Related
Here is code I am referring to.
// Person.h
#interface Person : NSObject {
NSString *firstName;
NSString *lastName;
}
#end
// Person.m
#implementation Person
- (id)init {
if (![super init]) return nil;
firstName = #"John";
lastName = #"Doe";
}
#end
// MyClass.m
#implementation MyClass
.....
- (NSArray *)getPeople {
NSMutableArray *array = [[NSMutableArray alloc] init];
int i;
for (i = 0; i < 10; i++) {
Person *p = [[Person alloc] init];
[array addObject:p];
}
return array;
}
.....
#end
Now, I know there is no memory-management going on in this sample code. What would be required?
In the getPeople loop, I am alloc'ing a Person (retainCount 1), then adding it to array. The retain count is now 2, right? If it is two, should I be [p release]'ing after adding it to the array, bringing the retainCount back down to 1?
Am I right in that it is the caller's responsibility to release the array returned by the method? (Which would also free the memory of the Person's, and their instance variables, assuming their counts are at 1).
I have read Apple's memory management document, but I guess what I am most unclear about, is what increases an objects retain count? I think I grasp the idea of who's responsibility it is to release, though. This is the fundamental rule, according to Apple:
You take ownership of an object if you create it using a method whose name begins with “alloc” or “new” or contains “copy” (for example, alloc, newObject, or mutableCopy), or if you send it a retain message. You are responsible for relinquishing ownership of objects you own using release or autorelease. Any other time you receive an object, you must not release it.
bobDevil's sentence "only worry about the retain counts you add to the item explicitly" made it click for me. After reading the Ownership policy at Apple, essentially, the object/method that created the new object, is the one responsible for releasing /it's/ interest in it. Is this correct?
Now, let's say I a method, that receives an object, and assigns it to a instance variable. I need to retain the received object correct, as I still have an interest in it?
If any of this is incorrect, let me know.
You are correct that the retain count is 2 after adding it to an array. However, you should only worry about the retain counts you add to the item explicitly.
Retaining an object is a contract that says "I'm not done with you, don't go away." A basic rule of thumb (there are exceptions, but they are usually documented) is that you own the object when you alloc an object, or create a copy. This means you're given the object with a retain count of 1(not autoreleased). In those two cases, you should release it when you are done. Additionally, if you ever explicitly retain an object, you must release it.
So, to be specific to your example, when you create the Person, you have one retain count on it. You add it to an array (which does whatever with it, you don't care) and then you're done with the Person, so you release it:
Person *p = [[Person alloc] init]; //retain 1, for you
[array addObject:p]; //array deals with p however it wants
[p release]; //you're done, so release it
Also, as I said above, you only own the object during alloc or copy generally, so to be consistent with that on the other side of things, you should return the array autoreleased, so that the caller of the getPeople method does not own it.
return [array autorelease];
Edit:
Correct, if you create it, you must release it. If you invest interest in it (through retain) you must release it.
Retain counts are increased when you call alloc specifically, so you'll need to release that explicitly.
factory methods usually give you an autoreleased object (such as [NSMutableArray array] -- you would have to specifically retain this to keep it around for any length of time.).
As far as NSArray and NSMutableArray addObject:, someone else will have to comment. I believe that you treat a classes as black boxes in terms of how they handle their own memory management as a design pattern, so you would never explicitly release something that you have passed into NSArray. When it gets destroyed, its supposed to handle decrementing the retain count itself.
You can also get a somewhat implicit retain if you declare your ivars as properties like #property (retain) suchAndSuchIvar, and use #synthesize in your implementation. Synthesize basically creates setters and getters for you, and if you call out (retain) specifically, the setter is going to retain the object passed in to it. Its not always immediately obvious, because the setters can be structured like this:
Person fart = [[Person alloc] init];
fart.firstName = #"Josh"; // this is actually a setter, not accessing the ivar
// equivalent to [fart setFirstName: #"Josh"], such that
// retainCount++
Edit:
And as far as the memory management, as soon as you add the object to the array, you're done with it... so:
for (i = 0; i < 10; i++) {
Person *p = [[Person alloc] init];
[array addObject:p];
[p release];
}
Josh
You should generally /not/ be worried about the retain count. That's internally implemented. You should only care about whether you want to "own" an object by retaining it. In the code above, the array should own the object, not you (outside of the loop you don't even have reference to it except through the array). Because you own [[Person alloc] init], you then have to release it.
