly more demanding method to call.
Never call dealloc on an object - releasing or autoreleasing is sufficient, and if required, the object will automatically be dealloced.
Make sure you never release objects you do not own, and if you do release objects, you release them after you no longer have any use of them - otherwise, you may try to access that object, and the device cannot find the object, and the situation results in your app crashing due to an EXEC_BAC_ACCESS error - to find out the root of the problem, open Instruments, add the Object Allocation instrument and enable NSZombie detection in its preferences (or add the Zombies instrument (only available in iPhone SDK 3.2+)). You can then view the entire history of the object which causes the problem, and jump to the exact line of code that caused the problem.
You can read more here.
Hope this helped
As others have pointed out Objective-C has no garbage collector on the iPhone, but it does have one for Mac OSX. Here is an article describing this in more detail: http://vasudevkamath.blogspot.com/2010/01/objective-c-my-opinions.html
You have to do your own memory management with Objective-C on the iPhone.
The answer isn't specific to Objective-C. If you're doing iPhone development, no...there is no garbage collector. If you're developing for Mac OSX, however, there is garbage collection.