If I declare a static object handle in a class file in global scope. And the object assigned to this handle is an autoreleased one. Then for how long will this object remain in memory during my application life cycle ?
If I declare a static object handle in a class file in global scope. And the object assigned to this handle is an autoreleased one. Then for how long will this object remain in memory during my application life cycle?
Short Answer: You want your global to be a strong reference. In MRC, you must add the retains and releases for globals. In ARC, the global is implicitly strong (and ARC adds them for you).
Long Answer:
Under ARC, your static global variable is a strong reference. In MRC, you would retain such a variable when set and then release the previous object. If you did not, then you could still access it after it were deallocated (dangling pointer).
Because it is a strong reference, your object will remain valid until a) the strong reference by the global variable is given up and b) the autorelease pool is drained c) and of course any other strong references are given up.
So if you are using strong references for that global and you never reassign it (logically, giving up the global's strong reference), then your object would never be dealloc'ed.
When you use unsafe nonretained semantics (by decoration in ARC, or the default for a static in MRC), the object would be -dealloced when the current autorelease pool is drained and all strong references are removed. This is easiest to illustrate with a program (MRC);
static MONObject * GlobalObject;
//
// In MRC, you must add the reference counting to ensure you do not end up with a dangling
// pointer, so false is what how your program should be written in MRC.
//
// In ARC, your program will look like NonRetainedOwnership because it treats the global
// as a strong reference.
static const bool NonRetainedOwnership = ...T/F...;
...
// assume GlobalObject is accessed from one thread only -- i won't bother with
// the supporting code to illustrate how this should be made thread safe.
- (MONObject *)sharedObject
{
if (nil == GlobalObject) {
if (NonRetainedOwnership) {
// don't do this! lifetime will be determined by strong client references
// and the reference you rely on now will be lost when the thread's current
// autorelease pool is drained -- leaving you with a dangling pointer which
// will cause issues when -sharedObject is called again.
GlobalObject = [[MONObject new] autorelease];
}
else {
// Good - keep a strong reference:
GlobalObject = [MONObject new];
}
}
return GlobalObject;
}
- (void)disposeSharedObject
{
if (NonRetainedOwnership) {
GlobalObject = nil;
}
else {
// remove our reference, which was established at [MONObject new]
// assuming there are no strong external references, the object
// will be dealloc'ed -- but you should not make that assumption
// if you return it.
[GlobalObject release], GlobalObject = nil;
}
}
So if NonRetainedOwnership is true and you use MRC, then your object would typically be -dealloc-ed shortly after -sharedObject returns (assuming whoever called -sharedObject holds no strong reference). In this case, 'shortly' means the pool will often be drained a few frames back in your current callstack, often in AppKit or UIKit when you are on the main thread since I don't see many people explicitly create autorelease pools on the main thread. borrrden said after "1/60th of a second", and the assumption there is that the object is created and autoreleased in the main thread's run loop (correct me if I am wrong). The 'Kits create autorelease pools at each iteration of the main run loop, but your program could be running on a secondary thread or there could be an inner autorelease pool, so the lifetime has the possibility to be shorter or longer. Normally, you don't need to think about these things in much depth -- just use a strong reference here since you really have no other way to ensure the right thing happens with this global variable.
If you simply wrote: #autoreleasepool{[[MONObject new] autorelease];} NSLog("Hi"); then the object would (under normal circumstances) be released by the time NSLog were called.
The declaration of variable doesn't matter as such, The thing what matters is when did you assigned an autoreleased object to it, If it is assigned under any autorelease pool than it would drain it else it would be released on program termination by autorelease pool in main method!
The Variable is just a pointer and does not retain the object unless done explicitly, thats why retaining static objects is preferred: Why retain a static variable?
When the pool is 'drained'. This may not happen right away.
Please see a similar question
It depends. If the variable is initialized only once, and should stay around for the lifetime of the application, then no, it shouldn't be released (its memory will essentially be freed when the application exits, anyway). If, however, the value of the static variable changes, then yes, the previous object should be released when the static variable is set to a new object.
