I'm just beginning to have a look at Objective-C and Cocoa with a view to playing with the iPhone SDK. I'm reasonably comfortable with C's malloc and free concept, but Cocoa's references counting scheme has me rather confused. I'm told it's very elegant once you understand it, but I'm just not over the hump yet.
How do release, retain and autorelease work and what are the conventions about their use?
(Or failing that, what did you read which helped you get it?)
Let's start with retain and release; autorelease is really just a special case once you understand the basic concepts.
In Cocoa, each object keeps track of how many times it is being referenced (specifically, the NSObject base class implements this). By calling retain on an object, you are telling it that you want to up its reference count by one. By calling release, you tell the object you are letting go of it, and its reference count is decremented. If, after calling release, the reference count is now zero, then that object's memory is freed by the system.
The basic way this differs from malloc and free is that any given object doesn't need to worry about other parts of the system crashing because you've freed memory they were using. Assuming everyone is playing along and retaining/releasing according to the rules, when one piece of code retains and then releases the object, any other piece of code also referencing the object will be unaffected.
What can sometimes be confusing is knowing the circumstances under which you should call retain and release. My general rule of thumb is that if I want to hang on to an object for some length of time (if it's a member variable in a class, for instance), then I need to make sure the object's reference count knows about me. As described above, an object's reference count is incremented by calling retain. By convention, it is also incremented (set to 1, really) when the object is created with an "init" method. In either of these cases, it is my responsibility to call release on the object when I'm done with it. If I don't, there will be a memory leak.
Example of object creation:
NSString* s = [[NSString alloc] init]; // Ref count is 1
[s retain]; // Ref count is 2 - silly
// to do this after init
[s release]; // Ref count is back to 1
[s release]; // Ref count is 0, object is freed
Now for autorelease. Autorelease is used as a convenient (and sometimes necessary) way to tell the system to free this object up after a little while. From a plumbing perspective, when autorelease is called, the current thread's NSAutoreleasePool is alerted of the call. The NSAutoreleasePool now knows that once it gets an opportunity (after the current iteration of the event loop), it can call release on the object. From our perspective as programmers, it takes care of calling release for us, so we don't have to (and in fact, we shouldn't).
What's important to note is that (again, by convention) all object creation class methods return an autoreleased object. For example, in the following example, the variable "s" has a reference count of 1, but after the event loop completes, it will be destroyed.
NSString* s = [NSString stringWithString:#"Hello World"];
If you want to hang onto that string, you'd need to call retain explicitly, and then explicitly release it when you're done.
Consider the following (very contrived) bit of code, and you'll see a situation where autorelease is required:
- (NSString*)createHelloWorldString
{
NSString* s = [[NSString alloc] initWithString:#"Hello World"];
// Now what? We want to return s, but we've upped its reference count.
// The caller shouldn't be responsible for releasing it, since we're the
// ones that created it. If we call release, however, the reference
// count will hit zero and bad memory will be returned to the caller.
// The answer is to call autorelease before returning the string. By
// explicitly calling autorelease, we pass the responsibility for
// releasing the string on to the thread's NSAutoreleasePool, which will
// happen at some later time. The consequence is that the returned string
// will still be valid for the caller of this function.
return [s autorelease];
}
I realize all of this is a bit confusing - at some point, though, it will click. Here are a few references to get you going:
Apple's introduction to memory management.
Cocoa Programming for Mac OS X (4th Edition), by Aaron Hillegas - a very well written book with lots of great examples. It reads like a tutorial.
If you're truly diving in, you could head to Big Nerd Ranch. This is a training facility run by Aaron Hillegas - the author of the book mentioned above. I attended the Intro to Cocoa course there several years ago, and it was a great way to learn.
If you understand the process of retain/release then there are two golden rules that are "duh" obvious to established Cocoa programmers, but unfortunately are rarely spelled out this clearly for newcomers.
If a function which returns an object has alloc, create or copy in its name then the object is yours. You must call [object release] when you are finished with it. Or CFRelease(object), if it's a Core-Foundation object.
If it does NOT have one of these words in its name then the object belongs to someone else. You must call [object retain] if you wish to keep the object after the end of your function.
You would be well served to also follow this convention in functions you create yourself.
(Nitpickers: Yes, there are unfortunately a few API calls that are exceptions to these rules but they are rare).
If you're writing code for the desktop and you can target Mac OS X 10.5, you should at least look into using Objective-C garbage collection. It really will simplify most of your development — that's why Apple put all the effort into creating it in the first place, and making it perform well.
