Let's say I have a method that returns an object pointer
-(MyObj *) returnMyObj {
MyObj *obj = [MyObj alloc] init];
return obj;
}
If I call this function without assigning the pointer like this
Scenario 1
[self returnMyObj];
and if I call this function with assignment to a pointer like this
Scenario 2
MyObj* obj = [self returnMyObj];
Compiler can release the object at the end of the returnMyObj method call in the Scenario 1 but it cant do the same in Scenario 2.How does ARC decide if it needs to retain the created object at the end of method invocation or not in both cases?
Here is what the ARC article in the documentation has to say:
To make sure that instances don’t disappear while they are still needed, ARC tracks how many properties, constants, and variables are currently referring to each class instance. ARC will not deallocate an instance as long as at least one active reference to that instance still exists.
To make this possible, whenever you assign a class instance to a property, constant, or variable, that property, constant, or variable makes a strong reference to the instance. The reference is called a “strong” reference because it keeps a firm hold on that instance, and does not allow it to be deallocated for as long as that strong reference remains.
ARC decides which objects will be kept in memory and which will be deallocated by counting the strong references to the objects.
In your second scenario you are creating a strong reference to the MyObj instance and ARC will not deallocate it while the object is in use. In this case it will be deallocated when the method that uses this object finishes.
The optimizer may absolutely release the object in scenario 2 at the end of the statement if obj is not referenced later in the block.
The whole point is that the optimizer can see when the pointer is referenced, and may release it as soon as the last reference is completed. obj does not have precise lifetime semantics, so it only extends the life of the object until the last time obj is referenced, not when obj goes out of scope.
In scenario 1, the reference is not assigned to any variable, so it clearly has no later references, and may be immediately released. That said, it may not immediately release the object, since returnMyObj does not transfer ownership to the caller (due to its name). So the object may not actually be released until the autorelease pool drains.
Related
Without using ARC, for objects initialized using instance method we do release it via release call.
example:
Sample *obj = [[Sample alloc]init] ;
// some code here
[obj release] ;
Now for objects allocated using class methods should release be called?
example:
TestSample *testobj = [TestSample initCustom] ;
// some code here
should there be [testobj release]?
Let's consider this standard function provided by apple.
endpointWithHostname is a class method.
NWHostEndpoint *pNetHost = [NWHostEndpoint endpointWithHostname:#"0.0.0.0" port:#"21"];
so, should there be a [pNetHost release] for above call?
You said:
for objects initialized using instance method we do release it via release call
The fact that you called an instance method isn’t relevant. You own any object returned by any method whose name starts with alloc, new, copy, or mutableCopy. In your example, you called alloc, and therefore ownership has been transferred to you. That’s why you are now responsible for calling release.
However, if you called some instance method that didn’t start with one of those four prefixes, though, ownership would not be transferred to you, and you would not call release.
This is all summarized in the Advance Memory Management Programming Guide, which says:
Basic Memory Management Rules
The memory management model is based on object ownership. Any object may have one or more owners. As long as an object has at least one owner, it continues to exist. If an object has no owners, the runtime system destroys it automatically. To make sure it is clear when you own an object and when you do not, Cocoa sets the following policy:
You own any object you create
You create an object using a method whose name begins with “alloc”, “new”, “copy”, or “mutableCopy” (for example, alloc, newObject, or mutableCopy).
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).
When you no longer need it, you must relinquish ownership of an object you own
You relinquish ownership of an object by sending it a release message or an autorelease message. In Cocoa terminology, relinquishing ownership of an object is therefore typically referred to as “releasing” an object.
You must not relinquish ownership of an object you do not own
This is just corollary of the previous policy rules, stated explicitly.
You go on to ask:
let's consider this standard function provided by apple. endpointWithHostname is a class method.
NWHostEndpoint *pNetHost = [NWHostEndpoint endpointWithHostname:#"0.0.0.0" port:#"21"];
so should there be a [pNetHost release] ; for above call
The fact that it is a class method isn’t relevant. The key is that the method name doesn’t start with alloc, new, copy, or mutableCopy. That means that ownership has not been transferred to you and therefore you don’t need to call release. In this case, it is returning an autorelease object, so it will be released for you when the autoreleasepool is drained.
Obviously, if you want to claim ownership of the pNetHost object, you would do so by calling retain (and of course, at that point you’d then be responsible for eventually calling release to offset that retain).
Now, let’s consider your example:
TestSample *testobj = [TestSample initCustom];
// some code here
should there be [testobj release]?
No. But let’s answer this question by looking at the implementation of the method you called initCustom. It might be implemented like so:
+ (instancetype)testSample {
TestSample *obj = [[self alloc] init];
// perhaps more customization of `obj` here
return [obj autorelease];
}
But, there are two key observations in the above:
I didn’t start the method name with init. That prefix should only be used with initialization methods, which this method isn’t.
The convention is to start the method name with the name of the object type, using camelCase (i.e. starting with a lowercase letter). Hence, this TestSample class method name might be called testSample, like above, not initCustom.