Related
The book named Pro Multithreading and Memory Management for iOS and OS X writed that
Why does the object need to be registered in autoreleasepool in order to use the object via the __weak qualified variable? Because a variable, which is qualified with __weak, does not have a strong reference, the object might be disposed of at any point. If the object is registered in autoreleasepool, until #autoreleasepool block is left, the object must exist. So, to use the objects via __weak variable safely, the object is registered in autoreleasepool automatically.
I do not understand why __weak need the variable is exist. __weak does not retain the variable.if the variable is not exist.The __weak variable should be nil.Why this book say that "If the object is registered in autoreleasepool, until #autoreleasepool block is left, the object must exist".
And if it is registered in autoreleasepool .Which pool does it is registered?I do not alloc a autoreleasepool when I use __weak.Who can tell me why.
The book you are reading is at least confusing, and may be sometimes outright wrong, in its presentation of __weak and __autoreleasing qualifiers. (The book is a translation, this may account for the sometimes confusing language used.)
References stored into __weak qualified variables are not also automatically added to the autorelease pool, that is contrary to the purpose of weak variables.
What does happen is that under some circumstances when a reference is loaded from a weak variable it is placed in the autorelease pool by the compiler to avoid a race condition (between an object being released and its reference being loaded from a weak variable). You do not need to concern yourself, at least directly, with this - the compiler is simply insuring that a reference you have loaded from a weak variable is valid for the time you use it.
You can find other comments on this book in the question Objective-C - weak object is registered in autoreleasepool automatically?
HTH
I have been using blocks and familiar with the memory management when using self inside blocks in
Non-ARC environment
But I have two specific questions:
1) I understand I can use __block to avoid the retain cycle a retained block which in turn using self can create, like below:
__block MyClass *blockSelf = self;
self.myBlock = ^{
blockSelf.someProperty = abc;
[blockSelf someMethod];
};
This will avoid the retain cycle for sure but I by doing this I have created a scope for self to be released and eventually deallocated by someone else. So when this happen self is gone and blockSelf is pointing to a garbage value. There can be conditions when the block is executed after the self is deallocated, then the block will crash as it is trying to use deallocated instance. How do we can avoid this condition? How do I check if blockSelf is valid when block executes or stop block from executing when self is deallocated.
2) On the similar lines suppose I use block like below:
__block MyClass *blockSelf = self;
self.myBlock = ^{
[blockSelf someMethod:blockSelf.someProperty];
};
// I am taking someProperty in an method argument
-(void) someMethod:(datatype*)myClassProperty
{
myClassProperty = abc;
}
Now there can be situations where self is not released but someProperty is released before someMethod's execution starts (This could happen when there are multiple threads). Even if I do self.someProperty = nil; when it is release, myClassProperty is not nil and pointing to some garbage, hence when someMethod is executed the first line will lead to crash. How do I avoid this?
This is the same issue as non-zeroing weak references everywhere else in non-ARC code, e.g. delegates etc. (MRC doesn't have zeroing weak references; so these are the only kind of weak reference in MRC. Yet people were still able to write safe code in the pre-ARC days.)
Basically, the solution is that you need a clear map of ownership. Either self is responsible for keeping the block alive; or some other object is responsible for keeping the block alive.
For example, with delegates, usually, a "parent" object is the delegate of a "child" object; in this case the "parent" object is responsible for keeping the "child" object alive, so the "parent" object will outlive the "child" object, thus the back reference can be weak and is safe (because the child object's methods could only possibly be called by the parent object while the parent object is alive).
On the other hand, if you have an asynchronous operation, and the block is given to the operation as a callback, then usually the operation is responsible for holding onto the block. In this case, self would not hold onto the block. And the block would hold a strong reference to self, so that when the operation is done, it can still safely do whatever it is that it needs to do on self. In fact, whatever object self is, it doesn't even need to be retained by whoever uses it, since it is indirectly retained by the asynchronous operation -- it can just be a create, fire, and forget kind of thing.
If you have something where the block is sometimes kept alive by self, and sometimes kept alive by something else, then you should re-think your design. You say "There can be conditions when the block is executed after the self is deallocated"; well, you should describe your whole design and how this can happen. Because usually, for something that is executed asynchronously, self need not hold onto the block.
