I need help with deallocation of my NSMutableArray of custom objects. I need to retain the array and so I have added a property in .h and I release it in dealloc in .m file. When I add objects to the array, I do the following:
myarray = [[NSMutableArray alloc] init];
[myarray addObject:[[mycustomObject alloc]initWithObject:obj1]];
[myarray addObject:[[mycustomObject alloc]initWithObject:obj2]];
Now, I don't know how to release mycustomobject. If I do the following:
[myarray addObject:[[[mycustomObject alloc]initWithObject:obj1] autorelease]];
I run in to problems when I access the array later. Please advice.
I don't think you understand how memory management in Cocoa works. The array will retain the objects you add to it, and it will release them by itself when the array no longer needs them (such as when you release the array).
In other words, add the autoreleased object to the array, and don't worry about its retain count after that. If you want to remove it from the array simply remove it (using removeObjectAtIndex: or something similiar). If you think you want to release the object without removing it from the array then you are doing something wrong, since that may leave a dangling pointer in your array that will cause you to crash later.
You should really really go over the documentation again, particularly the section on Object Ownership and Disposal.
The proper way to do this is to let the array maintain ownership of the custom object:
NSMutableArray * array = [[NSMutabelArray alloc] init];
for (id obj in anArrayOfObjects) {
mycustomObject * customObj = [[mycustomObject alloc] initWithObject:obj];
[array addObject:customObj];
[customObj release];
}
If you're having difficulties accessing your array later, then you're doing something wrong with the memory management of the array.
Related
So I had this code, and it did not work:
for (NSDictionary *item in data){
[self.resultsArray addObject:item];
}
self.resultsArray is nil. But then I changed it to this:
NSMutableArray *myDataArray = [[NSMutableArray alloc] init];
for (NSDictionary *item in data){
[myDataArray addObject:item];
}
self.resultsArray = myDataArray;
[myDataArray release];
and now it worked. self.resultsArray is now populated
So I'm a beginner in Objective C and I was wondering why can I not just directly use it in the property's addObject. Why did I have to create another mutable array, populate it, assign it to the resultsArray property and release the mutable array I made?
Thanks in advance!
EDIT: Also, in a lot of books I've been working on, this is done a lot.
simple answer
You didn't initialize self.resultArray before adding objects to it. It is just a pointer to the value which is nil until you alloc it.
self.resultArray = [[NSMutableArray alloc] init]; before adding objects to it will solve the issue.
However, this way of alloc'ing will create a memory leak, therefore it is not shown in books and examples. Memory leak can happen if the self.resultArray property is marked as retain and by calling alloc it will be retained 2 times.
If self.resultsArray is nil, then [self.resultsArray addObject:item] will NOT add an object to the array, it will just do nothing (because the array will be nil by default, and sending messages to nil is a no-op in Objective-C). When you create a mutable array as a local variable, you can add things to it — then if you assign it to the property, well, everything works as you expect and self.resultsArray will no longer be nil.
Typically when you have properties like this, you'd set them up in your init method:
- (id)init {
// ...
self.resultsArray = [NSMutableArray array];
// or access the ivar directly:
// _resultsArray = [[NSMutableArray alloc] init];
// ...
}
Then as soon as your object is initialized you'll be able to add things to the array. Again, if you don't do this, it will be nil by default, and [self.resultsArray addObject:item] will have no effect.
Chances are you are not initializing the array (I'm going to assume myDataArray is an NSMutableArray).
In your init method, call myDataArray = [NSMutableArray array]; and it'll work
The important thing to note is that you're not creating another mutable array as you didn't have an array to start with. Merely declaring a property or variable does not create an object to go along with it. That's why self.resultsArray starts out as nil.
The working code you have is designed to allow you to explicitly release the array as you are retaining it twice: once when you alloc it and once when you assign it to your property. You only want one of those retains, so you release once.
You could just do:
self.resultsArray = [[NSMutableArray alloc] init];
[self.resultsArray release];
for (NSDictionary *item in data){
[self.resultsArray addObject:item];
}
This is less code, but it's not as clear. Clarity is important.
I am currently working on an demo app so I was a little sloppy how to get things done, however I run the "Build and Analyze" to see how many leaks I get,... well and there are a lot.
