Suppose that i have UIView viewLoading declared in .h. and i do not directly initialize it (in the first code).
The first Code.
UIView *viewLoading2 = [[[UIView alloc] initWithFrame:CGRectMake(75 , 155, 170.0, 170.0)]];
viewLoading = viewLoading2;
[viewLoading2 release]
The second code:
viewLoading = [[[UIView alloc] initWithFrame:CGRectMake(75 , 155, 170.0, 170.0)]];
The third Code:
- (void) viewLoad:(UIView *) viewLoading2
{
viewLoading = viewLoading2;
//do i need to retain, alloc, or release something here?
}
2In the first code, do i need to release viewLoading in dealloc ? And what happen if i do not declare its property?
In the second code, does it have same effect from the first code? (need to dealloc or not).
For the third code, does it have same effect from the first code? and what should i do after i code that? (see the comment)
Do iPhone Code always need to have release for variable declared in .h? Or only if the variable declared in .h is allocated? if like in the first code, do i need to dealloc viewLoading?
what is the different between
self.viewloading = viewLoading2;
and
viewloading = viewLoading2;
Thanks
In the first example, you allocated the object (once), and you released it (once), so you don't need to do anything else. On the other hand, viewLoading is invalid as soon as you send the release to viewLoading2, so it's not very useful code.
In the second, you have not released viewLoading yet, so it does need to be done eventually.
In the third, whatever code allocated the object that was passed into this method via the parameter is responsible for releasing it. It should be valid for the duration of this method, but if you're saving it for later use you need to retain it here, then release it when you're done.
Edit:
I'm not sure I understand your question 4. A declaration in the interface (.h) file is just reserving space for a pointer. It's not an object declaration, so there's no release required until you actually do an object allocation.
self.viewloading = viewLoading2 is using the properties setter method to do the assignment. If the #property statement has "retain" in it, then a retain is done as part of that assignment. `viewloading = viewLoading2" is a straight assignment, no retain.
Related
Here's my current situation:
I have a NSMutableArray named dictKeyArray which I assign a property with #property(nonatomic,retain)NSMutableArray *dictKeyArray
I synthesize my mutable array in the implementation file.
Later, I have a dictionary name storeDict. I assign all the keys of the dictionary to the dictKeyArray like so:
dictKeyArray = [[storeDict allKeys] mutableCopy];
Now I use this dictionary later in my implementation file. However, when it comes to releasing it, I release it once in my dealloc method. When checking with instruments, a leak shows up! Why is dictKeyArray leaking? Should I be using assign instead of retain?
I'm still not clear on what the difference is exactly...
thank you!
You have to send it an
[[[storeDict allKeys] mutableCopy] autorelease];
Just to make this clear: mutableCopy does the same as alloc meaning you are claiming ownership of the object in question. You have to decrease the retainCount by one.
By the way: You should use the accessor you wrote for it. You are just assigning it to your iVar at the moment. If you want to make your accessors work, you will have to use something like
object.dictKeyArray = ...;
in general. Or here (as mentioned by dreamlax)
self.dictKeyArray = ...;
because you are referring to an object of this specific class the code is in.
Only this way you are ensuring your object is properly retained by your accessor. Otherwise writing the accessor code doesn't make sense at all because it never gets called.
Please note: As Josh said in the comments, your code should be valid (at least from my point of view). What I suggested is a solution that is not as error-prone as yours because you adhere to the rules (could save you from headache in the near future).
You should be using self.dictKeyArray = .... Without the self. you are accessing the instance variable directly, bypassing any memory management benefits of properties, but, remember that you own the result of mutableCopy, and assigning to a property that also takes ownership will result in double-ownership, so use:
self.dictKeyArray = [[[storeDict allKeys] mutableCopy] autorelease];
Under ARC, a block is suspected to cause a retain cycle if you're using self inside the block, for example.
I've seen a workaround here, like this:
How can this workaround prevent a retain cycle?
weakRequest is just a pointer to the exact same object referenced by request. When ARC modifies the retain count of weakRequest or request, it's affecting the same object.
Then, in the block, there is this strange thing going on:
__strong ASIHTTPRequest *strongRequest = weakRequest;
This is the eqivalent to saying:
ASIHTTPRequest *strongRequest = weakRequest;
[strongRequest retain];
But again: It's one and the same object. Why all these different variable names? They're just pointers!
