I am implementing an object reuse scheme using a singleton class.
What I do basically is:
MyClass* obj = [[MyClassBank sharedBank] getReusableItem];
The bank is just an NSMutableSet tweaked for optimum reusability. When I was happily implementing this Singleton, I had in mind that I will just do the following when I am done with "obj":
[[MyClassBank sharedBank] doneWithItem:obj];
Currently, My code would work if I where to use it this way, but I later realized that I sometimes add "obj" to an "NSCollection", and sometimes I call:
[theCollection removeAllObjects];
At first I thought about making my own class that is composed of a collection, then I would iterate the objects within the collection and call:
[[MyClassBank sharedBank] doneWithItem:obj];
But, that's too much of a hassle, isn't?
A neat idea (I think) popped into my mind, which is to override: -(oneway void)release;, so, I immediately jumped to Apple's documentation, but got stuck with the following:
You would only implement this method to define your own reference-counting scheme. Such implementations should not invoke the inherited method; that is, they should not include a release message to super.
Ao, I was reluctant to do that idea .. basically:
-(oneway void)release{
if ([self retainCount] == 1) {
//This will increment retain count by adding self to the collection.
[[MyClassBank sharedBank] doneWithItem:self];
}
[super release];
}
Is it safe to do that?
PS: Sorry for the long post, I want the whole idea to be clear..
EDIT:
How about overriding alloc alltogther and adding [[MyClassBank sharedBank] getReusableItem]; there?
Suggested method:
You're playing with the reference counting system. 99.9999999999999999% of the time this is a bad idea. I would highly recommend going with a different mechanism. Perhaps these objects could implement their own reference count that's independent of the retainCount? Then you could use that referenceCount to actually control when an object is ready to be re-used or not.
Not suggested method:
If, for some weird reason, you can't do that, then you could do the following thing that is still a bad idea and that i don't recommend you actually use:
You can override dealloc:
- (void)dealloc {
[ivar release], ivar = nil;
[anotherIvar release], anotherIvar = nil;
somePrimitive = 0;
// do not call [super dealloc]
}
- (void)_reallyDealloc {
[self dealloc]; // clean up any ivars declared at this level
[super dealloc]; // then continue on up the chain
}
Basically, the dealloc method would be the point at which the object is ready for re-use. When you're totally done with the object and finally want it to go away, you can use the _reallyDealloc method to continue on up the chain, eventually resulting in the object getting freed.
PLEASE don't do this. With things like Automatic Reference Counting, this is going to introduce you into a world of hurt and really bizarre debugging scenarios. A lot of the tools and classes and stuff depend on the reference counting mechanism to be working without alteration, so screwing around with it is usually not a Good Idea™.
For ppl who find this approach interesting/useful, Here is a cleaner way than calling [super dealloc]; directly (which is definitely bad)
//BAD!
//-(void)dealloc{
// for some reason, the retainCount at this point == 1
// if (![[BankStep sharedBank] purgeFlag]) {
// [self resetObject];
// [[BankStep sharedBank] doneWithItem:self];
// } else {
// [children release];
// [super dealloc];
// }
//}
by calling [[Bank sharedBank] purgeBank]; , set the flag to true, then remove all objects from the NSSet.
Adapted solution:
#Joe Osborn idea of using categories to implement a returnToBank Method!
Related
Question
In my ARC project I have a class that manages objects, called LazyMutableArray. Some of the objects are actually nil, but users of my collection will never know about this; therefore I made it a subclass of NSMutableArray, and it tries to do "the same thing". In particular, objects are retained when added.
Now let's take a look at a memory behavior of other methods. It turns out that the NSArray destruction methods are documented by Apple to be an exception to this rule, in that they release, not autoreleased object.
There is some debate as to whether the combination of addObject: + objectAtIndex: + array destruction is documented by Apple to be never autoreleasing or simply happens to be in the examples I tested and in the example Apple includes.
How can I create in my subclass a method with exact same memory semantics?
Last update
After some thought, I've decided implementation based on NSMutableArray is more appropriate in this case compared to NSPointerArray. The new class, I should note, has the same retain/autorelease pair as the previous implementation.
Thanks to Rob Napier I see that no modification of my objectAtIndex: method would change this behavior, which answers my original question about this method.