Source of teh proble is that I have a NSMutableArray and I add some Objects to it :
NSMutableArray *arr = [[NSMutableArray alloc] init];
[arr addObject:[[MyObject alloc] initWithText:#"Option1"]];
// I have like 100 lines like that and 100 complains
Now, xcode complains about a potential leak.
Can someone give me some advice how to handle that ?
Thanks.
The problem is that you're allocating an instance of MyObject which you have a responsibility to release. When you pass it to the array, the array also retains the object, so now both you and the array have to release it. You can simply autorelease the object, and the array will keep it retained until you remove the object from the array or destroy the array itself.
[arr addObject:[[[MyObject alloc] initWithText:#"Option1"]] autorelease];
Replace
[arr addObject:[[MyObject alloc] initWithText:#"Option1"]];
with
[arr addObject:[[[MyObject alloc] initWithText:#"Option1"] autorelease]];
Most collections (arrays, dictionaries) own the objects added to them. And, since you’ve sent +alloc to MyObject, you also own the object that’s just been instantiated. As the memory management rules say, you are responsible for relinquishing ownership of objects you own. Sending -autorelease to the newly instantiated object will do that.
umm So simple question here:
I have an instance of NSMutableArray declared in my header
NSMutableArray *day19;
#property (nonatomic, retain) NSMutableArray *day19
implementation:
#synthesize day19;
In my viewDidLoad
self.day19 = [[NSMutableArray alloc] init];
In the myMethod where I want to add objects to the array I:
NSObject *newObject = [[NSObject alloc] init];
[day19 addObject:newObject];
However... when i check the day19 array there is nothing in it. If I conversely add the newObject to a tempArray within the myMethod scope and then set the day19 array to the tempArray, day19 has the objects.
Super basic I know just must be a confused morning or something...
thanks for any help
Is day19 actually an instance variable? In the snippet, it's not clear when it's declared as an instance variable or just as a variable outside the scope of the class.
A couple of things:
Are you sure viewDidLoad is the right place to init your array? Confer here.
Also, at least from the code you've got posted, it looks like you're being sloppy with your retains. If your property is a retain type, you should not be writing:
self.myProperty = [[Something alloc] init]; // double retain here, bad
You should instead be writing something like:
self.myProperty = [[[Something alloc] init] autorelease]; // single, good
Also, with
NSObject *newObject = [[NSObject alloc] init];
[day19 addObject:newObject];
unless you have a
[newObject release];
down the pike, you've got a memory leak.
In my viewDidLoad
self.day19 = [[NSMutableArray alloc] init];
In the myMethod where I want to add objects to the array I:
NSObject *newObject = [[NSObject alloc] init];
[day19 addObject:newObject];
However... when i check the day19 array there is nothing in it. If I conversely add the newObject to a tempArray within the myMethod scope and then set the day19 array to the tempArray, day19 has the objects.
Let me guess: You checked the array with code like this:
NSLog(#"day19 contains %lu objects", [day19 count]);
Remember that a message to nil does nothing and returns nil, 0, or 0.0. That's why the output said 0 objects: You don't have an array in the first place. The most probable reason for that is that viewDidLoad hasn't been called yet, so you have not yet created the mutable array.
It's also possible that you have an array (i.e., the view has been loaded) at the time you examine the array, but you didn't have an array yet (the view hadn't been loaded yet) at the time you tried to add to the array, so your addObject: message fell on deaf ears.
Consider creating the array earlier. You probably should be creating it in init or initWithCoder:.
A third possibility is that you examined the array before you ever added to it. Make sure you log or break at both points, so you know which one happened first.
Whatever the problem is, you also need to either assign the array to the instance variable, not the property, or autorelease the array before assigning it to the property. Otherwise, you're over-retaining the array, which means you will probably leak it later on. You probably need to review the Memory Management Programming Guide for Cocoa.
I have a newbie question regarding when to release the elements of a NSArray. See following pseudo code:
NSMutalbeArray *2DArray = [[NSMutableArray alloc] initWithCapacity:10];
for (int i=0;i<10;i++) {
NSMutableArray *array = [[NSMutableArray alloc] initWithCapacity:5];
for (int j=0;j<5;j++) {
MyObject *obj = [[MyObject alloc] init];
[array addObject:obj];
[obj release];
}
[2DArray addObject:array];
[array release];
}
// use 2DArray to do something
[2DArray release]
My question here is, when I release 2DArray, do I need to explicitly release each of its element (array) first? Also, before I release the "array" object, do I need to release each of its element (MyObject) first?
