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.
Related
I wrote a class, which acts as a filter. I pass three objects:
An NSArray, which holds objects to filter (these objects have a timestamp property)
An NSMutableArray (which will hold the section names for a tableView, the periods based on timestamps). I need this array, because I have to sort the periods.
An NSMutableDictionary, in which the keys will be the section names, the values are NSMutableArrays, which hold the items for a given period.
In the class from which I pass these objects, there is a tableView, in which I display the items.
This class has it own NSMutableArray and NSMutableDictionary, I not initialize them, only retain the corresponding return values of the filter class. In the delloc method I release them. There is a method in the filter class:
+ (void)insertItem:(id)item forPeriod:(NSString *)period toContainer:(NSMutableDictionary *)container {
if ( ![[container allKeys] containsObject:period] ) {
// the period isn't stored, create and store it
NSMutableArray *periodArray = [[NSMutableArray alloc] init];
[container setObject:periodArray forKey:period];
[periodArray release];
periodArray = nil;
}
// store the item
NSMutableArray *arrayForPeriod = [container objectForKey:period];
[arrayForPeriod addObject:item];
arrayForPeriod = nil;
}
The instruments shows me leak when I set the newly allocated array as an object of the dictionary. At this point this is definitely true, because the dictionary retains again the array, so after the release, it retain count remains 1. But I think in the caller class when I release the dictionary, the array will be released too. Am I wrong?
Yes it is considered as a leak because your var is a local variable. Then you still have an object in memory but no reference to it. Remember the init makes a retain + the retain made by the dictionary = 2 retains. Just create your array using
NSMutableArray *periodArray = [[[NSMutableArray alloc] init]
autorelease]
Is it clear ?
You could switch to ARC. Alternatively, check what the static analyser thinks of your code. It is pretty good at finding memory leaks, better than most humans.
Once you have a few hundred objects in your dictionary, you waste an awful lot of time and memory. A dictionary doesn't have an array of all keys stashed away somewhere, it has to create it every time you call your method. That's copying a few hundred pointers (cheap) and retaining them (expensive). containsObject for an array compares the object with every object in the array calling isEqual: That's expensive. It's an NSString compare each time. The array is autoreleased, and when it finally goes away, all the keys in it get released. Again expensive.
NSDictionary uses a hash table, so [objectForKey ] will immediately go to the right object. One operation instead of possibly hundreds.
If an inited object comes to me retained, so I own it, and I store it in an NSArray, which retains that which gets stored in it, can I count on NSArray to see that it's already retained and not increase the count, or do I need to run through the array and decrement the retain count to insure no memory leak?
Sounds like you need to read the Memory Management Programming Guide. Your case is extremely simple. You own the object. You pass it to the array, which now also owns it. You need to release your ownership of it. Otherwise you'll leak it.
To make sure that the ownership of the object which was added into the NSArray is relinquished, send the -release message to the object right after you add it to the NSArray. If you do not do this, then you will indeed have a memory leak.
This is what happens:
NSString *str = [[NSString alloc] initWithFormat:#"%#", #"Blah"]; //retain count is 1, you own this object
[array addObject:str]; //retain count gets bumped to 2
[str release]; //retain count is 1 - relinquishing ownership here.
//There is no leak because when the NSArray is
//deallocated, the object will be sent the release message.
But if you don't send the owned inserted object the -release message, then even when the NSArray is deallocated, the object will only have a retain count of 1 and the memory obtained by the object will never be reclaimed, thereby resulting in a leak.
Whenever you release the NSArray, it'll release everything it retains.
As such, as long as you release the inited object once you've added it to the NSArray (so it's the only thing that retains it) or release it once you've finished with it outside of the array all should be fine.
Incidentally, there's a good blog post called "objective-c memory management for lazy people" that explains such things pretty well and is a handy reference if you're just starting out with such things.
You don't need to do that. NSArray takes ownership of any object that it stores. It will release its objects when it's deallocated. If you retain an object yourself, you take ownership too, and you are responsible for releasing it too.
NSArray will retain your object when you add it, and then release it when you remove it from the array. This is by design. This means that to ensure there's no memory leak, if you already retained the object before adding it to the array, you should release it after removing it from the array:
NSMutableArray *array = [[NSMutableArray alloc] initWithCapacity:10];
NSObject *object = [[NSObject alloc] init]; // retain count of 1 (because of alloc)
[object retain]; // useless, just for example, retain count of 2 (because of retain)
[array addObject:object]; // array is mutable, retain count of 3 (because of addObject:)
[array removeObject:object]; // retain count of 2
[object release]; // retain count of 1
[object release]; // retain count of 0, the object is dealloc'd afterwards
[array release]; // to be sure that we are not leaking an array, too
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.
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.
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.