Xcode: Working In ARC and NSArrays - ios

Okay so I know ARC auomatically releases objects in memory that don't have at least one retained pointer linked to it so just to clarify, if I were initialize object, objectA, inside a local block of code, then add it to a global array, _objects, and then proceed to remove objectA from that array outside of the scope in which objectA was initialized, would ARC automatically release objectA from memory, because I don't intend to ever refer to it again. See the example below
#implementation MXViewController {
NSMutableArray *_objects;
}
- (void)viewDidLoad {
_objects = [#[] mutableCopy];
NSObject *objectA = [NSObject new];
[_objects addObject:objectA];
NSObject *objectB = [NSObject new];
[_objects addObject:objectB];
}
- (void)someMethod {
[_objects removeObjectAtIndex:0];
// since objectA has no retained
// pointers linked to it, will it be
// released from memory as expected
// by ARC?
}
#end

Yes, it would. Arrays retain objects added to them, and release them when they are removed. Here is your example code with reference counts in comments:
#implementation MXViewController {
NSMutableArray *_objects;
}
- (void)viewDidLoad {
_objects = [#[] mutableCopy];
NSObject *objectA = [NSObject new]; //objectA has a reference count of 1
[_objects addObject:objectA]; //objectA now has a reference count of 2
NSObject *objectB = [NSObject new];
[_objects addObject:objectB];
} //objectA's reference count is 1 because it is out of scope
- (void)someMethod {
[_objects removeObjectAtIndex:0]; //objectA's reference count is 0 and it is deallocated
// since objectA has no retained
// pointers linked to it, will it be
// released from memory as expected
// by ARC?
}

Yes, it will be released in your example. To be sure: make a subclass of NSObject for which you implement the dealloc method; log something therein, and add objects of that type instead of NSObject to your array. Prove it to yourself!

Yes. When you add the object to the array, the retain count is incremented (the array retains the object). After removing the object from the array, the retain count will drop to zero and the object will be released.
It's important to understand that the release won't necessarily occur immediately after the object is removed from the array. If you were to access the removed object in your someMethod property, ARC would postpone the release of the object accordingly.

Yes, it will be deallocated.
The "history" of the file would be something like
+1 (alloc) (reference counter:1) - From the "NSObject *objectA = [NSObject new];" instruction
+1 (retain) (reference counter:2) - "[_objects addObject:objectA];"
-1 (autorelease) (reference counter:1) - At some unknown point that we don't need to know
-1 (release) (reference counter:0) - "[_objects removeObjectAtIndex:0];"
And, of course, when it gets to 0, it will be deallocated.

Related

ARC two-way relationship

So I want to have multiple nodes that are connected. Every node has outgoing and incoming connections. But the NSMutableArrays are creating leaks although i'm using ARC. How can i get the objects to be released properly? I'm already using an autoreleasepool.
The code so far is:
#interface TestObj()
#property(strong) NSMutableArray *incoming;
#property(strong) NSMutableArray *outgoing;
#end
#implementation TestObj
#synthesize incoming,outgoing;
- (id)init
{
self = [super init];
if (self) {
incoming = [NSMutableArray array];
outgoing = [NSMutableArray array];
}
return self;
}
-(void)addIncoming:(TestObj *)incomingN {
if([incoming indexOfObject:incomingN] == NSNotFound) {
[incoming addObject:incomingN];
[incomingN addOutgoing:self];
}
}
-(void)addOutgoing:(TestObj *)outgoingN {
if([outgoing indexOfObject:outgoingN] == NSNotFound) {
[outgoing addObject:outgoingN];
[outgoingN addIncoming:self];
}
}
With ARC, as with manual memory management on iOS, you need to avoid retain cycles. If you have one object that is retaining a second, and the second is retaining the first, those two will never be deallocated even if nothing else references them, so you have a memory leak.
You need to make it so that you aren't referencing them like this. NSArray and NSMutableArray keep strong references to other objects. You can do something like the following to create a weak reference that you can story in the array:
NSValue *val = [NSValue valueWithNonretainedObject:object];
If you store val in the array, it won't have a strong reference to the object, so it can be deallocated. However, you have to be careful that you aren't creating a situation where some of your objects have no strong references, or they will get deallocated before you want them to.
Well this may sound basic, but have tried setting them = to nil?