As for the memory management rules when not using GC:
If you create a new object using +alloc/+allocWithZone:, +new, -copy or -mutableCopy or if you -retain an object, you are taking ownership of it and must ensure it is sent -release.
If you receive an object in any other way, you are not the owner of it and should not ensure it is sent -release.
If you want to make sure an object is sent -release you can either send that yourself, or you can send the object -autorelease and the current autorelease pool will send it -release (once per received -autorelease) when the pool is drained.
Typically -autorelease is used as a way of ensuring that objects live for the length of the current event, but are cleaned up afterwards, as there is an autorelease pool that surrounds Cocoa's event processing. In Cocoa, it is far more common to return objects to a caller that are autoreleased than it is to return objets that the caller itself needs to release.
Objective-C uses Reference Counting, which means each Object has a reference count. When an object is created, it has a reference count of "1". Simply speaking, when an object is referred to (ie, stored somewhere), it gets "retained" which means its reference count is increased by one. When an object is no longer needed, it is "released" which means its reference count is decreased by one.
When an object's reference count is 0, the object is freed. This is basic reference counting.
For some languages, references are automatically increased and decreased, but objective-c is not one of those languages. Thus the programmer is responsible for retaining and releasing.
A typical way to write a method is:
id myVar = [someObject someMessage];
.... do something ....;
[myVar release];
return someValue;
The problem of needing to remember to release any acquired resources inside of code is both tedious and error-prone. Objective-C introduces another concept aimed at making this much easier: Autorelease Pools. Autorelease pools are special objects that are installed on each thread. They are a fairly simple class, if you look up NSAutoreleasePool.
When an object gets an "autorelease" message sent to it, the object will look for any autorelease pools sitting on the stack for this current thread. It will add the object to the list as an object to send a "release" message to at some point in the future, which is generally when the pool itself is released.
Taking the code above, you can rewrite it to be shorter and easier to read by saying:
id myVar = [[someObject someMessage] autorelease];
... do something ...;
return someValue;
Because the object is autoreleased, we no longer need to explicitly call "release" on it. This is because we know some autorelease pool will do it for us later.
Hopefully this helps. The Wikipedia article is pretty good about reference counting. More information about autorelease pools can be found here. Also note that if you are building for Mac OS X 10.5 and later, you can tell Xcode to build with garbage collection enabled, allowing you to completely ignore retain/release/autorelease.
Joshua (#6591) - The Garbage collection stuff in Mac OS X 10.5 seems pretty cool, but isn't available for the iPhone (or if you want your app to run on pre-10.5 versions of Mac OS X).
Also, if you're writing a library or something that might be reused, using the GC mode locks anyone using the code into also using the GC mode, so as I understand it, anyone trying to write widely reusable code tends to go for managing memory manually.
As ever, when people start trying to re-word the reference material they almost invariably get something wrong or provide an incomplete description.
Apple provides a complete description of Cocoa's memory management system in Memory Management Programming Guide for Cocoa, at the end of which there is a brief but accurate summary of the Memory Management Rules.
I'll not add to the specific of retain/release other than you might want to think about dropping $50 and getting the Hillegass book, but I would strongly suggest getting into using the Instruments tools very early in the development of your application (even your first one!). To do so, Run->Start with performance tools. I'd start with Leaks which is just one of many of the instruments available but will help to show you when you've forgot to release. It's quit daunting how much information you'll be presented with. But check out this tutorial to get up and going fast:
COCOA TUTORIAL: FIXING MEMORY LEAKS WITH INSTRUMENTS
Actually trying to force leaks might be a better way of, in turn, learning how to prevent them! Good luck ;)
Matt Dillard wrote:
return [[s autorelease] release];
Autorelease does not retain the object. Autorelease simply puts it in queue to be released later. You do not want to have a release statement there.
My usual collection of Cocoa memory management articles:
cocoa memory management
There's a free screencast available from the iDeveloperTV Network
Memory Management in Objective-C
NilObject's answer is a good start. Here's some supplemental info pertaining to manual memory management (required on the iPhone).
If you personally alloc/init an object, it comes with a reference count of 1. You are responsible for cleaning up after it when it's no longer needed, either by calling [foo release] or [foo autorelease]. release cleans it up right away, whereas autorelease adds the object to the autorelease pool, which will automatically release it at a later time.
autorelease is primarily for when you have a method that needs to return the object in question (so you can't manually release it, else you'll be returning a nil object) but you don't want to hold on to it, either.
