I have a setter like this:
- (UIImagePickerController *) foto {
if (_foto == nil) {
_foto = [[UIImagePickerController alloc] init];
_foto.delegate = self;
}
return _foto;
}
it is declared like
#property (nonatomic, retain) UIImagePickerController *foto;
with
#synthesize foto = _foto;
on my dealloc I have
[_foto release];
At some point in my code I want to do this
self.foto = nil;
but something in my soul says the object assigned to self.foto previously will leak, because it was alloc on the setter... how do I make it right?
thanks.
Edit: No, that should be fine. As long as you don't assign something else to _foto before you release, it should work.
Yup. You create an object, then loose the pointer to it. If you throw an autorelease on the init line, that will fix it. You could also use ARC.
The init line doesn't actually do anything... You assign the pointer to an object you create, then assign it to something else.
I don't think there is a leak there. When you assign to self.foto like this:self.foto = nil;, it will release the former one automatically. If you assign it by this way: _foto = nil;, you need to release it manually before the assignment.
Yes that works, and will not leak. When you set the value of _foto, its retain count is 1 (because you called alloc). As long as you release it (which you've said you do) in dealloc, you should be fine, as the retain count will go back to 0. UNLESS your setter is ALSO written by you, and written improperly. It needs to explicitly release the old value, if it's not nil. Something like this:
- (void)setFoto:(UIImagePickerController*)foto {
if (_foto) {
[_foto release];
_foto = nil;
}
if (foto)
_foto = [foto retain];
}
Related
Working on someone else's code. Came across a piece of code while analyzing the project
self.groupPicker = [[UIPickerView alloc] initWithFrame:CGRectMake(0,260,320,216)];
self.groupPicker.delegate = self;
self.groupPicker.showsSelectionIndicator = YES;
[self.view addSubview:self.groupPicker];
Where groupPicker is a UIPicker property. When analyzing the project I encountered a potential leak warning in this case. I have also noticed that the groupPicker property is not being released in the dealloc method. Nor is _groupPicker released anywhere in the project. What should be done in this case?
Should I remove the UIPicker property and just declare a UIPicker variable instead.
Should I just release groupPicker like [_groupPicker release];
What would be the retain count of groupPicker as it is retained once in the .h file and again being allocated as shown in the above piece of code.
1) No, it is perfectly fine to have the property, the problem is that it is being over retained. When you alloc/init the retain count is 1, then you use the retained property which increases the retain count again. The retain count is now 2 and assuming you release the object in dealloc, you end up with a retain count of 1, i.e. a leaked object.
There are many ways you can handle the problem. I think the best way is to autorelease the object on initialization. Like so
self.groupPicker = [[[UIPickerView alloc] initWithFrame:CGRectMake(0,260,320,216)] autorelease];
2) Anything you retain should be released in dealloc, so in dealloc you should
- (void)dealloc {
[_groupPicker release];
[super dealloc];
}
Watch out! When you set a property like
self.property1 = x;
and the property1 is declared as retain, the previous object in the property1 is released and the new object (x) is retained. This is why doing this:
self.property1 = [[x alloc] init];
when property1 is declared as retain, will retain x twice. (one for init, one for setting the property)
The correct way is declaring the object, setting to the property and then releasing
object x = [[class alloc] init];
self.property1 = x;
[x release];
This way, you give the "responsability" of releasing the object x to the property holder.
While using ARC for iOS5+ applications should be preferred, if you don't want to do that just use autorelease after init method.
You should use ARC (Automatic Reference Counting)
to do so got to edit>refactor>convert to objective c ARC
Either assign the UIPickerView to _groupPicker (or whatever the instance variable is named), or use an autorelease on the value as you assign it.
(The problem is that assigning to a retained property causes a retain, and there's already a retain on the object from the alloc.)
I'm going through all of my documentation regarding memory management and I'm a bit confused about something.
