I need to store an array of instances of a specific class. Which I can then access from a class method anywhere. (kind of like lots of singletons)
At first I thought to just have a static NSMutableArray and in init add self to the array and in dealloc remove it from the array. But because NSArrays retain their objects, dealloc will never be reached.
I was hoping there would be a function in something like runtime.h, to get all instances of a class, just as you can get a list of all classes.
So how can I keep track of all instances of a specific that are in memory.
Did you consider NSObject * array[3];?
to create a trivial non-retaining array:
NSMutableArray * nonRetainingArray = (NSMutableArray*)CFArrayCreateMutable(0,0,0);
It really doesn't matter if, when your application terminates, there are still lots of singleton objects which haven't been deallocated. Since your array is a singleton, there's no problem with it not going away.
The only problem is whith the objects in the array. If you add them to the array in -init, you can't remove them from the array in -dealloc because -dealloc will never be called. If you just want to keep a reference to all of the live objects of a certain type, you could use an NSPointerArray which is like an array but doesn't retain its elements.
Related
If I have an an atomic property in MyClass:
#property (readwrite, strong) NSMutableArray *atomicArray;
And a thread calls something like:
NSMutableArray* t = [myClassObject atomicArray];
Where is it documented that the object I get back is retained/autoreleased so that another thread can't call:
[myClassObject setAtomicArray:otherArray] and eliminate the object out from under me?
Another question mentioned this is true but without reference to any documentation from Apple.
Atomic properties vs thread-safe in Objective-C
See The Objective-C Programming Language, Property Declaration Attributes, Atomicity:
If you specify strong, copy, or retain and do not specify nonatomic, then in a reference-counted environment, a synthesized get accessor for an object property uses a lock and retains and autoreleases the returned value—the implementation will be similar to the following:
[_internal lock]; // lock using an object-level lock
id result = [[value retain] autorelease];
[_internal unlock];
return result;
If you specify nonatomic, a synthesized accessor for an object property simply returns the value directly.
Assuming you are using ARC, the reference obtained in t is a strong one to the object that existed at the time you retrieved it. If some other thread replaces the object, t would still reference the original object, and that object would stick around until t goes out of scope.
However t being a mutable array, some other thread could get its own reference and modify the mutable array itself by adding and removing objects. Those changes would be visible to your t, because it is the same object.
That is important, because if you try to iterate over the array while some other thread modifies it, you may encounter all kinds of unexpected results. Depending on your Xcode version it might produces warnings during compile and/or runtime to help identify those scenarios. One pattern to avoid the array changing from under you is to create an immutable copy, like NSArray *immutableT=[t copy]. This produces a shallow copy of the array that is immune to changes to the original mutable array, so you can safely iterate over immutableT.
This is a helpful hint, but can you try using Memory Graph Debugger feature of Xcode. Working on strong attributes with multi-threading is always a tedious task.
Memory graph debugger is an excellent tool to find out retain count, who is holding a strong reference of an object, etc.. It helps a lot if you play around with it.
I understand the whole business around reference counting and "owning an object" and that if you allocate an object in Objective-c, it's your responsibility to release it
However when exactly would you need to call alloc on a newly created object? Would it only be to retain the reference after the end of the scope or is there some other reason
You need to call alloc in order to allocate the memory for the object.
The typical setup of an object is something like:
Object *obj = [[Object alloc] init];
The alloc call allocates memory for the object, and the init call initialises it (gives it sensible default values for all attributes/properties).
Some object types come with factory methods, eg
NSArray *arr = [NSArray array];
In this case, the object is initialised and allocated by the single array call.
None of this has anything (directly) to do with reference counting, except that different ownership rules normally apply to the two methods.
I think you are misunderstanding a basic concept. sending alloc to a class will result in creating a new object of that class (not initialized yet) which you own (retain count will be 1).
from your question "when exactly would you need to call alloc on a newly created object?" -
if the object is newly created it means that someone already allocated it..
if you meant: when do you need to call retain on a newly created object? the answer is if you want to hold it yourself and not rely on whomever allocated it, and might release it sometime.. remember that alloc/new syntax raises the retain count by one, where as other creating methods (like [NSArray array]) return autorelease objects..
in general i would recommend using ARC and not be bothered by these issues..
