Question 1
Supposed I have this code:
MyClass * __strong foo = [MyClass new];
MyClass * __strong bar = foo;
// foo = nil; // by ARC?
In reference to this answer, does ARC automatically nil out foo on line 3 since bar already acquired the reference?
Question 2
//ECGService.m
#property (strong) MuttableArray *rriData;
(MuttableArray *)getRriData {
return _rriData;
}
//AlgorithmTest.m
// according to Apple docs, local variable are marked __strong by default
MuttableArray *rriData = [self.ecgService getRriData];
for(NSNumber *rri in rriData) {
// use rri!
}
// rriData = nil;
Should I nil out the local variable rriData after using it?
And, does the __strong attribute must only be applied to only one instance of an object?
Answer 1.
In principle, ARC doesn't “nil out” foo until it goes out of scope. When it goes out of scope, ARC releases its reference. It doesn't have to actually set foo to nil, but the effect is exactly as if ARC does set it to nil.
In practice, ARC is usually allowed to release the reference held by foo immediately after the last use of the foo variable, which may be long before it goes out of scope. In your example, if the only place where you use foo in the function is in the assignment to bar, then ARC is allowed to release foo's reference immediately after that assignment. Note that generally there's no way for you to know about this early release because you don't use foo again! Note also that bar will still reference the object unless bar is also not used later in the function.
There are some special attributes that can prevent ARC from performing this early release, called objc_precise_lifetime and objc_returns_inner_pointer but usually applied through the macros NS_VALID_UNTIL_END_OF_SCOPE and NS_RETURNS_INNER_POINTER. These are advanced features that you probably won't have to worry about any time soon, but if you want to see some examples of NS_RETURNS_INNER_POINTER, take a look at NSString.h.
Answer 2.
You don't need to “nil out” rriData. ARC will release its reference when it goes out of scope (or sooner—see answer 1).
You don't need to say __strong usually, because it's the default, and it's what you want usually. It is normal to have multiple strong references to an object. You use __weak explicitly when you need to prevent a retain cycle. There are many explanations of retain cycles on the web and on stack overflow, so if you need to learn about them, please visit your favorite search engine.
In reference to this answer, does ARC automatically nil out foo on line 3 since bar already acquired the reference?
No. ARC stands for Automatic Reference Counting. Obviously counting does not stop with 1.
It is the strength of ARC that every reference is handled independently of other references. A strong foo retains the referred object as long as it points to. It does not matter, whether other reference variables points to the same object. Track every reference isolated.
There is only one case that let ARC change the value of a reference variable: If it is marked with weak and there is no other strong reference to the object. This is the intended behavior.
As a result of optimization it is possible that retains and releases are omitted or different local variables in source code are put together. This is transparent for you.
Should I nil out the local variable rriData after using it?
You should not care about this. A strong reference variable automatically retains an object, whose reference is stored in the reference variable. And it automatically releases the object, if it does not point any longer to it. This includes releases on
a new value is assigned to the reference variable
nil is assigned to the reference variable
the reference variable loses it extent (lifetime).
Because of the last fact, there will be an automatically generated release, when rriData loses it extent. There is no reason to nil it out in advance.
Question 1. No, you simply end up with two references to the MyClass instance. Nothing gets nilled out.
Question 2. No, you don't need to nil out anything. ARC manages memory for you. That's the point. You also never need to say __strong. It is the default.
Related
I want to better understand strong and weak pointers implementation, and i figure out assumptions, about how their setter methods would look like (correct me if i wrong please).
First, strong pointer, look like:
- (void)setObj:(NSObject*)Obj // Setting Strong Obj
{
//First, check either we trying to set old value again
if (_Obj == Obj)
{
return;
}
NSObject* oldObj = _Obj;
_Obj = [Obj retain];
[oldObj release];
// Set pointer to old "chunk" of memory, containing old value,
// assign new value to backed instance variable and release
// old value
}
That is my assumption of construction, that strong setter may look like. So, my first question is - am i correct in my assumption?
Second, weak reference. I guess, it should look similar, but exclude retain.
- (void)setObj:(NSObject*)Obj // Setting Weak Obj
{
if (_Obj == Obj)
{
return;
}
NSObject* oldObj = _Obj;
_Obj = Obj; // setting value without incrementing reference count
[oldObj release];
}
Is that correctly assumption, about how weak reference work?
