The question is simple. Do I need to release a NSLocalizedString? For instance:
NSString *internetMessageTitle = NSLocalizedString(
#"You are currently not connected to a internet network"
#"Title of the message that tells the user there is no internet network");
Because I did this:
NSLog(#"Retain count of InternetMessageTitle is: %d",
[internetMessage retainCount]);
But it prints a retain count of 2. However I have read that the retainCount attribute is not very reliable. Should I release it twice?
And yes I have read the memory management rules and guide of the documentation but I don't see here any indication of NARC (NewAllocRetainCopy). I am still a beginner so I don't really know how NSLocalizedString makes strings.
Thank you!
EDIT1: I use this variable in a UIAlertView I don't know if the retainCount is increased there when I use it. And even when the alert is not used (inside an if, and if the if is skipped it isn't used) the retainCount is still 2 according to NSLog.
No, you must not release it. If you check how NSLocalizedString is defined you'll see:
#define NSLocalizedString(key, comment) \
[[NSBundle mainBundle] localizedStringForKey:(key) value:#"" table:nil]
That its normally a call to NSBundle's method that returns autoreleased string
I use this variable in a UIAlertView
I don't know if the retainCount is
increased there when I use it. And
even when the alert is not used
(inside an if, and if the if is
skipped it isn't used) the retainCount
is still 2 according to NSLog.
Yes, labels in UIAlert retain their content strings, but you should not worry about that - they will release them when get destroyed.
As you say, there's no NARC -- so you already know the answer is no.
And what you've read about retain counts? Heed it. Never look at the retain count as useful info. Never look at it all.
And FFS don't do something insane like calling release on an object several times just because you think it has a retain count > 1. That stuff is absolutely guaranteed to mess you up.
The Cocoa memory management rules are very simple. There's only one of consequence: all alloc/new*/*copy* calls must be balanced by a call to auto-/release. You're not calling a method or function named "alloc", starting with "new" or containing "copy", thus you shouldn't release.
Even simpler than following the memory rules is to use properties (object or class) when possible.
Related
I find I usually directly use a NSString like:
self.text = #"word";
label.text = #"word";
function(#"word");
I think it is a wrong way to directly use NSString, because the #"word" will have a retainCount of 2 after it has been used. Is that right?
What is exactly the retainCount of #"" ? Is it an autorelease object or has a retain of 1?
What`s more, I encountered some memory leak as the pic shown below:
I suspect it is related to the direct use of the NSString.
Essentially, the retain count of NSString literals is infinite. The memory for these objects is reserved by the compiler at compile time and never gets released for the duration that your app is running, no matter how often you try to retain or release them.
This is another good example why it is wrong to care about retain counts. You should only care about the golden memory management rules: if you take ownership of an object (with alloc, new, copy or retain), you have to release or autorelease it later. Otherwise, you don't.
Quoting from String programming guide
The simplest way to create a string object in source code is to use the Objective-C #"..." construct:
NSString *temp = #"/tmp/scratch";
Note that, when creating a string constant in this fashion, you should avoid using anything but 7-bit ASCII characters. Such an object is created at compile time and exists throughout your program’s execution. The compiler makes such object constants unique on a per-module basis, and they’re never deallocated, though you can retain and release them as you do any other object.
the #"word" will have a retainCount of 2 after it has been used. Is that right?
No. This is a perfect illustration of why it is bad to think in terms of retain counts which are an internal implementation detail. You can think of string literals as strings that are "owned" by the executable image so they will not go away. You should treat them like any other Objective-C object.
If you do take a sneaky peek at the retain count of a string literal, you'll find it is set to some really big number (something like the maximum value for an NSInteger). This is treated as a special value by retain and release that they don't change.
The memory for string literals is allocated as part of the binary image at compile time. It can never go away. Your memory leak is not because of this.
No, it's a good thing to declare an autoreleased NSString by using directly #"". But NSString are retained in a particular way and it's not really possible to know what's being done at runtime. You may have weird retain count sometimes. Don't bother about your leak of 48 Bytes.... It's not related to your nsstring.