I'm not exactly a objective-c guy, but I don't think you call alloc on any object, you call it on a class to allocate the object and call init on the newly allocated object.
You may want to retain to retain the reference after the release is performed by autorelease pool, if this is your setup. That often happens to the object created using [NSThing thingWithStuff:stuff] or some such.
I'm trying to learn/understand what happens and why when working with or creating various objects. (Hopefully to LEARN from the docs.)
I'm reading "Programming in Objective-C 2.0" (2nd edition, by Steven Kochan). On page 408, in the first paragraph is a discussion of retain counts:
Note that its reference count then goes to 2. The addObject: method does this automatically; if you check your documentation for the addObject: method, you will see this fact described there.
So I read the addObject: docs:
Inserts a given object at the end of the array.
There, the description is missing, while other items, like arrayByAddingObject:, state it:
Returns a new array that is a copy of the receiving array with a given object added to the end.
Where in the reference does it indicate that addObject: increases the retain count? Given the presence of ARC, I should still understand what these methods are doing to avoid bugs and issues. What does ARC bring to this? (Going to read that again...)
Great question, I'm glad to see someone actually reading the docs and trying to understand them!
Since you are looking for how to research answers using Apple's documentation more so than the actual answer itself, here is how I found the answer:
First I look at the class reference for addObject: which is a method of NSMutableArray and there is no mention of memory management.
Then I look at the Overview section at the top... Hmmm, still no luck.
Since the behavior might be inherited from a parent class, I look at the Inherits from section at the top of the class reference and see that NSArray is the most immediate parent. Let's check there:
Under the Overview There is one small section about retain's:
Special Considerations
In most cases your custom NSArray class should conform to Cocoa’s
object-ownership conventions. Thus you must send retain to each object
that you add to your collection and release to each object that you
remove from the collection. Of course, if the reason for subclassing
NSArray is to implement object-retention behavior different from the
norm (for example, a non-retaining array), then you can ignore this
requirement.
Okay, I'm still not happy... Where next? The parent class of NSArray is NSObject and I know that it won't be covered there in this case (from experience) so I won't bother checking that. (If the parent was another class or something that might be covered by NSObject, I would keep moving up the tree until I found something.)
The Companion Guides usually contains a lot of good information for these types of classes. Let's try the first one, Collections Programming Topics.
The first section (after Overview) is Accessing Indexes and Easily Enumerating Elements: Arrays. Sounds promising! Click on Relevant Chapters: “Arrays: Ordered Collections”
There it is under Array Fundamentals along with a link to even more information:
And when you add an object to an NSMutableArray object, the object
isn’t copied, (unless you pass YES as the argument to
initWithArray:copyItems:). Rather, an object is added directly to an
array. In a managed memory environment, an object receives a retain
message when it’s added; in a garbage collected environment, it is
strongly referenced. When an array is deallocated in a managed memory
environment, each element is sent a release message. For more
information on copying and memory management, see “Copying
Collections.”
The book must be referring to out of date documentation because you are correct it doesn't mention anything about the retain count. It does in fact retain the object though. The way you need to think of it is not in terms of retain counts (which are useless) but rather ownership. Especially so when using ARC.
When you add an object to an NSMutableArray, it is taking ownership of that object (in ARC terminology it has a strong reference to it).
"What does ARC bring to this?"
ARC does nothing different. All ARC does (besides some optimization) is add the same release, retain, and autorelease statements that you would add yourself without using ARC. All you need to care about is that once you add an object to the array, it will live at least as long as the array.
And the arrayByAddingObject: method creates a new NSArray (or NSMutableArray) containing the object you're passing, and keeps a strong reference to the passed object. The actual array object that it creates has no references yet unless you assign it to either an ivar, property, or local variable. What you assign it to determines it's lifespan.
Basically even without ARC, it's best to think of object life-cycles in terms of ownership, ARC just formalizes that. So because of that, when using the frameworks, it doesn't matter when retains happen or don't happen, you are only responsible for your objects until you pass ownership to another object and you can trust that the framework will keep the object alive as long as it needs it.
Now of course you have to intuit what constitutes ownership. For instance delegate properties are often assign, or in ARC unsafe_unretained or weak, to prevent circular retains cycles (where two objects each retain each other), though are sometimes retained/strong so you need to look into those on a case by case basis.