On a practical level, several people said that any method can tackle an extra retain/autorelease pair for no reason; it's not reasonable to expect otherwise and not reasonable to try to find out which methods do this and which do not. It's been therefore a great learning opportunity for me on several levels.
Code (based on NSMutableArray) is available at GitHub: implementation, header, test (that's -testLazyMutableMemorySemantics).
Thank you all for participating.
Why I try to subclass NSMutableArray:
Subclassing foundation objects, I agree, is not always an appropriate solution. In tho case I have objects (in fact, OData resources), most of which have subobjects. The most natural class for an array of subobjects is obviously NSArray. Using a different class doesn't seem to make sense to me.
But for an OData collection this "array of sub objects", while, being an NSArray, must have a different implementation. Specifically, for a collection of 1000 elements, servers are encouraged to return collection in batches of (say)20, instead of all at once. If there is another pattern appropriate in this case, I'm all ears.
Some more detail in how I found this
I unit test the hell out of this collection, and values can be put into array, read from the array, and so forth. So far, so good. However, I realized that returning the object increases its retain count.
How do I see it? Suppose I insert two objects into lazy array lazy, one held weakly, one held strongly (*see the code *). Then retain count of weakSingleton is, as expected, 1. But now I read element:
XCTAssertEqual(weakSingleton, lazy[0], #"Correct element storage"); // line B
And in the debugger I see the retain count go up to 2. Of course, -retainCount may give me wrong information, so let's try to destroy the reference in array by
lazy[0] = nil; // yep, does the right thing
XCTAssertNil(weakSingleton, #"Dropped by lazy array"); // line C <-- FAIL
indeed, we see that weakSingleton is not released.
By now you probably guess that it's not just a retain, it's an autoreleased retain — putting an #autorelease around line B releases the weakSingleton. The exact source of this pair is not obvious, but seems to come from NSPointerArray -addPointer: (and unfortunately not from ARC's [[object retain] autorelease]). However, I don't want to return an autoreleased object and make method semantics different from its superclass!
After all, the method I'm overriding, NSMutableArray -objectAtIndex:`, doesn't do that; the object it returns will dealloc immediately if an array is released, as noted in the Apple's example. That's what I want: modify the method around line A so that the object it returns does not have an extra retain/autorelease pair. I'm not sure the compiler should even let me do it :)
Note 1 I could turn off ARC for a single file, but this would be my first non-ARC Objective-C code. And in any case the behavior may not some from ARC.
Note 2 What the fuss? Well, in this case I could change my unit tests, but still, the fact is that by adding or deleting line B, I'm changing the result of unit test at line C.
In other words, the described behavior of my method [LazyMutableArray -objectAtIndex] is essentially that by reading an object at index 0, I'm actually changing the retain count of this object, which means I could encounter unexpected bugs.
Note 3 Of course, if nothing is to be done about this, I'll document this behavior and move on; perhaps, this indeed should be considered an implementation detail, not to be included into tests.
Relevant methods from implementation
#implementation LazyMutableArray {
NSPointerArray *_objects;
// Created lazily, only on -setCount:, insert/add object.
}
- (id)objectAtIndex:(NSUInteger)index {
#synchronized(self) {
if (index >= self.count) {
return nil;
}
__weak id object = [_objects pointerAtIndex:index];
if (object) {
return object;
}
}
// otherwise do something else to compute a return value
// but this branch is never called in this test
[self.delegate array:self missingObjectAtIndex:index];
#synchronized(self) {
if (index >= self.count) {
return nil;
}
__weak id object = [_objects pointerAtIndex:index];
if (object) {
return object;
}
}
#throw([NSException exceptionWithName:NSObjectNotAvailableException
reason:#"Delegate was not able to provide a non-nil element to a lazy array"
userInfo:nil]);
}
- (void)createObjects {
if (!_objects) {
_objects = [NSPointerArray strongObjectsPointerArray];
}
}
- (void)addObject:(id)anObject {
[self createObjects];
[_objects addPointer:(__bridge void*)anObject];
}
The complete test code:
// Insert two objects into lazy array, one held weakly, one held strongly.