Ok, one more question. Consider a situation like that (in Manual Memory Management):
- (void)testFunc
{
strongObj = val; // Retain count about >= 2
weakObj = val; // Retain count about >=1
}
// Now strongObj lives in memory with value of val, with retain count >=1
// weakObj is destroyed, since after end of a scope (function body) it retain count decreased by 1
So, actually i want to know, whether retain count decremented each time, when method that own variable finishes?
I know that question is familiar to many developers, but, i want clarification in that cases. Thanks.
Your strong implementation is correct.
The weak one is wrong. You are not allowed to release the value if you have not previously retained it. You would just set the new value without issueing memory management calls here.
Then again, that wouldn't really be weak, but assign. The special thing about weak is that the reference is zeroed out of the referenced object is deallocated.
For the first and second Q I refer to #rmaddy's comment and Christian's answer.
So, actually i want to know, whether retain count decremented each time, when method that own variable finishes?
First I want to be more precise: When you say "when method that own a variable finishes" you probably mean "when a local strong reference variable of automatic storage class loses its extent". This is not exactly the same. But it is what you likely wanted to say. ("A usual local var.")
In this case it is correct that the referred object is released.
But things are more difficult behind the scenes. I. e.: What happens if the local var (more precise again: the referred object) is returned? What happens in this case, if the method is ownership transferring or not?
The basic problem is that an automatic reference counting has to take edge cases formally into account, even in "usual" code things couldn't break. A human developer can say: "Oh, there is a very special situation the code can break, but I know that this never happens." A compiler cannot. So ARC typically creates very much memory handling calls. Fortunately many of them are optimized away.
If you want to have a deep view into what is done in which situation, you have two good approaches:
Read clang's documenation, which is more precise than Apple's by far, but it is more complicated.
Create a class in a separate file that implements the methods for manual reference counting (-retain, -release, …) and log the execution. Then compile it with manual reference counting, which is possible through compiler flags. Use that class in ARC code. You will see, what ARC does. (You should not rely on the results, because they are subject of optimization and the strategy can change in the future. But it is a good tool to understand, how ARC works.)
It may be helpful to think about strong and weak references in terms of balloons.
A balloon will not fly away as long as at least one person is holding on to a string attached to it. The number of people holding strings is the retain count. When no one is holding on to a string, the ballon will fly away (dealloc). Many people can have strings to that same balloon. You can get/set properties and call methods on the referenced object with both strong and weak references.
A strong reference is like holding on to a string to that balloon. As long as you are holding on to a string attached to the balloon, it will not fly away.
A weak reference is like looking at the balloon. You can see it, access it's properties, call it's methods, but you have no string to that balloon. If everyone holding onto the string lets go, the balloon flies away, and you cannot access it anymore.
I did work several times with blocks as with pointers to which i had strong reference
I heard that you should use copy, but what is the implication in working with blocks as pointers and not with the raw object?
I never got a complain from the compiler, that i should not use
#property (nonatomic, strong) MyBlock block;
but should use
#property (nonatomic, copy) MyBlock block;
as far as i know, the block is just an object, so why to preferrer copy anyway?
Short Answer
The answer is it is historical, you are completely correct that in current ARC code there is no need to use copy and a strong property is fine. The same goes for instance, local and global variables.
Long Answer
Unlike other objects a block may be stored on the stack, this is an implementation optimisation and as such should, like other compiler optimisations, not have direct impact on the written code. This optimisation benefits a common case where a block is created, passed as a method/function argument, used by that function, and then discarded - the block can be quickly allocated on the stack and then disposed of without the heap (dynamic memory pool) being involved.
Compare this to local variables, which (a) created on the stack, (b) are automatically destroyed when the owning function/method returns and (c) can be passed-by-address to methods/functions called by the owning function. The address of a local variable cannot be stored and used after its owning function/method has return - the variable no longer exists.
However objects are expected to outlast their creating function/method (if required), so unlike local variables they are allocated on the heap and are not automatically destroyed based on their creating function/method returning but rather based on whether they are still needed - and "need" here is determined automatically by ARC these days.
Creating a block on the stack may optimise a common case but it also causes a problem - if the block needs to outlast its creator, as objects often do, then it must be moved to the heap before its creators stack is destroyed.
When the block implementation was first released the optimisation of storing blocks on the stack was made visible to programmers as the compiler at that time was unable to automatically handle moving the block to the heap when needed - programmers had to use a function block_copy() to do it themselves.
While this approach might not be out-of-place in the low-level C world (and blocks are C construct), having high-level Objective-C programmers manually manage a compiler optimisation is really not good. As Apple released newer versions of the compiler improvements where made. Early on it programmers were told they could replace block_copy(block) with [block copy], fitting in with normal Objective-C objects. Then the compiler started to automatically copy blocks off stack as needed, but this was not always officially documented.