Your code is really confusing and doesn't make sense. For example, why would you assign to a parameter (myClassProperty)? What's the point of passing an argument when the parameter is going to be overwritten anyway? Why would you name a local variable "myClassProperty"?
What I think you are asking about is accessing a property that can be changed on different threads, and how to deal with the memory management of that. (This question is unrelated to blocks or ARC/MRC. It is equally an issue in ARC.)
The answer is that you need an atomic property. You can make a synchronized property atomic, or implement an atomic property manually if you know how. What an atomic property of object pointer type needs to do is its getter needs to not just return the underlying variable, but also retain and autorelease it, and return the result of that. The retrieval and retaining in the getter occurs in a critical section that is synchronized with a critical section in the setter that includes the release of the old value and retaining of the new. Basically, this synchronization guarantees that the value will not be released in the setter in between retrieving the value and retaining it in the getter. And the value returned from the getter is retained and autoreleased, so it is guaranteed to be alive for the duration of the scope.
Solution for 2)
__block MyClass *blockSelf = self;
self.myBlock = ^{
datatype* p = [blockSelf.someProperty retain];
[blockSelf someMethod:p];
[p release];
};
// I am taking someProperty in an method argument
-(void) someMethod:(datatype*)myClassProperty
{
myClassProperty = abc;
}
For 1) don't know how you use myBlock. If it's used only by self, then all will be fine. And if it's used by some other object, then it's also should have retained reference at self and then there is also all will be fine.
Right now I have something like this:
- (void)viewDidLoad
{
MyObject *myObject = nil;
#autoreleasepool
{
myObject = [[MyObject alloc] init];
[myObject doSomethingWithBlock:^{
NSLog(#"Something Happened");
}];
}
NSLog(#"End of method");
}
And the doSomethingWithBlock: has the following:
- (void)doSomethingWithBlock:(void(^)())aBlock
{
[self performSelector:#selector(something:) withObject:aBlock afterDelay:4.0f];
}
And the something:
- (void)something:(void(^)())aBlock {
aBlock();
}
I understand that the block is being passed between stacks frames, so it's kept alive until it actually is executed and it is disposed. But why when the "End of method" is called does myObjct still exists? I know that I am not referencing the object from within the block, so shouldn't it be released within the autorelease pool? Or is this a compiler detail (if it should actually release it there, or after the method returns), and I shouldn't care about it?
Someone pointed out that performSelector will retain the target so I used this instead:
dispatch_after(dispatch_time(DISPATCH_TIME_NOW, 4 * NSEC_PER_SEC), dispatch_get_main_queue(), ^{
aBlock();
});
It is not clear to me whether the question is about the lifetime of the block, as per the title, or the lifetime of the object referenced by myObject, so we'll cover both.
First, the stack allocation of blocks in an optimisation and something programmers should not really need to be aware of. Unfortunately when blocks were first introduced the compiler was not smart enough to handle this optimisation completely automatically, hence the need to know about Block_copy() etc. But the compiler is now much smarter and the programmer can pretty much forget about stack allocation of blocks.
This answer applies to Xcode 5.0.2/Clang 4.2. Use a different (earlier) version and YMMV.
Second, the block:
^{ NSLog(#"Something Happened"); }
is not actually stack allocated at all. As it references no values from the environment it is statically allocated and never needs to be placed on the stack or moved to the heap - another optimisation. To get a stack allocated block just change it to:
^{ NSLog(#"Something Happened: %p", self); }
which will be stack allocated (and don't worry about possible retain cycles, that's another topic...)
With that out of the way, let's look at the lifetimes:
The object:
The variable myObject has a lifetime of at most the call to viewDidLoad, the variable is created on entry to the method and destroyed on exit, if precise lifetime semantics are in force, or earlier if not needed and precise lifetime semantics are not in force - the latter is the default allowing the compiler to optimise storage use (independent of ARC). By default local variables have a strong ownership qualifier, so an ownership interest is asserted for any reference stored in one. ARC will release that ownership interest when another reference is stored in the variable or when the variable itself is destroyed. The latter will happen in this case and the variable is destroyed at some time after the last use of it and the end of the method - depending on how the compiler does its optimisations.