Thus
Person *p = [[Person alloc] init];
[array addObject:p];
[p release];
Also, the caller of "getPeople" should not own the array. This is the convention. You should autorelease it first.
NSMutableArray *array = [[[NSMutableArray alloc] init] autorelease];
You'll want to read Apple's documentation on memory management: http://developer.apple.com/documentation/Cocoa/Conceptual/MemoryMgmt/MemoryMgmt.html
I used to do this till once I found the retain count of one of my retained propery is zero before dealloc function. (This situation is normal or abnormal?)
NOTE: It's a RC condition, not ARC.
For example, I got 4 retained properties below, should they always be released in dealloc function?
If not, how could I know when to release, and when not to release? Manually judge the retainCount?
#property (nonatomic, retain) NSString *fileName;
#property (nonatomic, retain) UIImage *fullSizeImage;
#property (nonatomic, retain) UIImage *thumbnailImage;
#property (nonatomic, retain) UIImageView *checkedImageView;
- (void)dealloc {
[checkedImageView release];
checkedImageView = nil;
[fileName release];
fileName = nil;
[fullSizeImage release];
fullSizeImage = nil;
[thumbnailImage release];
thumbnailImage = nil;
[super dealloc];
}
Well, if the question is "always?", then Wain is almost right...
a SHORT answer is YES...
because in general, when someone set-up a property, it means he's going to use it as a property, that is he uses its setter method to initialize it.
BUT (LONG answer): NO, NOT ALWAYS:
what if you, somewhere in your code, initialize the private var associated to the property without it's setter method? Keep in mind that a property is not a var, but just a useful way to get methods from Xcode to get and set a var associated to it.
in other words, when you write in .h:
#property (nonatomic, retain) NSString *fileName;
and in .m:
#synthesize fileName;
you are declaring a var called fileName and are asking xcode to create 2 (invisible) methods for you:
a setter, used to set a new retained value in fileName:
-(void)setFileName:(NSString *)newString{
if (fileName == newString) {
return;
}
NSString *oldString = fileName;
fileName = [newString retain];
[oldString release];
}
and a getter, used to get the value of fileName:
-(NSString)fileName{
return fileName
}
so, when you somewhere in your code use:
self.fileName = #"ciao";
you are using the property setter method, exactly as if you'd call it directly (and you can do it, the invisible method setFileName: really exist):
[self setFileName:#"ciao"];
doing so, as you can see in the setter method, from now on fileName is retained, and so you should release it in dealloc.
BUT, to answer your question:
if you use the dot rule to set a new string in your var, ok, everything is fine,
but you may decide to set it in the standard way, somewhere, maybe just for mistake:
fileName = #"ciao";
// code
fileName = #"Hallo";
// code
fileName = #"Bye";
this way you are not using the property setter method, but you are using the var directly, and so fileName is not retained, and if you try to release it, well you may get a crash...
PS:
Manually judge the retainCount?
no, never do that
Yes, they should always be released in dealloc. If you get to dealloc and something is already released and not set to nil then you did something wrong with your memory management elsewhere in the app.
Technically in dealloc you don't need to set to nil after releasing but setting to nil after releasing is a generally good idea.
Your dealloc is unnecessarily calling the getter for each property and then immediately releasing it. Just assign nil to release the properties:
- (void)dealloc {
self.checkedImageView = nil;
self.fileName = nil;
self.fullSizeImage = nil;
self.thumbnailImage = nil;
[super dealloc];
}
Although if you are following the current trend of letting clang auto-generate your backing instance variables, then this is better, as it won't cause KVO side-effects:
- (void)dealloc {
[_checkedImageView release];
[_fileName release];
[_fullSizeImage release];
[_thumbnailImage release];
[super dealloc];
}
Yes, they should normally all be released. If you have a retain count of zero, that usually means you've made a mistake somewhere in your memory management code.
You ask: If not, how could I know when to release, and when not to release? Manually judge the retainCount?
Possibly, but you could also let Xcode help you, using static analysis. Go to Product -> Analyze. It will quite often help you find erroneous releases, etc.