I have some code which I think has extra release statements.
Is the code incorrect?
What is the end result?
I don't understand memory management well yet - even after reading lots of articles and stackoverflow answers. Thanks for straightening me out.
Update: The attached snippet works fine, but other code has the over-release problem
NSMutableArray *points = [NSMutableArray new];
for (Segment *s in currentWorkout.segments) {
[points addObjectsFromArray:[s.track locationPoints]];
}
[routeMap update:points];
[points release];
Your code is correct, but inadvisable. new acts as an implied alloc, which creates the object with a retain count of 1.
I think the last time I used new was in 1992; it's not wrong, but alloc/init is considered better practice, because it is clearer what you are doing. Please read Apple's guide to memory management, it is a comprehensive summary of the situation.
No messages can safely be sent to a deallocated object. Once an object has been released a sufficient number of times, it's deallocated. Any further messages sent to that object are going to an object that isn't there anymore. The precise result isn't completely predictable, but it usually ends in a crash. If you're less lucky, it could end in much stranger ways — for example, you could theoretically wind up with an Object A getting dealloced early and Object B allocated in the same memory location, then Object B receiving messages meant for Object A that Object B does understand but isn't supposed to receive at that time.
Basically, follow the rules. Think of it in terms of ownership. If you've claimed ownership, you need to release that ownership. If you don't own the object, you must not release it.
Take a look at this article online: http://weblog.bignerdranch.com/?p=2 .
It seems to imply that calls to release without a corresponding preior call to retain will result in a BAD_ACCESS error.
A short answer is, if you increasing the retain count of an object and you no longer are using it you should release it, otherwise you shouldnt...
So when ever you do a [objectName alloc] you are increasing the count by 1, when you use such methods as [NSString stringWithString:] these methods return an autoreleased object so you dont need to release it...if you instead did something like [[NSString stringWithString:]retain] then you are increasing the strings retain count and you should release it after you are done using it.
Im not too sure if new increases the reference count (i suspect that it would), you can always check your retain count by doing [object retainCount]... though note that even if the retain count is greater than 0, it does not mean you need to release the object, because some other class might have a reference to the object and therefore has its retain count increased by one and its the responsibility of the other class holding the reference to release it.
Hope this helps
you should use:
NSMutableArray *points = [[NSMutableArray alloc] init];
[...]
[routeMap update:points]; //if routemap stores the points, it will need it's own release retain
[points release]; //if there is a retain in the method above, reference will not be cleared
if unsure, use the build->analyze command, it will search your code for leaked references
you can get the official memory management guide from https://developer.apple.com/library/archive/documentation/Cocoa/Conceptual/MemoryMgmt/Articles/MemoryMgmt.html
Example: I have a view controller and get rid of it. But there's still an variable holding it's memory address. Accessing that results in EXEC_BAD_ACCESS. Of course. But: Is there any way to check if that variable is still valid? i.e. if it's still pointing to something that exists in memory?
You need to read this again:
Cocoa Memory Management Guidelines
In short, if you want something to stick around you must retain it.
If you want something to go away and you have previously retained it, you must release or autorelease it.
You must never call dealloc directly (except [super dealloc]; at the end of every one of your dealloc methods).
You must never release or autorelease an object that you did not retain.
Note that some methods do return retained objects that you must release. If you alloc an instance of a class, that implies a retain. If you copy and instance, the copy is retained.
If you are ever tempted to use the retainCount method, don't. It isn't useful. Only consider retain counts as a delta; if you add, you must subtract, but the absolute value is an implementation detail that should be ignored.
(In other words, even if there were ways to check for an object's validity definitively -- there aren't -- it would be the wrong answer.)
Oh, and use the Build and Analyze feature in Xcode. It does a very good -- but not quite perfect -- job of identifying memory management problems, amongst other things.
That's what the entire memory management model is set up for - if you call retain at the right times, and release and autorelease at the right times, that can't happen. You can use NSZombie to help you debug.