And also in cases like key value observing and NSNotification observing the object you are observing does not retain the observer.
But those are really exceptions to the rule. Generally you can assume a strong reference.
Regarding this sentence above: "The actual array object that it creates has no references yet unless you assign it to either an ivar, property, or local variable. What you assign it to determines it's lifespan." I'll try to explain:
When you run this piece of code: [someArray arrayByAddingObject:someObject]; you've instantiated a new NSArray or NSMutableArray object (depending on which object type someArray is) but you haven't actually assigned it to any reference. That means that if you're using ARC, it may be immediately released afterwards, or if not using ARC, it will be released when it's autoreleasepool is drained (probably on the next iteration of that thread's runloop).
Now if instead you did this: NSArray *someOtherArray = [someArray arrayByAddingObject:someObject]; you now have a reference to the newly created array, called someOtherArray. In this case, this is a local variable who's scope is only within whichever set of { } it resides (so it could be inside an if statement, a loop, or a method. Now if you do nothing else with it, it will die sometime after it's scope ends (it isn't guaranteed to die right away, but that isn't important, you just can't assume it lives longer).
Now if in your class you have an iVar (instance variable) declared in the header like NSArray *someOtherArray; (which is strong by default in ARC) and you run someOtherArray = [someArray arrayByAddingObject:someObject]; somewhere in your class, the object will live until you either remove the reference (someOtherArray = nil), you overwrite the reference (someOtherArray = someThirdArray), or the class is deallocated. If you were not using ARC, you would have to make sure to retain that to achieve the same effect (someOtherArray = [[someArray arrayByAddingObject:someObject] retain]; which is essentially what ARC is doing behind the scenes).
Or you may have a property declared instead like #property (nonatomic, strong) NSArray *someOtherArray in which self.someOtherArray = [someArray arrayByAddingObject:someObject]; would achieve the same effect but would use the proprety accessor (setSomeOtherArray:) or you could still use someOtherArray = [someArray arrayByAddingObject:someObject]; to set the iVar directly (assuming you #synthesized it).
Or assuming non-ARC, you might have declared the property like #property (nonatomic, retain) NSArray *someOtherArray in which self.someOtherArray = [someArray arrayByAddingObject:someObject]; would behave exactly as ARC would, but when setting the iVar directly you would still need to add that retain manually.
I hope that clears things up a bit, please let me know if there's anything I glossed over or left out.
As you mentioned in your comment, the key here is intuitively knowing when an object would be considered owned by another one or not. Luckily, the Cocoa frameworks follow a pretty strict set of conventions that allow you to make safe assumptions:
When setting an NSString property of a framework object (say the text property of a UILabel for example) it is always copied (if anyone knows of a counter-example, please comment or edit). So you don't have to worry about your string once you pass it. Strings are copied to prevent a mutable string from being changed after it's passed.
When setting any other property other than delegate, it's (almost?) always retained (or strong reference in ARC)
When setting delegate properties, it's (almost?) always an assign (or weak reference) to prevent circular retain cycles. (For instance, object a has a property b that is strong referenced and b has a strong referenced delegate property. You set a as the delegate for b. Now a and b are both strongly referencing each other, and neither object will ever reach a retain count of 0 and will never reach it's dealloc method to dealloc the other object. NSURLConnection is a counter-example that does strongly reference it's delegate, because it's delegate is set via a method -- see that convention below -- and it's convention to nil out or release an NSURLConnection after it completes rather than in dealloc, which will remove the circular retain)
When adding to an array or dictionary, it's always retained (or strong reference).
When calling a method and passing block(s), they are always copied to move them from the stack (where they are initially created for performance purposes) into the heap.
Methods that take in object parameters and don't return a result immediately are (always? I can't think of any that don't) either copying or retaining (strong referencing) the parameters that you pass to ensure that the method can do what it needs to with them. For instance, NSURLConnection even retains it's delegate because it's passed in via a method, whereas when setting the delegate property of other objects will not retain, as that is the convention.
It's suggested that you follow these same conventions in your own classes as well for consistency.
Also, don't forget that the headers of all classes are available to you, so you can easily see whether a property is retain or assign (or strong or weak). You can't check what methods do with their parameters, but there's no need because of the convention that parameters are owned by the receiver.