Because the method name doesn’t start with alloc, new, copy, or mutableCopy, it should return an autorelease object. And because it is an autorelease object, the caller would therefore would not call release (unless, of course, it claimed ownership by calling retain).
By the way, the static analyzer (shift+command+B; or “Product” » “Analyze”) is exceptionally good at analyzing manual reference counting code.
For example, let’s imagine that I forgot to add the autorelease in the above testSample example. If I let Xcode analyze the code, the compiler would tell me about the problem:
If I add the autorelease back in and re-analyze, this warning will go away.
When writing manual reference counting code, the static analyzer is an invaluable tool. Always make sure you have a clean bill of health from the analyzer.
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.
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.
We know about the complete pattern of alloc/init that alloc and init must be combined.
NSObject *myObj = [[NSObject alloc] init];
1- init method receives the object from another source(not from a alloc,new,copy or alike or retained) so according to the fundamental memory management rule its not the owner and it must not release it. However, "Allocating and Initializing Objects / The Returned Object" article says that init could free the receiver.
How could this be possible when its against the fundamental rule?
2- Also, from the same article, init could return another object or nil. So, in this case, when we use the complete pattern of alloc/init, we could not release the object returned by alloc but we could only release the object returned from init and, init releases the object it received from alloc instead of us.
But init is not a alloc,new,copy or alike method so we must not release the object returned from it as it does not give us the ownership of object.
How could we release the object returned from init although this is against the fundamental rule?
3- Or, to adhere to the last paragraph of the same article, must we accept init method as a special case and use alloc/init pattern as an exception to the fundamental rule?
Memory Management Fundamental Rule:
You only release or autorelease objects you own.
You take ownership of an object if you create it using a method whose name begins with “alloc” or “new” or contains “copy” (for example, alloc, newObject, or mutableCopy), or if you send it a retain message.
You use release or autorelease to relinquish ownership of an object. autorelease just means “send a release message in the future” (to understand when this will be, see “Autorelease Pools”).
http://developer.apple.com/library/ios/#documentation/Cocoa/Conceptual/MemoryMgmt/Articles/mmRules.html
Allocating and Initializing Objects / The Returned Object:
However, in some cases, this responsibility can mean returning a different object than the receiver. For example, if a class keeps a list of named objects, it might provide an initWithName: method to initialize new instances. If there can be no more than one object per name, initWithName: might refuse to assign the same name to two objects. When asked to assign a new instance a name that’s already being used by another object, it might free the newly allocated instance and return the other object—thus ensuring the uniqueness of the name while at the same time providing what was asked for, an instance with the requested name.
In a few cases, it might be impossible for an init... method to do what it’s asked to do. For example, an initFromFile: method might get the data it needs from a file passed as an argument. If the file name it’s passed doesn’t correspond to an actual file, it won’t be able to complete the initialization. In such a case, the init... method could free the receiver and return nil, indicating that the requested object can’t be created.
Because an init... method might return an object other than the newly allocated receiver, or even return nil, it’s important that programs use the value returned by the initialization method, not just that returned by alloc or allocWithZone:. The following code is very dangerous, since it ignores the return of init.
id anObject = [SomeClass alloc];
[anObject init];
[anObject someOtherMessage];
Instead, to safely initialize an object, you should combine allocation and initialization messages in one line of code.
id anObject = [[SomeClass alloc] init];
[anObject someOtherMessage];
http://developer.apple.com/library/ios/#documentation/Cocoa/Conceptual/ObjectiveC/Articles/ocAllocInit.html
The init method doesn't receive the object; the object receives the init message. The object does not possess itself; rather, it always knows about itself (through the implicit self argument in every message).
You're right that the object does not own itself, though. If alloc and init were fused in a single new method, that method would be its own (super) caller, so it would own the object (until it returns) and so be unquestionably right in releasing it. Since they are separate, and init is not the caller of alloc, you're right that it does not own the object, so you are right to question this practice.
This is one of the few cases where it's OK for one object to release an object (in this case, itself) on behalf of another. The alternative is not to release it, which, if you're going to either return nil or throw an exception, will be a leak.
In general, anytime you have an object retain or release itself, you should feel dirty. In this specific case, it's OK, because you are preventing a bug (a leak) rather than probably creating one.
2- Also, from the same article, init could return another object or nil. So, in this case, when we use the complete pattern of alloc/init, we could not release the object returned by alloc but we could only release the object returned from init and, init releases the object it received from alloc instead of us.
But init is not a alloc,new,copy or alike method so we must not release the object returned from it as it does not give us the ownership of object.
As init releases the old object on behalf of its caller, if it creates a new object, it does that on behalf of its caller. The caller does own the substitute object that init creates or retrieves for it.
As a corollary to this, if init retrieves a previously existing object, it must retain that, so that the caller will own it.
Again examining the hypothetical* new method, it would also need to both release the old object and create (owningly) or retain the substitute.
In all of these cases, it's init acting on behalf of its caller. It's normally dodgy for one method to do another's memory management, but for these cases, init doing it on behalf of its caller is necessary.
*The new method does exist, but simply sends alloc and init, so there's no need to implement it separately.