If you acquire an object where you did not call alloc/init to get it -- for example:
foo = [NSString stringWithString:#"hello"];
but you want to hang on to this object, you need to call [foo retain]. Otherwise, it's possible it will get autoreleased and you'll be holding on to a nil reference (as it would in the above stringWithString example). When you no longer need it, call [foo release].
The answers above give clear restatements of what the documentation says; the problem most new people run into is the undocumented cases. For example:
Autorelease: docs say it will trigger a release "at some point in the future." WHEN?! Basically, you can count on the object being around until you exit your code back into the system event loop. The system MAY release the object any time after the current event cycle. (I think Matt said that, earlier.)
Static strings: NSString *foo = #"bar"; -- do you have to retain or release that? No. How about
-(void)getBar {
return #"bar";
}
...
NSString *foo = [self getBar]; // still no need to retain or release
The Creation Rule: If you created it, you own it, and are expected to release it.
In general, the way new Cocoa programmers get messed up is by not understanding which routines return an object with a retainCount > 0.
Here is a snippet from Very Simple Rules For Memory Management In Cocoa:
Retention Count rules
Within a given block, the use of -copy, -alloc and -retain should equal the use of -release and -autorelease.
Objects created using convenience constructors (e.g. NSString's stringWithString) are considered autoreleased.
Implement a -dealloc method to release the instancevariables you own
The 1st bullet says: if you called alloc (or new fooCopy), you need to call release on that object.
The 2nd bullet says: if you use a convenience constructor and you need the object to hang around (as with an image to be drawn later), you need to retain (and then later release) it.
The 3rd should be self-explanatory.
Lots of good information on cocoadev too:
MemoryManagement
RulesOfThumb
As several people mentioned already, Apple's Intro to Memory Management is by far the best place to start.
One useful link I haven't seen mentioned yet is Practical Memory Management. You'll find it in the middle of Apple's docs if you read through them, but it's worth direct linking. It's a brilliant executive summary of the memory management rules with examples and common mistakes (basically what other answers here are trying to explain, but not as well).
I'm in need of understanding how memory is managed in objective C.
I know the basics, if you create it and own it, you must release it yourself.
However, when it gets to code such as:
self.storeDict = [NSMutableDictionary dictionaryWithContentsOfFile:plistPath2];
Do I own this? Must I release this memory?
self.storeDict = [NSMutableDictionary dictionaryWithContentsOfFile:plistPath2];
//73.3% leak
totalCharacters = [storeDict count];
tagCounter = 1;
dictKeyArray = [[storeDict allKeys] mutableCopy];
//13.3% leak
When Instruments puts a bunch of percentages next to the highlighted leaks, what does that tell me? Does it tell me the size of the leak relative to the total amount of memory leaked?
And one last thing.. Is it normal for the amount of allocated memory to continuously rise? Or should it stabilize somewhere?
Thanks for all the help! Everything is greatly appreciated!
In most cases, you only own objects returned by methods whose names begin with "alloc", "new", "copy", or "mutableCopy". Of course, you also own anything to which you send -retain. Exceptions to these rules should be called out in the documentation for the non-conforming methods.
See https://developer.apple.com/library/mac/documentation/Cocoa/Conceptual/MemoryMgmt/Articles/mmRules.html#//apple_ref/doc/uid/20000994-SW1
Instruments attributes a leak to the line where the object was created. However, that's not necessarily the code which leaked the object. If the pointer to the object was passed to some other code and that code did not balance its retains and releases, then that code is responsible for the leak. Instruments can show you the history of retains and releases for a specific object, and you'll have to review those to see which code is not discharging its ownership responsibilities properly.
Also, if an object is owned by another object and it's really that second object that was leaked, then everything it owned will have leaked "transitively" as it were. So, look for higher-level objects which have leaked before trying to track down low-level objects which have leaked. Often, it is the objects which have leaked fewer instances which are the root of a graph of leaked objects.
Whether it is normal for memory to keep rising or to stabilize, that depends a little. Usually, memory usage should stabilize. However, if your app really is doing more and more, then it may be normal for its memory usage to keep increasing. For example, if an app is receiving data over the network and accumulating results as it does so, then its memory usage would likely rise as more data arrives. But if it doesn't stop at some reasonable point, that's a problem. On an iOS device, the system will eventually kill it.
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.