In general, you should look in the "most global" spot for information about anything in the Cocoa APIs. Since memory management is pervasive across the system APIs and the APIs are consistent in their implementation of the Cocoa memory management policy, you simply need to read and understand the Cocoa memory management guide.
Once understood, you can safely assume that all system APIs implement to that memory management policy unless explicitly documented otherwise.
Thus, for NSMutableArray's addObject: method, it would have to retain the object added to the array or else it would be in violation of that standard policy.
You'll see this throughout the documentation. This prevents every method's documentation from being a page or more long and it makes it obvious when the rare method or class implements something that is, for whatever reason (sometimes not so good), an exception to the rule.
In the "Basic Memory Management Rules" section of the memory management guide:
You can take ownership of an object using retain.
A received object is normally guaranteed to remain valid within the
method it was received in, and that method may also safely return the
object to its invoker. You use retain in two situations: (1) In the
implementation of an accessor method or an init method, to take
ownership of an object you want to store as a property value; and (2)
To prevent an object from being invalidated as a side-effect of some
other operation (as explained in “Avoid Causing Deallocation of
Objects You’re Using”).
(2) is the key; an NS{Mutable}Array must retain any added object(s) exactly because it needs to prevent the added object(s) from being invalidated due to some side-effect. To not do so would be divergent from the above rule and, thus, would be explicitly documented.
I am trying to subclass NSArray, but it crashes the app when trying to access the count method. I know that NSArray is a class cluster.
But what does this mean?
Is there a work around to be able to subclass an NSArray?
I know that I can simply subclass NSObject and have my array as an instance variable but I would rather subclass NSArray.
EDIT:
Reason:
I am creating a card game, I have a class Deck which should subclass NSMutableArray to have a couple of extra methods (-shuffle, -removeObjects:, -renew, etc), and I think it will look cleaner to subclass NSArray rather than having a var.
The problem with adding a category on a class like this is that all instances of the class will inherit the additional methods. That's both unnecessary (since not every array needs to be able to be shuffled, etc.) and dangerous (because you can't benefit from typechecking to be sure the NSArray you're currently referring to is really one that was expected to be shuffled).
An alternative would be to create your own Deck class that has an NSMutableArray as an instance variable. There you can define actions on your deck exactly as you would like, and the fact that you are using an NSMutableArray becomes an implementation detail. This lets you take advantage of typechecking at compile-time and it lets you change the internal implementation of your Deck class without changing its clients. For instance, if you decided for some reason that an NSMutableDictionary would be a better backing store, you can make all those changes within the implementation of your Deck class without changing any of the code that creates and uses the Deck.
You usually won't need to subclass it, but in any case the suggestions made by Apple are:
Any subclass of NSArray must override the primitive instance methods count and objectAtIndex:. These methods must operate on the backing store that you provide for the elements of the collection. For this backing store you can use a static array, a standard NSArray object, or some other data type or mechanism. You may also choose to override, partially or fully, any other NSArray method for which you want to provide an alternative implementation.
Did you actually override countmethod? As they say you have to provide your own backing structure to hold array elements, and override suggested methods considering this..
If you're just adding new methods, and using the existing backing store, then a better approach is to add a category to NSArray. Categories are a really powerful part of objective-C - see cocoadev for some samples.
NSMutableArray already has a - (void)removeObjectsInArray:(NSArray *)otherArray;
You're going to be best off making an NSObject subclass with a mutable array property.
In this particular case, I'd shuffle the array using -sortedArrayUsingComparator: and make your comparator randomly return NSOrderedAscending or NSOrderedDescending.
E.G:
NSArray *originalArray; // wherever you might get this.
NSArray *shuffledArray = [orginalArray sortedArrayUsingComparator:
^(id obj1, id obj2) {
return random() % 2 ? NSOrderedAscending : NSOrderedDescending;
}];
When I make an NSArray using +[NSArray arrayWithObjects:], does it copy those objects? If I release the objects after adding them to the array, will I run into problems?
No, it doesn't copy them. It retains them. Yes, you can safely release the objects after adding them to the array.
The docs, as always, spell this out very clearly:
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. If you want a collection with different object ownership semantics, consider using CFArray Reference, NSPointerArray, or NSHashTable instead.