NSMutableArray * lazy = [LazyMutableArray new];
id singleton = [NSMutableArray new];
[lazy addObject:singleton];
__weak id weakSingleton = singleton;
singleton = [NSMutableDictionary new];
[lazy addObject:singleton];
XCTAssertNotNil(weakSingleton, #"Held by lazy array");
XCTAssertTrue(lazy.count == 2, #"Cleaning and adding objects");
// #autoreleasepool {
XCTAssertEqual(weakSingleton, lazy[0], #"Correct element storage");
XCTAssertEqual(singleton, lazy[1], #"Correct element storage");
// }
lazy = nil;
XCTAssertNotNil(singleton, #"Not dropped by lazy array");
XCTAssertNil(weakSingleton, #"Dropped by lazy array");
The last line fails, but it succeeds if I change first line to lazy = [NSMutableArray new] or if I uncomment #autoreleasepool.
First, I would not make this subclass. This is exactly what NSPointerArray is for. Wrapping that into an NSArray obscures important details that this approach can break. For example, what is the correct behavior for [NSArray arrayWithArray:lazyMutableArray] if lazyMutableArray includes NULLs? Algorithms that assume that NSArray can never include NULL need to be wary of the fact that this one can. It's true that you can get similar issues treating a non-retaining CFArray as an NSArray; I speak from experience that this is exactly why this kind of subclass can be very dangerous (and why I stopped doing that years ago). Don't create a subclass that cannot be used in every case that its superclass can be used (LSP).
If you have a collection with new semantics, I would subclass it from NSObject, and have it conform to <NSFastEnumeration>. See how NSPointerArray is not a subclass of NSArray. This was not an accident. Faced with the same problem, note the direction Apple chose.
By now you probably guess that it's not just a retain, it's an autoreleased retain — putting an #autorelease around line B releases the weakSingleton. This seems to be because line A under ARC translates to [[object retain] autorelease]. However, I don't want to return an autoreleased object and make caller remember this!
The caller should never assume anything else. The caller is never free to assume that a method does not add balanced autoreleases. If a caller wants the autorelease pool to drain, that is their responsibility.
All that said, there is some benefit to avoiding an extra autorelease if it's not required, and it's an interesting learning opportunity.
I would start by reducing this code to the simplest form, without your subclass at all. Just explore how NSPointerArray works:
__weak id weakobject;
#autoreleasepool
{
NSPointerArray *parray = [NSPointerArray strongObjectsPointerArray];
{
id object = [NSObject new];
[parray addPointer:(__bridge void*)object];
weakobject = object;
}
parray = nil;
}
NSAssert(!weakobject, #"weakobject still exists");
My structure here (such as the extra nesting block) is designed to try to avoid accidentally creating strong references I don't mean to make.
In my experiments, this fails without the autoreleasepool and succeeds with it. That indicates that the extra retain/autorelease is being added around or by the call to addPointer:, not by ARC modifying your interface.
If you're not using this implementation for addObject:, I'd be interested in digging deeper. It is an interesting question, even if I don't believe you should be subclassing this way.
I'm going to elaborate on why I said this "looks a lot like a homework assignment." This will likely earn me many down votes, but it will also server as a good learning case for others who later find this question.
Subclassing NSMutableArray not a goal of a program. It is a means to achieve something else. If I were to venture a guess, I expect you were trying to create an array that lazily creates the object when they are accessed. There are better ways to do this without dealing with memory management yourself.
Here's an example of how I would implement a lazy loading array.
#interface LazyMutableArray : NSMutableArray
- (id)initWithCreator:(id(^)(int))creator;
#end
#interface LazyMutableArray ( )
#property (nonatomic, copy) id (^creator)(int);
#property (nonatomic, assign) NSUInteger highestSet;
#end
#implementation LazyMutableArray
- (id)initWithCreator:(id(^)(int))creator
{
self = [super init];
if (self) {
self.highestSet = NSNotFound;
self.creator = creator;
}
return self;
}
- (id)objectAtIndex:(NSUInteger)index
{
id obj = nil;
if ((index < self.highestSet) && (self.highestSet != NSNotFound)) {
obj = [super objectAtIndex:index];
if ([obj isKindOfClass:[NSNull class]]) {
obj = self.creator(index);
[super replaceObjectAtIndex:index withObject:obj];
}
} else {
if (self.highestSet == NSNotFound) {
self.highestSet = 0;
}
while (self.highestSet < index) {
[super add:[NSNull null]];
self.highestSet += 1;
}
obj = self.creator(index);
[super add:obj];
self.highestSet += 1;
}
return obj;
}
Fair Warning: I'm not compiling or syntax checking any of this code. It probably has a few bugs in it, but it should generally work. Additionally, this implementation is missing an implementation of add:, count, removeObjectAtIndex:, insertObject:atIndex:, and possibly replaceObjectAtIndex:withObject:. What I show here is just to get you started.