When to release? Quite obviously, if your object was holding a reference to another object, and your object goes away, then it should stop holding a reference to the other object. Why would you even look at the retain count? Retain count is about other people holding on the same object, but they are none of your business. They should know what they are doing. So you release the object. You do your job; everyone else has to do theirs. The easiest way, as others said, is to assign
self.someproperty = nil;
If your object was the only one holding a reference, that other object will go away. If others held a reference, it won't go away. Just as everyone would expect. The "release" method should be the only one ever caring about what the retain count of an object is.
I'm trying to switch views in my app using this chunk of code:
self->variable1 = [[NSNumber alloc] initWithInt:0];
self->variable2 = [[NSMutableArray arrayWithCapacity:1];
self->variable3 = [[NSMutableArray arrayWithCapacity:1];
[self presentModalViewController:titleScreen animated:YES];
If I comment out all of the allocated variable lines, the code works fine. If it leave just 1 line in the code crashes with the "EXC_BAD_ACCESS" error. Why is this happening? The variables aren't being used at all, just declared for later use. I'm not getting any compile errors on the lines either. What am I doing wrong?
UPDATE:
Thank you everyone for the help. I change the way I declare my variables to #property/#synth to clean up my code, but it didn't fix the problem. After a long time of fiddling I fixed it. I changed the code from this:
self.variable1 = [[NSNumber alloc] initWithInt:0];
to this:
self.variable1 = [NSNumber alloc];
[self.variable1 initWithInt:0];
and it worked! Can someone explain why this worked and the first line didn't?
Update:
Thank you Peter Hosey for showing me my evil ways. This time I'm pretty sure it's fixed. I was storing my variable Releases in
-(void)release
I didn't realize xCode will release when it needs to. I moved all the variable releases to
-(void)Destroy
so I can release everything on MY command. Now the code works. Thanks again!
I suggest that you declare variable1, variable2, and variable3 as properties, not instance variables. Then, use self.variable1, self.variable2, and self.variable3 to access them.
The dot syntax (self.variable1, etc.) uses the memory management policy you declared on each property; the arrow syntax (self->variable1, etc.) will access the variables directly. The crash is because you created two arrays in away that doesn't leave you owning them, and then did not assign the arrays to a property that would retain them.
You may also want to upgrade your project to use ARC. Then there is no memory-management difference; assigning to the instance variables rather than the properties will not cause the object to be prematurely released, because ARC considers instance variables to be ownerships by default. You may still want to switch to using properties after you switch to ARC, but not to prevent a crash.
In response to your edit:
I change the way I declare my variables to #property/#synth to clean up my code, but it didn't fix the problem.
Then something else was wrong.
You never did say much about the problem itself. You said you got an EXC_BAD_ACCESS, but not what statement triggered the crash or on what grounds you blamed it on the code you showed.
I changed the code from this:
self.variable1 = [[NSNumber alloc] initWithInt:0];
That's the correct code, though. That's what you should be using.
to this:
self.variable1 = [NSNumber alloc];
[self.variable1 initWithInt:0];
Noooo! That code is wrong, wrong, wrong, on multiple levels.
init methods (including initWithWhatever: methods) are not guaranteed to return the same object you sent the message to. NSNumber's initWithInt: very probably doesn't.
That object creates an uninitialized NSNumber object and assigns that to the property. Then it sends initWithInt: to that object, which will return an initialized object, which can be and very probably will be a different object. Now you are holding an uninitialized object (which you will try to use later) and have dropped the initialized object on the floor.
Never, ever, ever send alloc and init(With…) in separate expressions. Always send them in the same expression. No exceptions. Otherwise, you risk holding the uninitialized object rather than the initialized object. In your case (with NSNumbers), that is almost certainly what will happen.
What you should be doing is declaring and synthesizing a strong property that owns the NSNumber object, and creating the NSNumber object in a single statement: either [[NSNumber alloc] initWithInt:] or [NSNumber numberWithInt:]. If you're not using ARC, you'll want the latter, since the property will retain the object. If you are using ARC, they're effectively equivalent.
And if you get a crash with that code, then something else is wrong, so please tell us—either in this question or in a new question—about the crash so we can help you find the true cause of it.
variable2 and variable3 are being autoreleased before you actually access them (presumably) later after presenting the modal view.
