I'm coding a generic Swift application (not for iOS, it will later run on raspbian) and i noticed a constant increase of memory. I checked for memory leaks also with the inspector, and there are none.
To dig deeper, I created a blank application for macOS, and I just wrote those lines of code, which are only for testing:
var array = [Decimal]()
while(true) {
array = [Decimal]()
for i in 0..<10000
{
array.append(Decimal(string: i.description)!)
}
sleep(1)
}
As I know, at beginning of every cycle of the while loop the entire array that was filled in the previous cycle should be deleted from memory. But seems that this is not happening, with those lines of code the process memory rises indefinitely.
I also tried the same code on an iOS project putting it on the application function (the one that is called at the beginning in the app delegate) and I noticed that in this case, the memory remains constant and do not rises up.
Am I missing something on the non iOS project?
The Decimal(string:) is creating autorelease objects. Use an autoreleasepool to drain the pool periodically:
var array = [Decimal]()
while true {
autoreleasepool {
for i in 0..<10_000 {
array.append(Decimal(string: i.description)!)
}
sleep(1)
array = []
}
}
Normally the autorelease pool is drained when you yield back to the runloop. But in this case, this while loop never yields back to the OS, and therefore the pool is not getting drained. The use of your own autoreleasepool, like above, solves that problem.
FWIW, Apple has been slowly excising the use of autorelease objects in the Foundation/Cocoa classes. (We used to experience this problem with far more Foundation objects/APIs than we do now.) Clearly Decimal(string:) still is creating autorelease objects behind the scenes. In most practical cases, this isn't a problem, but in your example, you will need to introduce your own autoreleasepool to mitigate this behavior in this while loop.
I am wondering if there's any benefit of using #autoreleasepool on an ARC code inside a method.
I mean this. Suppose I have a memory intensive method that is called several times in sequence. Something like
// this is my code
for (id oneObject in objects {
[self letsUseMemory];
}
and then
- (void) letsUseMemory {
// heavy use of memory here
}
and I do this
- (void) letsUseMemory {
#autoreleasepool {
// heavy use of memory here
}
}
Is there any benefit? I mean, the method variables will be deallocated anyway when the method finishes, so adding an autoreleasepool there, in theory, will do any benefit, right?
Or will autoreleasepool inside that method speed the deallocation?
thanks.
Is there any benefit? I mean, the method variables will be deallocated anyway when the method finishes, so adding an autoreleasepool there, in theory, will do any benefit, right?
It depends. Any autoreleased temporary objects will not be deallocated until the pool drains, regardless of whether you're using ARC. I.e.:
NSString* foo = [NSString stringWithFormat:#"Bar: %#", baz];
Without an enclosing #autoreleasepool, that object instance may hang around until you return to the run-loop. If that line of code exists within a loop, you may be accumulating a large number of these temporary objects.
The general rule of thumb is that if you have a potentially large loop that may create autoreleased objects, wrap the inside of the loop in with an #autoreleasepool.
It's less common and perhaps somewhat meaningless to wrap a single method in an #autoreleasepool because it would usually only have meaningful effect if the method was called many times in a loop. Putting the #autorelease pool in the loop makes the intent more clear.
There are a number of "it depends" things going on there, I believe.
Starting with the obvious, if there are no autoreleased objects, it doesn't matter. If your code goes right back to the run loop after your enumeration finishes, it doesn't matter.
That leaves the case where the method containing the enumeration is used to do a bunch of initialization and then continues on with more processing. For that one, you could benefit by getting rid of temporary objects that were marked for later release.
I created
object *Obj = [[Obj alloc] init];
Obj retain count is 1. After I release it, the object is deallocated.
If I try to release the object again, what will happen?
EXT_BAD_ACCESS most likely since your object reference is no longer valid.
The code may crash. But it may just as well work most of the time.
You brake a rule, you may get caught. But you may just as well get away with it, living in constant fear that you may get caught later on.
There’s an important distinction to be made here: you can’t release the object again, because the object no longer exists. If you send another message to the variable (be it release or any other message), the behaviour is undefined because the variable is no longer known to point to a valid object. (It’s possible that the address the variable now points to will have been reused for a different object, in which case it may not crash, but of course that’s still a bug.)
Once the retain count of an object reaches 0, it is released, and all further attempts to access it will result in random behaviour.
If you use autorelease instead, the retain count will not be lowered, and the object will be put in the autoreleasepool. The object will only lower its retain count once it reaches the autoreleasepool drain command, which is usually done on a much higher level in a much broader scope. If you really need the object after the autoreleasepool is drained, you should retain it before drain is executed, or else it will have exactly the same behaviour as in my first paragraph.
Get EXT_BAD_ACCESS. Because of you are already release it and now try to release again.
your object reference is no longer valid.
I know this question looks like a dupe: I checked and it's not
In talking about NARC, the author of this blog says, "Personally, I like to immediately autorelease anything I NARC-ed, on the same line." This goes completely counter to all the examples I've seen on the Apple site and in books, where autorelease is only used when the object must be returned and cannot be released immediately. In general, the idea is that autorelease is memory intensive and can gum up your program (though it makes code cleaner). From Appress Beginning iPhone 3 Development:
these objects can have a detrimental
effect on your application’s memory
footprint. It is OK to use
autorelease, but try to use it only
when you really need to, not just to
save typing a line or two of code.
I am not asking if autorelease is worse than explicitly calling release (it is), but rather:
In most 'normal' situations on iPhone, just how bad is it to replace a later release with an earlier autorelease (in the same method)? Also, in what situations would it be absolutely prohibitive to do this?
My guess is that, compared to using a garbage collector (as MonoTouch apps do sucessfully), autorelease will hardly make a dent in your memory footprint, and that Vincent's advice it right on, and can make for cleaner code and less accidental memory-leaks.
There's nothing wrong with using autorelease, but when you allocate objects in a loop, you should always call release explicitly.
Using autorelease:
for (int i=0;i<1000;i++) {
NSString *s = [[[NSString alloc] init] autorelease];
}
// at this point, there are 1,000 unreleased string objects in memory
Using release:
for (int i=0;i<1000;i++) {
NSString *s = [[NSString alloc] init];
[s release];
}
// at this point, no string objects are "alive"
As you can see, you have to be really careful when using autorelease in loops.
You should be aware how autorelease works. Each thread in your application normally has a single autorelease pool. Objects can be registered in the pool. At the time they are registered, the pool determines the stackframe they belong to and will automatically pop the from the pool whenever that stackframe is left.
While this may seem costly (and it certainly is compared to direct retain/release), I don't think it even close to the cost a generation mark and sweep garbage collector can have.
Where autorelease really shines is in all situations where exceptions may be raised and there's no try/catch around. Autorelease is definitely preferable to a direct release in such cases.
There are, however, situations where you should avoid autorelease (the same goes for garabge collected environments where you should try to avoid these situations too). Creating temporary, autoreleased objects in a loop which runs a huge number of times is such a scenario, which puts significant stress on a garbage collector or the autorelease pool.
Replacing release with autorelease should be avoided in worker threads that are very simple and can live without the overhead of an autorelease pool. So the guideline is: Whenever you can avoid it, you should, whenever you're unsure autorelease.
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).