In objective c, suppose I have an object Obj stored in a NSMutableArray, and the array's pointer to it is the only strong pointer to Obj in the entire program. Now suppose I call a method on Obj and I run this method in another thread. In this method, if Obj sets the pointer for itself equal to nil will it essentially delete itself? (Because there will be no more strong pointers left) I suspect the answer is no, but why? If this does work, is it bad coding practice (I assume its not good coding, but is it actually bad?)
It is highly unlikely that an object would be in a position to cause its own release/deallocation if your code is designed properly. So yes, the situation you describe is indicative of bad coding practice, and can in fact cause the program to crash. Here is an example:
#interface Widget : NSObject
#property (retain) NSMutableArray *array;
#end
#implementation Widget
#synthesize array;
- (id)init
{
self = [super init];
if(self) {
array = [[NSMutableArray alloc] init];
[array addObject:self];
}
return self;
}
- (void)dealloc
{
NSLog(#"Deallocating!");
[array release];
[super dealloc];
}
- (void)removeSelf
{
NSLog(#"%d", [array count]);
[array removeObject:self];
NSLog(#"%d", [array count]);
}
#end
and then this code is in another class:
Widget *myWidget = [[Widget alloc] init];
[myWidget release]; // WHOOPS!
[myWidget removeSelf];
The second call to NSLog in removeSelf will cause an EXC_BAD_ACCESS due to the fact that array has been deallocated at that point and can't have methods called on it.
There are at least a couple mistakes here. The one that ultimately causes the crash is the fact that whatever class is creating and using the myWidget object releases it before it is finished using it (to call removeSelf). Without this mistake, the code would run fine. However, MyWidget shouldn't have an instance variable that creates a strong reference to itself in the first place, as this creates a retain cycle. If someone tried to release myWidget without first calling removeSelf, nothing would be deallocated and you'd probably have a memory leak.
If your back-pointer is weak (which it should be since a class should never try to own it's owner, you will end up with a retain-cycle) and you remove the strong pointer from the array the object will be removed from the heap. No strong pointers = removed from memory.
You can always test this.
If you need a class to bring to a situation where its deleted, the best practice is to first retain/autorelease it and then make the situation happen. In this case the class won't be deleted in a middle of its method, but only afterwards.
I think we can say it might be bad coding practice, depending on how you do it. There are ways you could arrange to do it safely, or probably safely.
So let's assume we have a global:
NSMutableArray *GlobalStore;
One approach is to remove yourself as your final action:
- (void) someMethod
{
...
[GlobalStore removeObject:self];
}
As this is the final action there should be no future uses of self and all should be well, probably...
Other options include scheduling the removal with a time delay of 0 - which means it will fire next time around the run loop (only works of course if you have a run loop, which in a thread you may not). This should always be safe.
You can also have an object keep a reference to itself, which produces a cycle and so will keep it alive. When its ready to die it can nil out its own reference, if there are no other references and that is a final action (or a scheduled action by another object) then the object is dead.
in my project I'm managing several Drawing objects. I'm trying to add a copy of a SmartPath object to a drawing object. This works. But when the drawing gets deallocated the SmartPath does not. I have put some extra code in the dealloc of the Drawing to explicitly set clear the pointer to the SmartPath. For some reason this works (the retain count was 1). I know I can probably copy the SmartPath and assign that to a strong parameter to fix this leak. But I'm relatively new to IOS and want to know how to use the copy parameters properly in combination with ARC.
