With an instance variable myArray:
#interface AppController : NSObject
{
NSArray *myArray;
}
Sometimes I see myArray initialized like this:
- (void)init
{
[super init];
self.myArray = [[NSArray alloc] init];
return self;
}
and sometimes I see it with a more complicated method:
- (void)init
{
[super init];
NSArray *myTempArray = [[NSArray alloc] init];
self.myArray = myTempArray
[myTempArray release];
return self;
}
I know that there's no difference in the end result, but why do people bother to do the longer version?
My feeling is that the longer version is better if the instance variable is set up with a #property and #synthesize (possibly because the variable has already been alloced). Is this part of the reason?
Thanks.
If myArray is a property and it's set to retain or copy (as it should be for a property like this), then you'll end up double-retaining the variable when you do this:
self.myArray = [[NSArray alloc] init];
The alloc call sets the reference count to 1, and the property assignment will retain or copy it. (For an immutable object, a copy is most often just a call to retain; there's no need to copy an object that can't change its value) So after the assignment, the object has retain count 2, even though you're only holding one reference to it. This will leak memory.
I would expect to see either a direct assignment to the instance variable
myArray = [[NSArray alloc] init];
Or proper handling of the retain count:
NSArray *newArray = [[NSArray alloc] init];
self.myArray = newArray;
[newArray release];
Or the use of autoreleased objects:
self.myArray = [[[NSArray alloc] init] autorelease]; // Will be released once later
self.myArray = [NSArray array]; // Convenience constructors return autoreleased objects
This is an idiom used in mutators (sometimes called "setters"), but I think you typed it slightly wrong. Usually it looks like this:
-(void)setMyName:(NSString *)newName
{
[newName retain];
[myName release];
myName = newName;
}
The new name is retained, since this instance will need to keep it around; the old name is released; and finally the instance variable is assigned to point to the new name.
I have a feeling you mean this:
NSArray* a = [NSArray arrayWithObjects:#"foo", #"bar", nil];
and this
NSArray* a = [[NSArray alloc] initWithObjects:#"foo", #"bar", nil];
//...
[a release];
With the first style, the static method performs an alloc/init/autorelease on it for you so you don't have to. With the second style, you have more control over when the memory is released instead of automatically releasing when you exit the current block.
That code will crash your application. The second version only copies the pointer then releases the instance. You need to call [object retain] before releasing the reference.
Related
If I have an nsarray full of custom objects and I make a second array using:
NSArray *temp = [NSArray arrayWithArray:original];
then work with some properties of the objects inside the original array, then decide to roll back, I am then using the reverse:
original = [NSArray arrayWithArray:temp];
I am finding the objects I changed in the array also effected my temp array. I also tried implementing copyWithZone on my custom class, and using copyItems and it did not help. What else should I try?
To be clear, in order to use copyWithZone, I changed my array creation command to:
NSArray *temp = [[NSArray alloc] initWithArray:original copyItems:YES];
My copyWithZone:
-(id)copyWithZone:(NSZone *)zone{
CustomObject *ret = [[CustomObject allocWithZone: zone] init];
//copy properties
return ret;
}
I'm beginning to get my head round all the memory management stuff, but I'm a bit puzzled by the use of properties with arrays. If I declare the property in the interface like so -
#property (nonatomic,retain) NSMutableArray *myArray;
then synthesize it in the implementation, do I need to alloc it when I create the array? Like so -
self.myArray = [[NSMutableArray alloc] init];
or does this result in an extra retain count? Should I just do -
self.myArray = [NSMutableArray array];
and let the setter do the retaining?
Many thanks to anyone who can clarify this for me!
In both cases you are letting the setter retain your instance.
In this case you are overretaining:
self.myArray = [[NSMutableArray alloc] init];
The setter does and the alloc message sent.
This can be fixed with:
self.myArray = [[[NSMutableArray alloc] init] autorelease];
or
NSMutableArray *newInstance = [[NSMutableArray alloc] init];
self.myArray = newInstance;
[newInstance release];
This is fine
self.myArray = [NSMutableArray array];
however not every class has a convenience class method to return an autoreleased instance.
Have a look at the Memory Management Programming Guide / Object Ownership and Disposal, this will give you a good understanding about when the retain counts increases and when you should release.
I just ran my app through the Leaks in Instruments and I am being told that the following code causes leaks, but I don't see how.
I allocate some NSMutableArrays in my viewDidLoad with this code:
- (void)viewDidLoad {
[super viewDidLoad];
self.currentCars = [[NSMutableArray alloc] init];
self.expiredCars = [[NSMutableArray alloc] init];
}
Then I populate these arrays inside of my viewWillAppear method with the following:
[self.currentCars removeAllObjects];
[self.expiredCars removeAllObjects];
for (Car *car in [self.dealership cars]) {
if ([car isCurrent])
[self.currentCars addObject:car];
if ([car isExpired])
[self.expiredCars addObject:car];
}
And later in the code I release these arrays here:
- (void) viewWillDisappear:(BOOL)animated {
if (currentCars != nil) {
[currentCars release], currentCars = nil;
}
if (expiredCars != nil) {
[expiredCars release], expiredCars = nil;
}
[super viewWillDisappear:animated];
}
Any ideas? Thanks!