All this means that the object referenced by myObject may or may not be still around at your call to NSLog(). In the case of using performSelector:withObject:afterDelay: that call will assert ownership over the object until after the selector has been executed -= so the object will live. In the case of using dispatch_after the object is not required and so will not stay alive.
The block:
As mentioned above as written the block is statically allocated so lifetime is easy - it always exists. More interesting is modifying it as above to make it stack allocated.
If the block is stack allocated it will exist in the stack frame of the viewDidLoad call and unless moved or copied to the heap it will disappear when that call returns.
The call to doSomethingWithBlock: will be passed a reference to this stack allocated block. This is possible as doSomethingWithBlock: expects a block and code within it can move it to the heap if needed as it knows it is a block.
Now it gets interesting. Within doSomethingWithBlock: in the performSelector:withObject:afterDelay: case the block is passed as the withObject: parameter which is of type id - which means the called code expects a standard Objective-C object and those live on the heap. This is a case of type information loss, the caller has a block, the callee only sees an id. So at this point the compiler inserts code to copy the block to the heap and passes that new heap block to performSelector:withObject:afterDelay: - which treats it as it would any other object. [Older compilers did not always do this, the programmer had to know that type information loss was about to occur and manually insert code to move the block to the heap.]
In the case of dispatch_after, the function argument is block typed so the compiler just passes a stack allocated block on. However the dispatch_after function copies the block to the heap, as per its documentation, and there is no problem.
So in either case a stack allocated block is copied to the heap before viewDidLoad finishes and its stack frame is destroyed taking with it the stack allocated block.
HTH.
It is declared outside of the autorelease pool, so it should exist after it. It's all about scope where the object is declared, not allocated. By your logic class variables allocated inside an autorelease pool should be nil'd at the end of the pool and not usable elsewhere.
Example
-(void)function
{
MyObject *object; //MyObject declared here
if(someBoolean)
{
object = [[MyObject alloc] init]; //MyObject allocated here
}
//This is outside the scope of allocation,
//but it's still inside the scope of declaration.
//If ARC referenced the scope of allocation your
//object would not be valid here, as ARC would
//release it as out of scope.
}
I am new to IOS development and I have started to learn objective c to program towards IOS 7. and as I know, it is way easier to code now than it has been before because of the Automatic reference counting.
there are a couple of things I do not understand . in MAIN method we have the autoreleasepool block, so my first question is that in order to enable ARC , the code has to be inside this block? if no, then what is the difference between the code that is inside autoreleasepool and the rest those aren't?
my second question is that when I am writing my IPHONE programs , I have bunch of classes and non of those codes are inside "autoreleasepool" , only the code inside the MAIN method.
int main(int argc, char * argv[])
{
#autoreleasepool {
return UIApplicationMain(argc, argv, nil,
NSStringFromClass([HomepwnerAppDelegate class]));
}
}
so , does this mean that this block somehow magically gets applied to all lines of code inside any other classes of the same program?
My last question is that whether with ARC or without it, if we had a declared pointer variable inside a method, does the object gets released/destroyed when the method returns/exit?
assume we have a method like this :
- (void)doSomething {
NSMutableArray *allItems = [[NSMutableArray alloc] init];
NSString *myString = #"sample string";
[allItems addObject:myString]
}
then when we call this method and it exits, what would happen to those local variables defined inside the method ? is there any difference in the outcome if we are using ARC or not ? (Object are still in the memory or not)
Autorelease pools predate ARC by about 15 years. Cocoa uses a reference-counting memory management scheme, where (conceptually, at least) objects are created with a reference count of 1, retain increases it by 1 and release decreases the count by 1, and the object is destroyed when the count gets to 0.