Here is the code:
Drawing.h:
#interface Drawing : NSObject{
#private
SmartPath* rawLinePath; //path that is build up from straight lines between input points
SmartPath* smoothLinePath; //smoothened version of rawLinePath
}
#property(atomic,copy)SmartPath* rawLinePath;
#property(atomic,copy)SmartPath* smoothLinePath;
Drawing.m
#implementation Drawing
#synthesize rawLinePath;
#synthesize smoothLinePath;
-(id)init
{
if (self = [super init])
{
[NSThread detachNewThreadSelector:#selector(pointMonitor)
toTarget:self
withObject:nil];
}
return self;
}
-(void)dealloc{
rawLinePath=nil;
}
SmartPath.m
/*
* Init - set all variables in the correct state
*/
-(id)init
{
if (self = [super init])
{
visible=TRUE;
color = [UIColor redColor].CGColor;
width = SMARTPATH_LINE_WIDTH;
path = CGPathCreateMutable();
lock = [[NSLock alloc]init];
}
return self;
}
/*
* dealloc - clean up after self
*/
-(void)dealloc{
CGPathRelease(path);
}
/*
* copy method to be able to pass a SmartPath to a copy property
*/
-(id)copyWithZone:(NSZone *)zone{
SmartPath *pathCopy = [[SmartPath allocWithZone: zone] init];
pathCopy.visible =visible;
pathCopy.color = color;
pathCopy.width = width;
return pathCopy;
}
I hope any of you knows the answer to this problem.
Best regards
Your problem is your call to -detachNewThreadSelector:toTarget:withObject:. This retains target, which is self, and does not release it until pointMonitor exits. I suspect this never happens, so you've effectively created a retain loop.
You should almost never use -detachNewThreadSelector:toTarget:withObject:. It can create an unbounded number of threads. Instead, you should generally use dispatch queues, NSTimer, NSOperation or other async mechanisms. NSThread objects are generally only appropriate for long-lived producer/consumer threads (and usually those are still handled better with the newer tools like dispatch queues).
I'm not certain what pointMonitor does, but is there any reason it needs its own thread at all? You can do a lot of very good Cocoa development and never fork a thread. Could you use an NSTimer here? Note that most of these techniques retain their target until they fire (just like NSThread). If they didn't, you'd crash when they fired.
Without knowing what you're trying to do, I'm not certain which approach to recommend. You may want to put that together as a new question.
By not starting instance variable names with an underscore, you end up with code where you never know whether you are using an accessor method or an instance variable. As a result, you can never be sure whether a copy is made or not.
If you do that in other places, there's a good chance that a reference to your SmartPath object gets stuck somewhere. And what are you doing creating NSLock objects? Do you need to do anything that #synchronized can't do with much less code?
And if you use a newer Xcode version, get rid of all the instance variables and #synthesize statements. Just declare the properties.
And excuse me, but detaching a thread from an init method is just sick.
Stick with me. I'm visually impaired, have never used this site before, and will probably not post this in precisely the format that you are all used to. I apologize for any unintentional faux pas's herein.
Using Objective-C in an iOS project…
I have a singleton class, set up in what appears to be the usual way for Objective-C. It is, in the main, a series of methods which accept NSString values, interprets them, and return something else. In the code below, I'm simplifying things to the barest minimum, to emphasize the problem I am having.
From the singleton class:
- (NSUInteger) assignControlState:(NSString *)state {
// excerpted for clarity...
return UIControlStateNormal; // an example of what might be returned
}
Now, an instance of another class tries to use this method like so:
- (void) buttonSetup:(UIButton*)button {
[button setTitle:#"something" forState:[[SingletonClass accessToInstance] assignControlState:#"normal"]];
}
This code actually works. HOwever, when the system goes to draw the UI which includes the button whose title was set in this way, an EXC_BAD_ACCESS error occurs.
If the assignControlState method is moved into the same class as the buttonSetup method, no error is generated.
I'm guessing this is something about Apple's memory management that I'm not fully understanding, and how things go in and out of scope, but for the life of me, I can't figure out where I'm going wrong.
HOpe someone can help. Thanks.
The problem is in your accessToInstance method. I'll bet you are under-retaining. The implementation should be more like this:
static SingletonClass *sSingletonClass = nil;
#implementation
+ (id)accessToInstance {
if (sSingletonClass == nil) {
sSingletonClass = [[[self class] alloc] init];
}
return sSingletonClass;
}
#end
Now, if your program is following normal memory management rules, the singleton will stay around. You can check by writing:
- (void)dealloc {
[super dealloc]; // <-- set a breakpoint here.
}
If the debugger ever stops at this breakpoint, you know something in your program has over-released the singleton.