Your leak is here:
self.currentCars = [[NSMutableArray alloc] init];
self.expiredCars = [[NSMutableArray alloc] init];
Assuming that you declared property accessores like this:
#property(nonatomic, retain) NSMutableArray *currentCars;
#property(nonatomic, retain) NSMutableArray *expiredCars;
In my opinion, the best way to find leaks (other than using Instruments) is to keep track of the retain count manually.
If you were to do that with for example currentCars, you would find your leak easily. Here is what happens:
self.currentCars = [[NSMutableArray alloc] init];
// The 'init' makes the retain count 1.
// 'self.currentCars = ..' translates to the setCurrentCars: method.
// You probably did not implement that method yourself,
// but by synthesizing your property it is automatically implemented like this:
- (void)setCurrentCars:(NSMutableArray *)array {
[array retain]; // Makes the retain count 2
[currentCars release];
currentCars = array;
}
// In your viewWillDisappear: method
[currentCars release], currentCars = nil; // Makes the retain count 1 so the object is leaked.
The solution is simple. Use this:
NSMutableArray *tempMutableArray = [[NSMutableArray alloc] init];
self.currentCars = tempMutableArray;
[tempMutableArray release];
A little sidenote. You shouldn't release your objects in viewWillDisappear:. The recommended place to do that is dealloc. So your code would be:
- (void)dealloc {
[currentCars release], currentCars = nil;
[expiredCars release], expiredCars = nil;
[super dealloc];
}
The problem is (probably) that you are using the property accessors for the initial setting of the arrays in -viewDidLoad. Since well-implemented property accessors will retain the object, you are getting 1 retain from the +alloc and another retain from assigning it. To fix this, you should release your arrays after assigning them or use [NSMutableArray array] to get an autoreleased one to use for your initial assignments.
Unless you're doing something very odd in currentCars, expiredCars, dealership or cars, no, there's no leak there.
Instruments' pointer to the location of a leak isn't necessarily where the object is actually leaked, per se. If I were to guess, I'd say you're probably neglecting to release either currentCars or expiredCars in your dealloc method.
Does the release, recursively releases all inner objects? or must it be done manualy?
Can I do just this?
NSMutableArray *list = [[NSArray alloc] init];
// ...
// fill list with elements
//...
[list release];
Or must I release all inner objects one by one before releasing the NSMutableArray? // Suposing there isn't any other reference to the contained objects, except on the list itself.
Yes it does. It retains them when added, and releases them when dealloc'd. This is actually one of the most common questions I see here.
If you are owning the object then you will have to release it.
NSMutableArray *list = [[NSArray alloc] init];
NSString *str = [[NSString alloc] init] // you are the owner of this object
[list addObject:str];
[str release]; // release the object after using it
[list release];
If you are not the owner of the object then you should not release.
NSMutableArray *list = [[NSArray alloc] init];
NSString *str = [NSString string]; // you are not owning this object
[list addObject:str]; // str retain count is incremented
[list release]; // str retain count is decremented.
This is the concept which even array also uses. When you add any object to the array, array will retain it. In the sense it becomes the owner of that object and It will release that object when you release the array.
I have the following method which is spawned by a call for a new thread (using NSThread):
- (void) updateFMLs {
NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
NSArray *temp = [[NSArray alloc] initWithArray:someArrayFromAnotherProcess];
[self performSelectorOnMainThread:#selector(doneLoading:) withObject:temp waitUntilDone:NO];
[pool release];
}
My doneLoading: method looks like this:
- (void) doneLoading:(NSArray *)obj {
myArray = [[NSArray alloc] initWithArray:obj copyItems:NO];
}
The contents of myArray become invalid. How can I preserve the contents of myArray so I can use them later in my app?
P.S. myArray is defined in the class header file.
If your background thread does some work and needs to 'pass' an NSArray to your main thread, then all doneLoading needs to do is:
-(void)doneLoading:(NSArray *)obj
{
[myArray release]; // release the previous array and its objects
myArray = [obj retain];
// now use myArray, refresh tables, etc.
}
There's (likely) no need to make another copy of the array, and that might be the underlying issue. You should also call [temp release] after your performSelector call, since arguments to that are retained already.
If the contents of myArray are becoming valid somehow, then they are being doubly released somewhere. myArray will retain any objects that are added to it. You mentioned that myArray itself is becoming invalid, so try rewriting your background thread and your doneLoading method with this pattern.
Finally you should use [pool drain] in place of [pool release].
The code you posted looks fine, apart from the memory leak in updateFMLs. You're probably over-releasing the objects somewhere else. I'm guessing it would be wherever someArrayFromAnotherProcess is made.
An alternative to the options above would be to declare myArray as an atomic property in the header
#property (atomic,retain) NSArray *myArray;
Then in updateFMLs you should be able to simply call the setter from the secondary thread. Obviously this only works if you are willing to pay the performance penalty for an atomic property.
- (void) updateFMLs {
NSAutoreleasePool *pool - [[NSAutoreleasePool alloc] init];
NSArray *temp = [[NSArray alloc] initWithArray:someArrayFromAnotherProcess];
[self setMyArray:temp];
[temp release];
[pool drain];
}