Use "NSZombieEnabled" break point.
For this reason only all strongly recommend us to use accessors. If your object is released anywhere, it will get assigned to nil, and there will be no harm if you call any API or method on Nil object. So please make a habit of using Accessors.
you just add this NSZombieEnabled Flag as an argument to your application in build settings. and enable it. Now you run your application in debug mode. If any such crash is about to occur, this breakpoint will show you which object is freed and where it is crashing.
Cheers,
Manjunath
If by variable, you mean whether the pointer to your object still references valid memory then:
MyClass *myVariable = [[MyClass alloc] init];
//Tons of stuff happens...
if (myVariable != nil)
//Do more stuff
I'm somewhat confused by the following behavior I'm seeing within an Xcode project compiled for the iPhone simulator (or device):
NSString *test = [[NSString alloc] initWithCString:"foo"];
NSLog(#"test retain count = %d", [test retainCount]); // prints 1
[test release];
NSLog(#"test retain count = %d", [test retainCount]); // also prints 1 instead of 0
However, any further attempts to access 'test' result in a crash of the Xcode enviroinment, whether it be another [test retainCount] NSLog statement or otherwise (even if only to check if test is equal to nil).
Thoughts? Compiled within a simple View based test project...code exists within project's applicationDidFinishLaunching method.
Clarification -- I know NOT to do the above in practice. This was just a test to see why in some debugging cases a retain count of 1 wasn't actually reflecting the real state of an object. Thanks for your responses. This was just a test stub to see why I was seeing certain behavior in a few cases. What I'm really trying to do is track down a very small memory leak (0.06MB) that is consistently being created whenever I destroy/recreate a custom view.
Retain counts are a debugging aid and can be misleading based on what Cocoa may be doing behind the scenes. This is particularly true with string literals where the data is permanently available and is never really deleted.
Concentrate of ensuring your code follows the Cocoa memory management rules with respect to object ownership. Where necessary, use Instruments to check for actual memory leaks.
You are calling retainCount on test after it has been released and possibly deallocated, so definitely the result is not reliable, not to mention you shouldn't be sending dealloced objects any messages.
Don't rely on retain counts...the framework may be retaining stuff on its own.
Instead, rely on the rules of memory management for objective-C. If you alloc, init, or new something, you own it and are responsible for releasing it. If you didn't, you aren't.
don't rely on the retain count; instead, make sure that you're balancing retains and releases. Here are some links on Cocoa (Touch) memory management:
http://iamleeg.blogspot.com/2008/12/cocoa-memory-management.html
When you send that release, the string is dealloced — it has no other owners retaining it. Sending further messages to it (including retainCount) is an error and won't return anything meaningful.
And as others have pointed out, don't rely on retain counts to be particularly useful. They're a piece of the picture you can look at, but they're often misleading. For example, autoreleases don't show up in the retain count, sometimes objects are retained extra times for caching purposes, etc. In this particular case, though, you're not sending the message to a valid object at all, so you're lucky to get back an answer rather than a crash.
You should read Apple's memory management guide if you're unclear on any of this. It's not too complicated and absolutely essential for programming Cocoa.
One trick to improve the code shown above would be to use the 'set to nil' convention.
EG:
NSString *test = [[NSString alloc] initWithCString:"foo"];
NSLog(#"test retain count = %d", [test retainCount]); // prints 1
[test release];
test = nil; // set to nil, as I have released the pointer, I should no longer use it.
NSLog(#"test retain count = %d, test foobarpro = %d", [test retainCount], [test foobarPro]); // will now print 0 and 0 - because any objective-c call on a nil object returns 0 (and will not crash)
By setting a pointer to zero after you release it, you future proof your code a little: Someone coming along later and editing code 10 lines below can accidentally use 'test' with likely no ill effects (or an obvious crash). I even set pointers to nil in dealloc calls, as some of the hardest debugging tasks happen when non - nil 'bad' pointers are used in destruction.
Also you can debug with zombies to look for cases where you use pointers like your invalid test.
--Tom
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).