There has been no need to manually copy blocks off the stack for a while, though Apple cannot shrug off its origins and refers to doing so as "best practice" - which is certainly debatable. In the latest version, Sept 2014, of Apple's Working with Blocks, they stated that block-valued properties should use copy, but then immediately come clean (emphasis added):
Note: You should specify copy as the property attribute, because a block needs to be copied to keep track of its captured state outside of the original scope. This isn’t something you need to worry about when using Automatic Reference Counting, as it will happen automatically, but it’s best practice for the property attribute to show the resultant behavior.
There is no need to "show the resultant behavior" - storing the block on the stack in the first place is an optimisation and should be transparent to the code - just like other compiler optimisations the code should gain the performance benefit without the programmer's involvement.
So as long as you use ARC and the current Clang compilers you can treat blocks like other objects, and as blocks are immutable that means you don't need to copy them. Trust Apple, even if they appear to be nostalgic for the "good old days when we did things by hand" and encourage you to leave historical reminders in your code, copy is not needed.
Your intuition was right.
You are asking about the ownership modifier for a property. This affects the synthesized (or auto-synthesized) getter and/or setter for the property if it is synthesized (or auto-synthesized).
The answer to this question will differ between MRC and ARC.
In MRC, property ownership modifiers include assign, retain, and copy. strong was introduced with ARC, and when strong is used in MRC, it is synonymous with retain. So the question would be about the difference between retain and copy, and there is a lot of difference, because copy's setter saves a copy of the given value.
Blocks need to be copied to be used outside the scope where it was created (with a block literal). Since your property will be storing the value as an instance variable that persists across function calls, and it's possible that someone will assign an unoccupied block from the scope where it was created, the convention is that you must copy it. copy is the right ownership modifier.
In ARC, strong makes the underlying instance variable __strong, and copy also makes it __strong and adds copying semantics to the setter. However, ARC also guarantees that whenever a value is saved into a __strong variable of block-pointer type, a copy is done. Your property has type MyBlock, which I assume is a typedef for a block pointer type. Therefore, a copy will still be done in the setter if the ownership qualifier were strong. So, in ARC, there is no difference between using strong and copy for this property.
If this declaration might be used in both MRC and ARC though (e.g. a header in a library), it would be a good idea to use copy so that it works correctly in both cases.
what is the implication in working with blocks as pointers and not with the raw object?
You are never using the raw value, you always have a pointer to a block: a block is an object.
Looking at your specific example, I am assuming you want to keep the block around, "so why to preferrer copy anyway"enter code here? Well, it's a matter of safety (this example is taken from Mike Ash blog). Since blocks are allocated on the stack (and not on the heap as the rest of the objects in objective-c), when you do something like this:
[dictionary setObject: ^{ printf("hey hey\n"); } forKey: key];
You are allocating the block on the stack frame of your current scope, so when the scope ends (for example your returning the dictionary), the stack frame is destroyed and the block goes with it. So you got yourself a dangling pointer. I would advise reading Mike's article fully. Anyway, you can go with a strong property if when you are assigning the block you copy it:
self.block = [^{} copy];
Edit: After looking at Mike's article date, I am assuming this was the behaviour Pre-ARC. On ARC it seems it's working as expected, and it won't crash.
Edit2: After experimenting with Non-ARC it doesn't crash as well. But this example shows different results depending on the use of ARC or not:
void (^block[10])();
int i = -1;
while(++i < 10)
block[i] = ^{ printf("%d\n", i); };
for(i = 0; i < 10; i++)
block[i]();
Quoting Mike Ashe on the different outcomes:
The reason it prints out ten 9s in the first case is quite simple: the
block that's created within the loop has a lifetime that's tied to the
loop's inner scope. The block is destroyed at the next iteration of
the loop, and when leaving the loop. Of course, "destroy" just means
that its slot on the stack is available to be overwritten. It just
happens that the compiler reuses the same slot each time through the
loop, so in the end, the array is filled with identical pointers, and
thus you get identical behavior.
As far as I understand copy is required when the object is mutable. Use this if you need the value of the object as it is at this moment, and you don't want that value to reflect any changes made by other owners of the object. You will need to release the object when you are finished with it because you are retaining the copy.
On the other hand, strong means that you own the object until it is needed. It is a replacement for the retain attribute, as part of ARC.