When you do a release, you do not immediately remove the memory. I used this code and I can see the memory before and after the use of release and it do not change. Ok, it will be release after some time.
But, what can I do for release all memory I can before start a library that will use a lot of memory? Or how can I immediately release memory?
Memory Management is a big thing in iOS but this tidbit of information helped me a lot during my development.
"Each object has a "retain count" which is increased by calling "retain" and decreased by calling "release". Once the retain count hits 0, the object is released and the memory can be used for something else.
You can "autorelease" objects. This means the retain count isn't immediately decreased, but is decreased the next time the current autorelease pool is drained.
iOS apps have an event loop in which your code runs. After each iteration of the event loop, the autorelease pool is drained. Any object with a retain count of 0 is released.
By default, autoreleased objects are returned by methods that don't begin with new, copy, mutableCopy, retain or init. This means you can use them immediately but if you don't retain them the object will be gone on the next iteration of the run loop.
If you fail to release retained objects but no longer reference them then you will have a memory leak, this can be detected by the leaks tool in Instruments.
One strategy is to autorelease everything returned by the above named methods and store objects in retain properties (or copy for strings). In your object's dealloc method, set all your properties to nil. Setting a retain/copy property to nil releases the object that it currently points to. As long as you don't have any circular references (avoided by not using retain properties for "parent" objects such as delegates), you will never encounter any leaks."
here is the link to the thread for this information
http://www.quora.com/What-is-the-best-way-to-understand-memory-management-in-iOS-development
It's a good thread with some useful code examples as well as other references.
Release frees the memory immediatly (assuming it's the last release). That means, it can be used by your application again when allocating.
Note that every applications have some chunks (pages) of free memory assigned by system and when allocating/deallocating part of a page, the released memory is not returned automatically to the system. It's just marked as free and can be used again by the application.
To understand all this, you need to learn something about how operating systems handle memory allocation, virtual memory etc.
Let's say that I have a loop that returns a bunch of autoreleased NSData objects...
NSData* bigData = ...
while(some condition) {
NSData* smallData = [bigData subdataWithRange:...];
//process smallData
}
Under ARC, should I still wrap an #autoreleasepool around the while condition?
NSData* bigData = ...
#autoreleasepool {
while(some condition) {
NSData* smallData = [bigData subdataWithRange:...];
//process smallData
}
}
The reason why I'm asking is I see the living allocation count in instruments going through the roof for my NSData objects that invoke a dataWith... method as opposed to an initWith... method. When I use initWith..., the living allocation count is much, much less.
Is it better to prefer the initWith... methods whenever possible?
Yes you should still use autorelease pools when using convenience methods in a tight loop. All the old memory management rules still apply under ARC, the compiler is merely injecting RRs for you. Checkout the great post by the awesome Mike Ash!
Link
I think your issue is that the autorelease pool is supposed to go inside the loop. With the loop inside the autorelease block rather than vice-versa, the accumulated objects won't be released until after the loop finishes.
Under ARC, should I still wrap an #autoreleasepool around the while condition?
Yes. Autorelease Pools are still in place, and grow and pop as before. The compiler just adds and coalesces the necessary retains and releases operations when ARC is enabled (echoing Logan), based on the methods that are visible to the TU and default naming conventions.
Execution in ARC is nearly identical the manual reference counting: Autorelease pool stacks still exist. One difference is that the compiler may order the reference counting operations slightly different from the way you wrote it (not in an incorrect way), and may omit unnecessary retain cycles.
Is it better to prefer the initWith... methods whenever possible?
WRT minimizing heap growth compared to the autoreleased counterparts: Yes. That's always been the case. It's especially important on iOS devices, where memory is quite limited.
The exception to this is when the object may avoid an allocation. Example:
NSString * copy = [NSString stringWithString:arg];
in this case, copy may be [[arg retain] autorelease]. Note that in this case, copy is still autoreleased, but you should not usually go to great lengths to test the presence of such optimizations. Note: It's also better to use copy = [arg copy]...[arg release] here.
The other bonus is that your ref count imbalances are often caught earlier when the object is never autoreleased, and closer to the call site (rather than when the Autorelease Pool is finally popped).
Performance with large autorelease pools is actually much worse than most people would suppose. If you can avoid depending on them heavily (e.g. using alloc+init...+release), you can make your program noticeably faster. Explicitly creating autorelease pools is cheap, and can help minimize this problem. When allocations are large and/or numerous, avoid using autorelease on them where possible, and do wrap these sections in explicit autorelease pools.