A problem with this scheme is that it makes returning an object kind of awkward, because you can't release the object before you return it — if you did, it might be destroyed before the other method got to use it — but you don't want to require the other method to release the object either. This is where autorelease pools come in. An autorelease pool lets you hand an object to it, and it promises to release the object for you later. Under manual retain/release (the way we used to do things before ARC), you did this by sending autorelease to an object.
OK, so then ARC comes into the picture. The only thing that really changes with ARC is that you aren't the one writing retain, release and autorelease anymore — the compiler inserts them for you. But you still need an autorelease pool for autoreleased object to go into.
As for your second question:
My last question is that whether with ARC or without it, if we had a declared pointer variable inside a method, does the object gets released/destroyed when the method returns/exit?
assume we have a method like this :
- (void)doSomething {
NSMutableArray *allItems = [[NSMutableArray alloc] init];
NSString *myString = #"sample string";
[allItems addObject:myString]
}
then when we call this method and it exits, what would happen to those local variables defined inside the method ? is there any difference in the outcome if we are using ARC or not ?
If you're using ARC, the compiler will release any objects referenced by local variables. If you're not using ARC, you'd need write [allItems release] yourself, because the variable going out of scope does not magically cause the object it references to be released.
new to IOS development
Best not to worry, Automatic means that you mostly concentrate on other things ^)
does this mean that this block somehow magically gets applied to all lines of code inside any other classes of the same program
Yes. You're in main function, so all the code that is executed has to be inside this function - your app will terminate once it ends. Unless you create a separate thread, but it's hard to do that by accident.
the code has to be inside this block
As said above, all of your code on main thread will execute within this block.
what would happen to those local variables defined inside the method
You're guaranteed that they will be destroyed before returning.
in MAIN method we have the autoreleasepool block, so my first question is that in order to enable ARC, the code has to be inside this block? if no, then what is the difference between the code that is inside autoreleasepool and the rest those aren't?
ARC is enabled by corresponding Objective-C compiler setting. If you create a new project in the latest version of Xcode it will be enabled by default.
The #autorelease keyword places code inside the curly brackets into autorelease pool scope. Autorelease pools are used both with ARC and manual memory management.
my second question is that when I am writing my IPHONE programs , I have bunch of classes and non of those codes are inside "autoreleasepool" , only the code inside the MAIN method.
iOS applications are event based. Main thread starts event loop when you call UIApplicationMain function processing touch events, notifications etc. This event loop has its own autorelease pool that autoreleases objects at the end of the loop iteration. This autorelease pool has nothing to do with the autorelease pool you see in main function.
My last question is that whether with ARC or without it, if we had a declared pointer variable inside a method, does the object gets released/destroyed when the method returns/exit?
If you use ARC the objects will be released (unless you return a reference to an object from the method). In MMR you would need to manually send release message to destroy the objects.
I've people who use [variable release] and some other times variable = nil to clean up memory?
When do you use each one? and what are the differences?
variable = nil; will not release memory. self.property = nil; will release memory if [self setProperty:nil]; would, for example a synthesized property with the retain attribute. Calling [variable release]; will always release one reference of an object.
Depends on what you mean by "clean up memory".
release is the only thing that frees dynamically allocated memory allocated by alloc. alloc should always be paired with a call to release or autorelease somewhere.
Setting a varible to nil does not necessarily free any memory (see drawnonward's answer), and can be a source of memory leaks.
When you see a variable set to nil, it's about preventing it from accidentally being used later after its memory has been freed (this can cause crashes). While you can always set a variable to nil after a call to release, it's somewhat a matter of style when it's actually necessary. For example, you don't often see variables set to nil in the dealloc method of a class, since by that point an object won't be able to accidentally misuse such a variable anymore, since it's being nuked.
If a property is set to retain, then these 3 are equivalent:
[self setProperty:nil];
self.property = nil;
[property release]; property = nil;
In each case, the object will be released, and then set to nil so that all access to the object from then on will not be allowed. "nilling" the instance variable is handy since it ensures you can only ever release the object once in this context because calling self.property = nil twice will do nothing the second time, but calling [property release] will release the object twice even though you likely only retain it once.
Most of the time I find it least bug prone to let retain properties do their thing and try to stay away from explicit retain and release calls most of the time.