NSCopying arrays of custom objects

I have a Singleton object that manages all my lists. We'll call it ListStore.
ListStore has a mutable array, which stores Lists.
#interface ListStore : NSObject
#property (nonatomic, copy) NSMutableArray *lists; // an array of List objects
end
Lists has a mutable array, which stores Things.
#interface Wanderlist : NSObject <NSCoding, NSCopying>
#property (nonatomic, copy) NSMutableArray *things; // an array of Thing objects
#end
At any time, a background process might go through ListStore and loop through and process all Lists, while a user might be interacting with a List.
To guard against "object was mutated while being enumerated" type errors, I do this:
// all of this is in a background thread
NSArray *newLists = [[ListStore sharedStore] lists] copy];
for (List *list in newLists) {
// yay, no more crashes, because I'm enumerating over a copied object, so the user
// can do whatever they want while I'm here
for(Thing *thing in list.things) {
// oh crap, my copy and the original object both reference the same list.things,
// which is why i'm seeing the 'mutation while enumerating" errors still
...
}
}
I originally thought that because I made a copy into newLists that all of its members would be properly copied. I now understand that not to be the case: I'm still seeing the "object was mutated while enumerated" errors, but this time it's happening on list.things.
Can I use NSCopying with my setup so that when I say:
[[ListStore sharedStore] copy];
It calls copyWithZone: on Lists, so I can then copyWithZone: on things?
I tried to set it up like this but copyWithZone: wasn't getting called.
I know I could simply say NSArray *newList = [list.things copy] but I'd like to get a better understanding of NSCopying at the very least.
Right before submitting this question I clicked on a question in SO's list of related questions, and found my solution.
Figured it doesn't hurt to post my solution.
Instead of this:
NSArray *newLists = [[ListStore sharedStore] lists] copy];
I had to do:
NSArray *newLists = [[NSArray alloc] initWithArray:[[ListStore sharedStore] lists] copyItems:true];
From the NSArray docs:
- (id)initWithArray:(NSArray *)array copyItems:(BOOL)flag
flag:
If YES, each object in array receives a copyWithZone: message to create a copy of the object—objects must conform to the NSCopying protocol. In a managed memory environment, this is instead of the retain message the object would otherwise receive. The object copy is then added to the returned array.
Once I used initWithArray:copyItems:, it automatically sent copyWithZone to all my List objects, and I was able to then manually perform a copyWithZone on list.things.

how to manage memory with using properties

my.h file
#interface myObject : NSObject {
NSMutableDictionary *myDictn ;
}
i have a property
#property (nonatomic,retain) NSMutableDictionary *myDictn ;
then in .m File
i have a allocated it from id
- (id)init {
if (self=[super init]) {
myDictn= [NSMutableDictionary alloc]init];
}
}
my Question is i have mentioned Retain in Property and i have allocated memory also (is Retain Count goes to 2 in (id)init )
so how to manage memory in this case ?
i am new to this so dnt have much idea regarding memory management .
one more thing if i have a method x in my code and i also allocated memory to myDictn then in that case also how can i use release .??
Thanks in Advance .
The proper way to initialize a retained property is:
NSMutableDictionary *newDict = [[NSMutableDictionary alloc] init];
self.myDictn = newDict;
[newDict release];
or alternatively:
self.myDict = [NSMutableDictionary dictionary];
The first method makes sure not to increase the already retained property's retain count by performing alloc,init (an action that increments retain count) on a temporary object.
The second method uses a convenience method for obtaining an autoreleased instance of the dictionary.
You should first refer to Cocoa Fundamentals Guide. Then, if you're targeting iOS > 4.3, you can rely on Automatic Reference Counting for memory management. But be sure to understand the base concepts and read the fundamental guide or you'll waste a lot of time programming in the dark.