At the very least change the lines to:
self->variable2 = [[NSMutableArray arrayWithCapacity:1] retain];
self->variable3 = [[NSMutableArray arrayWithCapacity:1] retain];
or
self->variable2 = [[NSMutableArray alloc] initWithCapacity:1];
self->variable3 = [[NSMutableArray alloc] initWithCapacity:1];
variable1 should be fine.
Best would be to use #property and #synthesize so you can use dot notation:
.h
#interface MyClass : SuperClass
#property (nonatomic,retain) NSMutableArray *variable2;
#property (nonatomic,retain) NSMutableArray *variable3;
#end
.m
#implementation MyClass
#synthesize variable2,varible3;
- (void)foo {
self.variable2 = [NSMutableArray arrayWithCapacity:1];
self.variable3 = [NSMutableArray arrayWithCapacity:1];
}
#end
By default, all instance variables in objective-c have protected scope. So unless you have explicitly declared them public in your interface file as:
#interface MYClass {
#public
NSNumber *variable1;
NSMutableArray *variable2;
NSMutableArray *variable3;
}
//...
#end
then they will not be accessible using the struct dereferencing operator. This is likely the cause of those EXC_BAD_ACCESS errors.
I want to know that difference between following two lines
name1 = [[NSString stringWithUTF8String:(char *)sqlite3_column_text(compiledStatement,1)] retain];
name1 = [NSString stringWithUTF8String:(char *)sqlite3_column_text(compiledStatement,1)];
What will be effect on name1 if I do use retain at the last,
I face once problem, and couldn't use name1 in a method that is being call by NSTimer, and when I use retain, they it worked fine for me.
If I do call value from database in viewDidLoad, and use in a method that is being called by NSTimer after each second, then it will give bad-exec, but when I do use retain then it will work properly,
I want to know the reason
Here is the difference
- (void)func1 {
name1 = [[NSString stringWithUTF8String:...] retain];
name2 = [NSString stringWithUTF8String:...];
}
- (void)func2 {
NSLog(#"%#", name1); //OK, name1 is still there
NSLog(#"%#", name2); //Would be crashed because name2 could be released anytime after func1 is finished.
}
I wrote this answer to another question, but it explains what you're asking:
Objects in objective c have a retain count. If this retain count is greater that 0 when the object goes out of scope (when you stop using it), it leaks.
The following things increase the retain count
[[alloc] init]
new
copy
[retain]
adding an object to an array
adding an object as a child (e.g. views)
There are likely more, but you don't appear to use any others in your code
The following decrease the retain count
[release]
removing an object from an array
if you dealloc an array, all of its objects are released
You should go through your code and ensure each of the retains or additions to an array are matched with a corresponding release. (You can release member variables in the dealloc method).
Another user made a valid point that my answer doesn't
Once you add an object to an array, it takes ownership and will release the object when it is done with it. All you need to do is make sure you release anything you own according to the memory management rules
There are also autorelease objects, have a look at this example;
-(init){
...
stagePickerArray = [[NSMutableArray alloc] init];
for (int i = 0; i < 3; i++)
{
//this string is autoreleased, you don't have call release on it.
//methods with the format [CLASS CLASSwithsomething] tend to be autorelease
NSString *s = [NSString stringWithFormat:#"%d", i);
[stagePickerArray addObject:s];
}
...
}
Your issue is that when you come to use your string later, it has a retain count of zero and has been released. By calling retain on it, you're saying 'I want to use this later'. Don't forget to match every retain with a release or you're objects will 'leak'
I bet your code wouldn't crash if your name1 was a property - either (nonatomic, retain) or just (copy) depending on your needs.
Second condition is to have name1 initialized to sth meaningful at the time your other function tries to do sth with it.
EDIT:
With a property you'd have to use synthesized setter in this case with: self.name1 = #"your string";.
Normally you don't have to manually retain/release a string created with stringWith... methods since there's nothing you created in memory yourself by using explicit alloc. Also please note that with code:
NSString *str = [NSString stringWithUTF8String:#"your string"];
your str (if not used to set a property) will stop being available when the function gets out of scope (iOS eventloop will autorelease it).