I never really cared much about blocks and tried to avoid them. But now this made me curious about what everyone is talking about when they say "a block captures the variables". Until today I thought this just means a block will retain every pointer you use which has been defined outside of the scope of the block, meaning that the block just retains whatever object you touch in it's scope.
I did this quick test:
UIView *v = [[UIView alloc] initWithFrame:CGRectMake(100, 100, 100, 100)];
[self.view addSubview:v];
v.backgroundColor = [UIColor orangeColor];
NSLog(#"self = %p", self); // 0x6a12a40
[UIView animateWithDuration:1.5
delay:0
options:UIViewAnimationOptionAllowUserInteraction
animations:^{
UIViewController *my = self;
NSLog(#"my = %p", my); // 0x6a12a40
v.frame = CGRectMake(200, 200, 100, 100);
}
completion:nil];
Like you can see the object itself stays exactly the same. The block does not create a copy. So I can safely assume all the years of C and Objective-C knowledge are still valid:
ASIHTTPRequest *strongRequest = internetRequest;
ASIHTTPRequest *foo = strongRequest;
ASIHTTPRequest *bar = foo;
if (bar == internetRequest) {
NSLog(#"exact same thing, of course");
}
So what is going on there? How can this resolve a retain count if all that's happening is create different pointers to the same object? Why the extra mile of creating those pointers?
Wouldn't this be totally the same thing?
[request setCompletionBlock:^{
NSString *respondeString = [request responseString];
if ([_delegate respondsToSelector:#selector(pingSuccessful:)]) {
[_delegate pingSuccessful:responseString];
}
}];
There must be some secret about Objective-C which explains why duplicating pointers solves memory management problems here. It just doesn't make any sense to me.
It actually has nothing to do with ARC, but rather how blocks capture variables. The pointer is duplicated so that the variable captured by the block has the correct ownership qualifier.
weakRequest is just a pointer to the exact same object referenced by request. When ARC modifies the retain count of weakRequest or request, it's affecting the same object.
Right, they both point to the same object, but weakRequest has the __unsafe_unretained ownership qualifier, which means that when that variable is captured by the block, its retain count is unchanged.
If request were captured by the block then it would be retained and you would have retain cycle, regardless of whether you're using ARC.
The conversion back to a __strong pointer simply keeps that object alive for the duration that block execution.
Well, you specify a variable as __weak precisely so that the block won't retain it so that you can avoid a retain cycle. However, creating a __strong Variable inside the block and pointing it to the __weak variable is completely superfluous. You designate it as weak so that the block won't retain it. Creating a new one and designating it as __strong doesn't mean anything because there's no instance whereby the block will need to retain it. __strong is only a compiler keyword to tell ARC to retain the value if the need arrives...and ARC will never find that need since it's already been passed into the block. In the end, you could simply use the weakRequest variable and do away with the strongRequest variable.
You may be confused because there are two different things going on, to prevent two different problems. You quoted this line:
__strong ASIHTTPRequest *strongRequest = weakRequest;
That line does not prevent a retain cycle.
The (potential) retain cycle is one problem. The retain cycle would contain three objects: self, the ASIHTTPRequest, and the block. Use of the weakRequest variable breaks that cycle, because the block captures the weakRequest, which does not own the ASIHTTPRequest object. In reference count terms, assigning te weakRequest does not increment the reference count of the ASIHTTPRequest.
The line you quoted is there to prevent the other problem, which is created by solving the first problem. The other problem is a potential dangling pointer. Since weakRequest doesn't own the ASIHTTPRequest, there's a risk that during the execution of the completion block, all owners of the ASIHTTPRequest will release it. Then weakRequest will be a dangling pointer - a pointer to an object that has been deallocated. Any use of it is likely to cause a crash or heap corruption.
In the line you quoted, the block copies weakRequest to strongRequest. Because strongRequest is __strong, the compiler generates code to retain (increment the reference count of) the ASIHTTPRequest, and code to release it at the end of the block. This means that even if all other owners of the ASIHTTPRequest release it while the block is running, the ASIHTTPRequest will stay alive because the block has temporarily made itself an owner of the request.
Note that this solution to the dangling pointer problem is not thread-safe. If the owners of the request can release it from other threads while block is executing, there is a race condition that can still lead to a dangling pointer. This is why you should try to use __weak instead of __unsafe_unretained for weak pointers: __weak references can be copied to __strong references without a race condition.