Source: Objective-C declared #property attributes (nonatomic, copy, strong, weak)
Note: You should specify copy as the property attribute, because a block needs to be copied to keep track of its captured state outside of the original scope. This isn’t something you need to worry about when using Automatic Reference Counting, as it will happen automatically, but it’s best practice for the property attribute to show the resultant behavior. For more information, see Blocks Programming Topics.
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'm working on a Objective-C project and we have ARC enabled. I'm currently dealing with two separate classes, let's call them A and B.
A has a strong reference to B and B has a block property with the copy attribute.
#interface A {
B *b;
...
}
#end
#interface B {
...
}
#property (copy) void (^myBlock)();
#end
Now, if inside a method owned by A I try to assign to myBlock:
b.myBlock = ^{ [self doSomething] };
The compiler (correctly) complains:
Capturing 'self' strongly in this block is likely to lead to a retain cycle.
I understand this behaviour and how to work around it, based on the answers to this question. (Basically, a cycle is formed because once the block is copied it will contain a strong reference to A, which has a strong reference to B, which in turn has a strong reference to the block itself.)
Now, if instead of directly assigning the block to the property I pass it to a method of B:
[b takeMyBlock:^{ [self doSomething] }];
And inside that method I do the assignment, the compiler will no longer complain.
My question is: Will that create a memory leak? If so, how to avoid it? Is it possible to annotate the method argument somehow so that the compiler knows the argument is going to be copied?
Thanks in advance.
I just encountered the same issue myself. While the issue didn't manifest itself as a leak per se, I actually found the problem while analysing heap growth using the Allocations tool in Instruments -- so it obviously does cause a memory issue.
I think the compiler would have to be pretty smart to pick up on the problem and issue a warning, so my guess is that one of two approaches is needed:
Documentation: As you suggested in your question, clearly document
that this method will copy the block passed to it. This might
involve naming the block parameter something like blockToCopy
(Xcode helpfully displays the block parameter name when using
autocompletion). Also, comment the method declaration. If you're
using a documentation tool like appledoc this is particularly nice,
as your documentation will also appear in Xcode's quick help dialog
when option clicking your code.
Never capture self strongly in a block: This is perhaps best
practise, as I don't think it's ever a good idea to keep a strong
reference to self inside a block. If you have to reference self,
use a weak reference. This will certainly avoid the situation you
describe. (N.B. referencing an instance variable will also keep a strong
reference to self).
In ARC, what it does is if a local variable or a WEAK variable loses it reference. The memory will be dealloced immediately since its reference count is zero. However, if you are reallocating a STRONG variable, it will definitely cause a memory leak for strong variable retains even if reference count decays to zero. In this case, you shall set this variable to NIL before reallocation or set the variable to weak if strong property is unnecessary.
Sometimes I set objects to nil after releasing them to avoid crashes due to unexpected messages sent to dealloc'd objects.
Why doesn't the Objective-c runtime do this automatically when an object is finally dealloc'd?
Because pointers are passed by value and not reference. All dealloc gets is a local copy of the pointer; any changes it makes are discarded. Same goes for C's free and C++'s delete.
How would the runtime be able to track down all such references?
MyClass* a = [MyClass new];
MyClass* aCopy = a;
[a release];
a = nil; // Tidy up
[aCopy crashNowPlease];
Sometimes you can catch this sort of thing with Build & Analyze.
Also, you will be able to use zeroing weak references in the future (or, Mike Ash created such a facility).
In objective-C, the objects are always accessed as pointer.
Setting an object to nil simply change the pointer value, not the object value.
In your method, you have only access to the object's value, not to the pointer pointing to it!
As #David Dunham says, you can have more than one pointer to an object, so how would the compiler knows?
And more than that, to make things easier, imagine the following code :
int a;
int* aPtr = &a;
If you change a value, you can access the changed value via *aPtr right? But you can change a value as long as you want, it won't change aPtr value, as it is not the same variable!
Thus, even if you only have one pointer to your object, if you modify the object's value, you pointer will still point to the same address value!
What you are describing is called a zeroing weak reference. This feature is available in OS X 10.7's Automatic Reference Counting (ARC) runtime. Prior to 10.7, you can use Mike Ash's zeroing weak reference implementation.
Without explicit runtime support (and some small but unavoidable overhead), standard C-style pointers (id in Objective-C is a pointer to a structure) are just a memory address (an integer) and not tracked by the runtime. There's nothing stopping you from making copies of this value elsewhere in memory or from storing the value in an integer of the appropriate size and casting to an id later on (of course if you do any of this, you kill a baby seal). Thus, when an object is dealloc'd there's no way to know where (or how many) references there are to that object.