I have the following example class:
Test.h:
#interface Test : UIButton {
NSString *value;
}
- (id)initWithValue:(NSString *)newValue;
#property(copy) NSString *value;
Test.m:
#implementation Test
#synthesize value;
- (id)initWithValue:(NSString *)newValue {
[super init];
NSLog(#"before nil value has retain count of %d", [value retainCount]);
value = nil;
NSLog(#"on nil value has retain count of %d", [value retainCount]);
value = newValue;
NSLog(#"after init value has retain count of %d", [value retainCount]);
return self;
}
Which produces the following output:
2008-12-31 09:31:41.755 Concentration[18604:20b] before nil value has retain count of 0
2008-12-31 09:31:41.756 Concentration[18604:20b] on nil value has retain count of 0
2008-12-31 09:31:41.757 Concentration[18604:20b] after init value has retain count of 2147483647
I am calling it like:
Test *test = [[Test alloc] initWithValue:#"some text"];
Shouldn't value have a retain count of 1? What am I missing?
Thanks for your help.
Don't look at retain counts. They're not useful and will only mislead you — you can't be certain that nothing else is retaining an object, that an object you get from somewhere isn't shared.
Instead, concentrate on object ownership and follow the Cocoa memory management rules to the letter. That way your memory management will be correct no matter what optimizations Cocoa may be doing behind the scenes for you. (For example, implementing -copy as just -retain for immutable objects.)
Furthermore, it's critical to understand the difference between properties of your objects and instance variables within your objects. In your question's code, you are assigning a value to an instance variable. That instance variable is just that: a variable. Assigning to it will behave like any other variable assignment. To use the property, you must use either dot syntax or bracket syntax to actually invoke the property's setter method:
self.value = newValue; // this is exactly equivalent to the next line
[self setValue:newValue]; // this is exactly equivalent to the previous line
The code generated for the dot syntax and the bracket syntax is identical, and neither will access the instance variable directly.
You are passing in a literal string. The compiler probably allocates it in static memory and sets the retain count to the maximum possible value.
Try a dynamically allocated string instead and see what happens.
NSString* string = [[NSString alloc] initWithString: #"some text"];
Test* test = [[Test alloc] initWithValue: string];
You've got a reference to an immutable string. Assignment doesn't need to copy the value (the string data) since it's immutable. If you do a mutable operation, like value = [newValue uppercaseString] then it should copy the bits into value, and value's retain count incremented.
You're passing in a string constant, which can't really be deallocated. I think that 2147483647 is probably UINT_MAX, which basically means that the object can't be released.
I think you want to do this:
self.value = newValue;
which will invoke the property setter and cause the copy to occur. "value = newValue" simply assigns a pointer value to the instance variable.
You shouldn't be paying attention to the retain counts, just follow the Cocoa memory management rules. http://iamleeg.blogspot.com/2008/12/cocoa-memory-management.html
hmm.. we're getting closer.
it appears that newValue's retain count is also 2147483647.
I tried dynamically allocating the string instead with the same retain count results.
I found a helpful article here: http://www.cocoadev.com/index.pl?NSString
FTA:
Does the NSString returned by #"" need to be released, or is it autoreleased?
Neither. #""-strings are of class NSConstantString?, and thus act like atoms in lisp; they hang around. That is, if you use #"cow" in two separate places in your code, they will be referencing the very same object.
I don't think -release or -autorelease does anything to either of them.
If I have "copy" on the property though, shouldn't it copy the contents of the target memory into new memory with a retain count of 1? It would seem the copy attribute does nothing in this case?
#import <Foundation/Foundation.h>
int main (int argc, const char * argv[]) {
NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];
char *cstr = "this is a c string";
NSString *str = [[NSString alloc] initWithUTF8String:cstr];
NSLog(#"rc1: %d", [str retainCount]);
[pool drain];
return 0;
}
If you run the above code, it will display a retain count of 1
In Cocoa, many immutable objects will simply retain themselves when you ask for a copy within the same zone. If the object is guaranteed not to change (i.e. its immutableness) then an exact duplicate is redundant.
In Objective-C, the constant string class is separate to Cocoa's NSString class, although it may be a subclass of NSString (I'm not too sure). This constant string class may override NSObject's methods like retain, release and dealloc so that they do nothing, and also override retainCount so that it always returns the same number, UINT_MAX or so. This is because an Objective-C constant string is created in static memory. It must have the overall general behaviour of a Cocoa object (when using Cocoa) so that it can be added to arrays, used as keys to a dictionary etc, except in regards to its memory management, since it was allocated differently.
Disclaimer: I don't actually know what I'm talking about.