I have been working with Objective-C for a month approximately but regretfully I'm still a complete dummy in memory management so I need your advice. I pass an array from one file to the other like this
BidView *bidView = [[[BidView alloc] init] autorelease];
NSLog(#"%i",[bidView.seatsForTableCreated retainCount]);
bidView.seatsForTableCreated = [NSArray arrayWithArray:seats];
NSLog(#"%i",[bidView.seatsForTableCreated retainCount]);
[self.navigationController pushViewController:bidView animated:YES]; `
NSLog tells me that retain count of seatsForTableCreated has raised from zero to two. Then, when I quit the BidView screen (without doing anything with seatsForTableCreated array) I' m doing the following:
NSLog(#"%i",[seatsForTableCreated retainCount]);
[seatsForTableCreated release];
NSLog(#"%i",[seatsForTableCreated retainCount]);
it's quite unclear for me. Now NSLog tells me (both times) that retain count is 1. Then I repeat this procedure (running the same application I mean) and each time things are the same:0-2-1-1. So my questions are:
1)Why 0 to 2? Why retain count increases to 2 not to 1?
2)why then it drops to 1 without being impacted in any way?
3)Why it still remains 1 after i've released it?
4)How would you manage the memory in such a case?
Great thanks in advance
First and foremost. Don't call nor use retainCount for nothing, think about this property as private and only the OS can call. To check if you have a memory leak you should use Instruments.
Seems like you've created an autoreleasing ([NSArray arrayWithArray:seats]) object, so you can't manually release it.
And use the Allocations Instrument to really check if you have a memory leak.
My advice assumes you are using Xcode 4+ and you are not using ARC,
command+shift+B will analyse your memory management (and dead stores and such). I think you got it right. Don't worry about the retain counts so much until you get a complaint from Analyze or find leaks with Instruments. I am not sure how reliable retain counts are. I have seen comments on SO saying not to rely on them.
You are following the rules well
New, Alloc, Copy, Retain --> You will need to release this object when you are done with it.
I am also assuming in BidView.h your property is declared as
#property(nonatomic, retain) NSArray * seatsForTableCreated;
So releasing that in the dealloc method in BidView.m is good memory management
EDIT
It works when even though you don't allocate seats for table created because.
self.seatsForTableCreated = ... will retain whatever object you are setting there.
So if you have a property with (retain) in the declaration, you can consider
self.property = object;
as setting property and retaining it. The properties were added to objective-C to reduce similar code being in every class.
A property in .h
#property (nonatomic,retain) NSObject * property; // don't name your properties property..
Compiler will create 2 methods for you automatically when you #synthesize in the .m
-(void)setProperty:(NSObject*)newP
{
[newP retain]; // retains the new object so it sticks around for line 3
[property release]; // releases previous property
property = newP; // set the property to the object retained in line 1
// property is now same as newP and you are responsible for releasing it
// -(void) dealloc is where you should release it
}
// Note, the compiler may not create the exact same code as above when creating the //setProperty method. If it does, it could be subject to change.
-(NSObject*)property
{
return property;
}
I tried to figure out why Analyze isn't catching the issue when you don't release your property, but haven't. That is confusing and I want to explore it further.
I just wrote some code. It has my own custom made class. In that custom made class called 'WonderfulNumbers' I have a method that looks like this
- (NSString *)storedNumberAsString {
NSString *stringToReturn = [[NSString alloc]initWithFormat:#"Hello World"];
return [stringToReturn autorelease];
}
I obviously then #import "WonderfulNumbers" into the main and etc
then in my main I have an IBAction.
it reads like the following
-(IBAction)displaySomeText:(id)sender {
WonderfulNumbers *myNumber = [[WonderfulNumbers alloc]init];// Step 1
NSString *numberString = [myNumber storedNumberAsString];// Step 2
[textView insertText:numberString];// Step 3
//textView is a variable of NSTextView.
[myNumber release];// Step 4
}
I get the step by step process, and the logic behind this.
What I like to know and try to reassure to my self is what exactly is happening at step 2.
I understand Step 1, 3 and 4.
But step 2, I crated an NSString variable called 'numberString' and it obviously holds the object called 'myNumber' with the method described above applied to it. Makes sense.
What I want to know is, I never allocated or initialized the 'numberString' object of NSString class. How am I able to then use it.
I know I don't release it since it's never allocated .. but did I initialize it by just doing [myNumber storedNumberAsString]; ?
A slight explanation would be awesome. Thank's to those that reply.