If initialization fails for some reason and must return null, then you must release the object in order to avoid leaking memory.
Similarly, init may decide to swap in a different object and return it - in that case you must also release the object in order to avoid leaking memory.
In both cases it's necessary because the original object isn't being returned by init, and will be orphaned after the method returns. Alloc has automatically retained the object, so if you don't release it its retain count will be stuck at 1 forever.
[Would another perspective help?]
The init method (and its siblings initWith... and similar) is a bit of an odd case but is not a special case of memory allocation rules. Init is odd because it has a name that sounds like it is going to change the internals of the instance but in fact it may do more than that (it may substitute some other object and initialize that object, for example). The tip-off is in the declaration of init:
- (id) init // the way it is
vs
- (void) init // in some other universe
The init method returns an object, so it might have been better named something like 'return an object that is (class-wise) an equivalent object and that has been initialized'. Most methods do not perform this kind of switcheroo, which makes init a bit different/odd.
There is nothing 'magic' about the alloc / init nesting -- it's just the simplest way to handle the fact that the object that you get back from alloc may not be the same object you get back from init. This works perfectly fine:
NSSomeClass* s = [NSSomeClass alloc];
s = [s init]; // that 's =' part is really important ;-)
and is exactly equivalent to the 'standard' idiom:
NSSomeClass* s = [[NSSomeClass alloc] init];
This is potentially problematic:
NSSomeClass* s = [NSSomeClass alloc]
[s init]; // bad! you ignored the result of init
The implementation of an init method must be done particularly carefully when the implementation returns a different object than the one it receives as the incoming 'self'. In such a case the init method takes on the responsibility of memory management of the 'self' object (because it's not going to return that object - so who else could be expected to do the management?)
It's possible to do some pretty ugly trickery, BTW. Don't try this at home!
// don't do this!
S* s = [S alloc]
[s retain]; // needs to survive 2 init's
S* ss = [s init......]; // s.retain goes 2-->1
S* sss = [s init.....]; // ... goes 1-->0;
Doing this is extremely poor practice because it depends on the init..... method always returning a new object (instead of the one it receives). That's an obviously bad assumption!
NOTE that by "a method receiving an object in 'self'" I mean that the method was invoked upon/by an object, and that object is made available by convention through the 'self' pointer.
The fundamental rule just doesn't apply in this special situation.
In fact, don't worry about it - unless you plan to do Posing, you won't need to write code that does this, and it will have no impact at all on the code you write.
You should continue to follow the fundamental rule in all your code.
And thirdly, the code is more what you'd call "guidelines" than actual rules.
(Captain Barbosa)
alloc/init is a bit of a special case. You have to do the retain/release stuff inside init in such a way that whatever object the caller gets back is owned by the caller and there are no leaks.
I am still new to this Memory Management stuff (Garbage Collector took care of everything in Java), but as far as I understand if you allocate memory for an object then you have to release that memory back to the computer as soon as you are finished with your object.
myObject = [Object alloc];
and
[myObject release];
Right now I just have 3 parts in my Objective-C .m file: #Interface, #Implementation and main. I released my object at the end of the program next to these guys:
[pool drain];
return 0;
But what if this program were to be a lot more complicated, would it be okay to release myObject at the end of the program?
I guess a better question would be when do I release an object's allocated memory? How do I know where to place [myObject release];?
This is probably a little over-simplified, but in general, you are going to want to release it where you declared it.
If you declare an object INSIDE a particular method call, then by definition, you will be done with that object (or at least that handle to that object) at the end of that method call... release it then.
If you declare an object as an instance variable, then by definition you will be done with it when that instance is destroyed... release it in the dealloc method of that class.
Keep in mind that "release" does not equal "destroy." When passing objects around in your application, it may make sense to have more than one handle to that object stored in different places... in that case "release" means "I'm done with this object, but someone else may still be using it." Deallocation only occurs when the number of "handles" (retain count) reaches zero.
Apple has some fantastic documentation on memory management, I would check it out at developer.apple.com.
You essentially have three kinds of objects, each with a different pattern.
Transients Objects
In general, you should autorelease transient objects. These are objects that are allocated locally and do not need to exist beyond the method in which they are called. Or they are passed around from method to method.
Chain of Ownership
When one object exists as an instance field inside another, you should release the "owned" (or "child") object when the "owner" (or "parent") object goes out of existence. This is done in the dealloc method of the parent object:
- (void) dealloc {
[child release]; // child was declared as an instance variable
[super dealloc];
}
Lifetime of the Program
When an object is intended to exist for the lifetime of the program, it usually isn't necessary to call release at all, unless some kind of resource cleanup needs to occur. You can put this in applicationWillTerminate:, which you can look up in Apple's documentation.
(You should probably avoid having such objects, but that is a discussion for another question.)
You have to think in terms of ownership. When you take ownership of an object by calling alloc, new or retain, you're also responsible for releasing it, either by calling autorelease when you return an owned object to the caller, or by calling release.
A general rule is:
Local variable: release it within the same method. When you want to return it to the caller, use autorelease
Class member: release it in the dealloc method