iOS: Reference counting

I am doing some research on reference count increase. Please help on finding it.
Below is sample code and research i'm doing what would happen of reference counting for each line below.
.h file
NSArray *tempArray;
#property (nonatomic, retain) NSArray *tempArray;
.m file
#synthesize tempArray;
-(void) sampleFunction
{
NSArray *myArray = [[NSArray alloc] init]; // Thinking reference count increases to "1"
tempArray = myArray;// reference count increases and tempArray gets retain count "1" now.
tempArray = myArray;// reference count increases and tempArray gets retain count "2" now.
tempArray = [NSArray arrayWithObject:#"SomeString"]; // retain count = ?
}
I know this code may not be for functioning, but this is for only researching about what will happen on reference counting for such scenarios. I tried printing retainCount, but it doesn't show the correct result. Please advise me how does the reference count works on this each line?
In lines 2, 3 and 4 you are affecting the instance variable tempArray to the same object as myArray. But if you write it this way, you try to affect an instance variable. As a matter of fact, if you didn't write any #synthesize tempArray or #synthesize tempArray = tempArray in your code, by default the instance variable generated automatically to store the property value is the same name as the property name, but prefixed with an underscore. So as the property name is tempArray, the instance variable is named _tempArray. The instance variable tempArray itself does not exist and your line of code is invalid.
So if we suppose you wrote instead:
-(void) sampleFunction
{
NSArray *myArray = [[NSArray alloc] init]; // (1)
self.tempArray = myArray; // (2)
self.tempArray = myArray; // (3)
self.tempArray = [NSArray arrayWithObject:#"SomeString"]; // (4)
}
In (1) you are creating a brand new instance of NSArray. "alloc" always initialize new instance with a reference count of 1
In (2) you write self.tempArray = myArray (which is equivalent to [self setTempArray:myArray]; and thus call the property setter), so you set the property to point to the same array you created in (1). This array is thus retained by the property, and its retainCount increses by one, because it is retained by myArray and by the self.tempArray property.
In (3) you affect the property to the very same object as before. This the ref count does not change at all. You could understand that as if you replaced the value of the self.tempArray with another value, so the setter of the property release the old value (decrementing its ref count), then retain the new value (thus incrementing its ref count). As in your case the old and new values are the same object, you would decrement the ref count of your array then re-increment it again. In practice, the ref count does not even change at all (instead of decrementing+incrementing again) to avoid any potential dealloc of the object, because the default implementation of a property setter is as follow:
-(void)setTempArray:(NSArray*)newValue
{
// check if old and new value are different. Only do sthg if they are different
if (newValue != _tempArray)
{
[_tempArray release]; // release old value
[newValue retain]; // retain new value
_tempArray = newValue; // store new value in the backing variable associated with the property
}
}
In (4) you replace again the value of the property tempArray, but this time with a completely new object. So the property will release its old value and retain the new one. Thus the first array you created in (1) which had a refcount of 2 (retained by myArray and by self.tempArray) decrease its ref count to 1 (because the property won't retain it anymore), and the new instance you created [NSArray arrayWithObject:#"SomeString"] is retained by the property, so its ref count is +1.
If you replaced self.tempArray = ... (so the use of the property) with the direct use of the instance variable, using instance variables don't retain the objects they are affected to (except if you are using ARC but it seems you don't), so the ref count of the object wouldn't have changed at all in (2), (3) and (4).
Actually, retain count increase only in new, alloc, retain and copy condition but if we are providing ownership to an object through this for retain count will increase other than that there is no possibility to increase retain count.
First things first, don't even try to rely upon retainCount.
After that: you're wondering which scenario happens among the ones you enumerated. Well, neither one.
Why? Because, in first palce, you're assigning to an instance variable directly - that won't change retain count. At all. Except if you use ARC, but it seems you don't.
You probably wanted to assign stuff to the property of the object, that is, write
self.tempArray = myArray;
etc. Now because the property itself (and not its backing ivar!) is declared as retain, the corresponding accessor method will increase the reference count of the object being assigned to the property. However, in order not to leak memory, an accessor method is usually implemented by releasing the previously assigned object when assigning and thus retaining the new one, i. e.
- (void)setTempArray:(NSArray *)tmp
{
[tmp retain];
[tempArray release];
tempArray = tmp;
}
So basically, when you reassign myArray to the self.tempArray property, it looses and gains a reference, thus its reference count doesn't chnage at all.
When you assign another, new array to the property, then again myArray loses a refcount, dropping to 0 it is deallocated, then the new array, created using + [NSArray arrayWithObject:] is retained. Its exact reference count is supposed to be 1 after this, since it was created using alloc - init - autorelease (that's how the method is implemented), and it has been retained by the property. However, the value returned by - retainCount is still (and never) to be relied upon.
In your particular example, you are assigning to tempArray directly and not self.tempArray, so the retainCount will stay at 1 throughout. But let's go through what would happen if you did what I think you meant.
In objective-c, a synthesized retained property will have a setter functionally equivalent to this:
-(void) setTempArray:(NSArray *value)
{
if(tempArray != value) {
[tempArray release];
tempArray = [value retain];
}
}
This increases the retain count when a new object is assigned to it, essentially does nothing when it is set to the same object, and releases it when something else is assigned to it. So the retain counts in your example go something like this:
-(void) sampleFunction
{
NSArray *myArray = [[NSArray alloc] init]; // Retain count of 1
self.tempArray = myArray; // 2
self.tempArray = myArray; // still 2
self.tempArray = [NSArray arrayWithObject:#"SomeString"];
// myArray.retainCount is 1,
// tempArray.retainCount is 2 but with 1 autorelease
// myArray leaks
}

Can an autoreleased NSArray cause a memory leak?

I am trying to create an NSMutableArray using arrayWithArray, add two objects, sort, and store to an ivar as an NSArray. My code looks like this:
NSMutableArray *_mutableItems = [NSMutableArray arrayWithArray:[self.mainViewController.someDictionary allKeys]];
[_mutableItems addObject:#"Buildings"];
[_mutableItems addObject:#"Parking"];
self.curItems = [_mutableItems sortedArrayUsingSelector:#selector(localizedCaseInsensitiveCompare:)];
When I profile the app I get a memory leak for an NSArray after the view is popped. But what I don't understand is: aren't all of these objects autoreleased? Am I increasing the retain count when I assign it to the instance property?
Yes, setting the property is probably increasing the retain count. Specifically, _mutableItems will be autoreleased, but the array you create with sortedArrayUsingSelectoris retained by the property.
Does your property declaration include retain or copy?
#property (retain) NSArray *curItems;
If so, in your class dealloc method, make sure you call release on the array;
- (void)dealloc {
[curItems release];
[super dealloc];
}

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