P.S. I know that everything in objective-c is an object but just for the sake of this argument, since 'numberString' is not technically "created by allocate and init" is it right to call that a variable?
I think I know the differences between the two but just want reassurance.
Thanks.
You are initializing NSString *stringToReturn to the return value of [myNumber storedNumberAsString]. Inside storedNumberAsString you can see that it returns an NSString reference, properly allocated and all, so it's fine to use.
The key here is autorelease, which causes the object to be released automatically “sometime later” (actually when the topmost autorelease pool is released, which, unless you changed it yourself, tends to happen after each iteration of the event loop).
The convention in Objective-C is that if you alloc or retain an object, or get an object from a method whose name begins with new, copy, or mutableCopy, you are responsible for releasing it. Otherwise you can assume it will be released by someone else (e.g., later via autorelease). Since storedNumberAsString does not begin with new, copy, or mutableCopy, you don't need to release it. Meanwhile the implementation of storedNumberAsString must ensure that the object it allocs gets released -- in this case that is done by calling autorelease on it.
What is the difference between the following 2 ways to allocate and init an object?
AController *tempAController = [[AController alloc] init];
self.aController = tempAController;
[tempAController release];
and
self.aController= [[AController alloc] init];
Most of the apple example use the first method. Why would you allocate, init and object and then release immediately?
Every object has a reference count. When it goes to 0, the object is deallocated.
Assuming the property was declared as #property (retain):
Your first example, line by line:
The object is created by alloc, it has a reference count of 1.
The object is handed over to self's setAController: method, which sends it a retain message (because the method doesn't know where the object is coming from), incrementing its reference count to 2.
The calling code no longer needs the object itself, so it calls release, decrementing the reference count to 1.
Your second example basically does steps 1 and 2 but not 3, so at the end the object's reference count is 2.
The rule is that if you create an object, you are responsible for releasing it when you're done with it. In your example, the code is done with tempAController after it sets the property. It is the setter method's responsibility to call retain if it needs that object to stick around.
It's important to remember that self.property = foo; in Objective-C is really just shorthand for [self setProperty:foo]; and that the setProperty: method is going to be retaining or copying objects as needed.
If the property was declared #property (copy), then the object would have been copied instead of retained. In the first example, the original object would be released right away; in the second example, the original object's reference count would be 1 even though it should be 0. So you would still want to write your code the same way.
If the property was declared #property (assign), then self isn't claiming ownership of the object, and somebody else needs to retain it. In this case, the first example would be incorrect. These sorts of properties are rare, usually only used for object delegates.
As others have noted, the two code snippets you show are not equivalent (for memory management reasons).
As to why the former is chosen over the latter:
The correct formulation of the latter would be
self.aController= [[[AController alloc] init] autorelease];
Compared with the former, this adds additional overhead through use of the autorelease pool, and in some circumstances will lead to the lifetime of the object being unnecessarily extended (until the autorelease pool is released) which will increase your application's memory footprint.
The other "possible" implementation (depending on where the example is from) is simply:
aController = [[AController alloc] init];
However, setting an instance variable directly is strongly discouraged anywhere other than in an init or dealloc method. Elsewhere you should always use accessor methods.
This brings us then to the implementation shown in sample code:
AController *tempAController = [[AController alloc] init];
self.aController = tempAController;
[tempAController release];
This follows best practice since:
It avoids autorelease;
It makes the memory management semantics immediately clear;
It uses an accessor method to set the instance variable.
Note also that your desire to cut the code down to one line is why many people use Autorelease:
self.aController = [[[AController alloc] init] autorelease];
Though in theory on the iPhone autorelease is somehow more expensive (never heard a clear explanation why) and thus you may want to explicitly release right after you assign the object elsewhere.
If you're using Xcode, it can help you detect such code with the static analyzer.
Just hit Build >> Build and Analyze
This will show you a very helpful message at such pieces of code.
One other thing to note is that your example depends on the #property definition of aController also.
If it were defined as #property (readwrite, retain) id aController; then your example works, while if it is defined as #property (readwrite, assign) id aController; then the extra call to release would cause your object to be deallocated.
You could also do
#property (nonatomic, retain)AController *aController;
...
self.aController= [[AController alloc] init];
[aController release];
with a retaining property, and it would function the same way, but its better to use the other way (for retaining properties) because it's less confusing, that code makes it look like you assign aController and then it gets deleted from memory, when actually it doesn't because setAController retains it.