I am new to Objective C. Please help. thanks.
No, you don't need to tell each object to be released. When you send a release method to an NSArray, it automatically sends a release method to each item inside first.
So in your case, you send [2DArray release]. This automatically sends [array release] to every other array, which sends [obj release] to each object inside each array.
You don't need to release the kept objects. NSArray retains them when you add, and releases them when released. So if you allocate, add to the array, then release, the object in the array will have the retain count of 1. Once the array is freed, the object is released, therefore freed.
When an object is created, it has a retain count of 1. Whenever a object is added to an array, its retain count is increased (in this case to 2). After adding to the array, your code release its hold of the object, dropping its retain count by 1 (to 1 in this case). Then when you release the array, it calls release on everything in it dropping their retain counts by 1 (to 0 in this case). When retain count hits 0 the object is deallocated.
Your code looks correct from a memory management stand point.
In Objective-C, if array1 is copied onto array2 using mutableCopy, and suppose the code is done in main(), who is responsible for releasing the objects contained in the array? Is it main() or array2?
I think the previous answers have missed the point, or else the asker was pretty unclear. The actual question isn't talking about either array, but rather the array contents:
who is responsible for releasing the objects contained in the array? Is it main() or array2?
Both array1 and array2 are responsible for releasing the objects.
From the NSArray documentation:
"Arrays maintain strong references to their contents—in a managed memory environment, each object receives a retain message before its id is added to the array and a release message when it is removed from the array or when the array is deallocated."
To begin with, each of the objects are retained by the NSArray array1. When you create array2 via -mutableCopy, you get an NSMutableArray which points to the same objects, and retains each of them again. If you were to release array1 at this point, when its dealloc method were called it would release each of the objects it contains. However, array2 has retained them, so the objects won't be destroyed — only when their retain count reaches 0, which would happen if array2 were destroyed and nobody else has retained any of the objects (or when they are removed from array2).
Since collection classes (arrays, sets, dictionaries, etc.) handle retaining and releasing their contents, all you have to worry about is retaining or releasing the collection itself. Since you used -mutableCopy, remember that you have implicitly retained array2, so you should release it when you're done with it.
I reference this guide for Memory Management in Obj-C. He has a section on Arrays and Dictionaries, here's an excerpt:
Arrays, dictionaries etc. generally retain any objects added to them. (When dealing with 3rd party collection type objects, always check the documentation to see if they retain or not). This means that these collections will take ownership of the object, and you do not need to retain before adding.
The comments for the posting are also useful
The ownership responsibilities are not changed by storing objects in an array. Here's an example:
int main(int argc, char *argv[])
{
// ...
NSObject *obj1 = [[NSObject alloc] init]; // owned
NSObject *obj2 = [[NSObject alloc] init]; // owned
NSObject *obj3 = [[[NSObject alloc] init] autorelease]; // not owned
NSMutableArray *array1 = [NSMutableArray arrayWithObjects: obj1, obj2, obj3, nil]; // not owned
NSMutableArray *array2 = [array1 mutableCopy]; // owned
// ...
[array2 release];
[obj2 release];
[obj1 release];
// ...
}
This code directly allocates obj1 and obj2, so it owns them and must release them, but it autoreleases obj3, so it doesn't have to release that. In the same way, it doesn't own the result of arrayWithObjects:, so it doesn't release that, but it does own the result of mutableCopy, so it must release that. The objects being stored in an array is irrelevant—all you need to care about is ownership.
Both arrays keep strong references to their content, so obj1, obj2, and obj3 won't be deallocated as long as the arrays exist—but that's a detail of the NSArray contract, it doesn't affect how you manage the ownership of the objects or the arrays.
These are all details of Cocoa's memory management conventions, not arrays.
It wouldn't make sense for a mutable array to be tied to an immutable array. main() would be responsible for releasing array1.
In my experience however, releasing objects only causes applications to crash. ObjC is fairly good at automatically managing memory. My Cocoa apps don't seem to ever need more memory than they started with, even after running several hours.