You know that bit you excerpted for clarity? I think you need to show us what it is because there's probably an over release in it somewhere.
Specifically, I think you release an autoreleased object. If you do that and don't use the object again, everything will carry on normally until the autorelease pool gets drained. The autorelease pool gets drained automatically at the end of the event at about the same time as the drawing normally occurs.
That would also explain the delayed crash following the NSLogs.
I'm fairly new to Cocoa and Objective-C. Currently I'm developing a fairly basic application to test my knowledge and put some of the stuff I've been reading about into practice. Everything is working, but Leaks reports a number of issues.
None of these leaks seems to be directly applicable to code that I've written (I have read and tried to follow Apple's rules on memory allocation). Currently my project makes use of Garbage Collection and I'm developing on Snow Leopard. Running AnalysisTool finds no issues with my code (aside from a few naming convention warnings).
Currently my application makes use of an NSTableView which I have hooked up to an NSArrayController. Interacting with the NSTableView seems to cause leaks to report issues (actions such as sorting table columns and other standard user interaction). This leads me to believe that my use of the NSArrayController (and my implementation of its content source) is to blame.
Currently the NSArrayController receives its content from an NSMutableArray (timers) handled in my Application's delegate like so:
- (id) init
{
if (self = [super init])
{
timers = [NSMutableArray array];
}
return self;
}
- (void) dealloc
{
[timers release];
[super dealloc];
}
Within Interface Builder my NSArrayController has its Object Controller set to the Timing class, which is defined below:
#interface Timing : NSObject {
NSString *desc;
NSDate *timestamp;
bool active;
}
#end
#implementation Timing
-(id) init
{
if (self = [super init])
{
desc = #"New timing";
timestamp = [[NSDate alloc] init];
active = false;
}
return self;
}
-(void) dealloc
{
[timestamp release];
[super dealloc];
}
#end
I've used standard Cocoa bindings to hook up Add and Remove buttons to manipulate the TableView and these seem to work correctly (clicking Add will create a row in the TableView with the value of 'New timing', for instance).
Leaks reports that the libraries responsible are AppKit and CoreGraphics. Although, honestly, I'm still new to the Leaks tool - so I could be reading its output incorrectly. If it helps, I've placed a screenshot of its output here. If anyone could point me in the right direction, that would really be appreciated.
As an aside, I've also been experimenting with manually adding objects to the timers array without the use of Cocoa bindings. Here's what I came up with:
Timing *timingInstance = [[Timing alloc] init];
[timers addObject:timingInstance];
[timingInstance release];
[timersController setContent:timers];
[timersTableView reloadData];
Again, this seems to work, but I thought it best to ask the experts!
Your memory management for the timers array is not quite correct. Using the array factory method will return an instance of NSMutableArray that has already been autoreleased, so the lifetime of that object is (probably) limited to the end of the current run loop, and it will be over-released when you call release in your dealloc method. The proper way to do it is as follows:
- (id) init
{
if (self = [super init])
{
timers = [[NSMutableArray alloc] initWithCapacity:0];
}
return self;
}
This method will give you an instance of NSMutableArray with a retain count of 1, which will then drop to zero (and properly release the memory) when you call release in your dealloc method. The call to alloc in your init method is balanced out by the call to release in your dealloc method. I notice that this is the exact pattern that you used for your NSDate object in the Timing class, so you are already familiar with the idea.
Your code as written is not leaking. The Cocoa frameworks will sometimes generate false leak reports when run under leaks, as certain things such as singletons and caches which are used in the implementation of the frameworks will sometimes show up as leaks even though they're not.
You're probably better off running the ObjectAlloc and/or ObjectGraph instruments to get an idea of when your objects are being allocated and deallocated.
Currently my project makes use of
Garbage Collection and I'm developing
on Snow Leopard
I don't understand. You're using garbage collection right? If so then GC takes care of releasing objects for you so anywhere you use "release" does absolutely nothing. Release calls are ignored when GC is enabled. You only use release when you're managing the memory yourself which is when GC is off. Also, your dealloc methods do nothing too. In GC that method is never used. When you create an object and then you finish with it you tell GC that it's OK to get rid of the object by setting it to nil. That's all you have to do. There is no "release" needed or dealloc method needed. Just set things to nil or or not as needed.