When you use #property, it creates getters/setters for the object:
.h:
#property (retain, nonatomic) NSString *myString
.m:
#synthesize myString
I understand that, but where I get confused is the use of self. I see different syntax in different blogs and books. I've seen:
myString = [NSString alloc] initWithString:#"Hi there"];
or
self.myString = [NSString alloc] initWithString:#"Hi there"];
Then in dealloc I see:
self.myString = nil;
or
[myString release];
or
self.myString = nil;
[myString release];
On this site, someone stated that using self adds another increment to the retain count? Is that true, I haven't seen that anywhere.
Do the automatic getters/setters that are provided autorelease?
Which is the correct way of doing all of this?
Thanks!
If you are not using the dot syntax you are not using any setter or getter.
The next thing is, it depends on how the property has been declared.
Let's assume something like this:
#property (nonatomic, retain) Article *article;
...
#synthesize article;
Assigning something to article with
self.article = [[Article alloc] init];
will overretain the instance given back by alloc/init and cause a leak. This is because the setter of article will retain it and will release any previous instance for you.
So you could rewrite it as:
self.article = [[[Article alloc] init] autorelease];
Doing this
article = [[Article alloc] init];
is also ok, but could involve a leak as article may hold a reference to an instance already. So freeing the value beforehand would be needed:
[article release];
article = [[Article alloc] init];
Freeing memory could be done with
[article release];
or with
self.article = nil;
The first one does access the field directly, no setters/getters involved. The second one sets nil to the field by using a setter. Which will release the current instance, if there is one before setting it to nil.
This construct
self.myString = nil;
[myString release];
is just too much, it actually sends release to nil, which is harmless but also needless.
You just have to mentally map hat using the dot syntax is using accessor methods:
self.article = newArticle
// is
[self setArticle:newArticle];
and
myArticle = self.article;
// is
myArticle = [self article];
Some suggestions on reading, all official documents by Apple:
The Objective-C Programming Language
Dot Syntax
Declared Properties
Memory Management Programming Guide
Object Ownership and Disposal
Using Accessor Methods
When you create a retain setter, you're creating something like this:
- (void)setString:(NSString *)someString {
if (someString != string) {
[string release];
[someString retain];
string = someString;
}
}
If you don't use the setter, the new value is not getting that retain—you don't "own" that string, and because it's all references, if the original string is released, you might be facing a null reference, which will lead to an EXC_BAD_ACCESS. Using the setter ensures that your class now has a copy of that value—so yes, it does increment the retain count of the new value. (Note that using the getter is a convention of OOP—that outsiders should not be able to directly touch the ivar. Also in your getter you can modify the value, maybe returning an NSArray when your ivar is an NSMutableArray, for example).
You shouldn't autorelease in a setter—Apple has used it in their sample code, but a thing to keep in mind is that setters are called a lot—millions of times, potentially. All of those objects are going into the same autorelease pool, so unless you create your own and/or regularly flush it, you'll have a ton of elements in your pool, all unneeded but still taking up RAM. Much better to simply release.
As for dealloc, trace back through that setter. If you send a release directly, it's obvious—you release that object. But if you write self.string = nil;, what you're doing is this:
The nil value is not the same, so you enter the if block
You release the old value—what you want to do
You retain nil: messages to nil do nothing, and you don't crash
You set nil, which doesn't take up any memory, to the string, which is now effectively empty
As a matter of convention, I use release in my dealloc method, because release seems more final, and dealloc is the final method call your object will receive. I use self.string = nil; in viewDidUnload and the memory warning methods.
Hope this helps!
In addition to Nick's answer - synthesized getters/setters don't provide autorelease (btw, what's the big idea of doing this? Well, you can use getter as a factory, but it's not a common way in Objective C).
Then in dealloc I see:
self.myString = nil;
or
[myString release];
or
self.myString = nil; [myString
release];
In dealloc it doesn't really matter which form of release you're using. But the good way is to nil your fields when releasing them :) I prefer to use self.myString = nil; in dealloc
.h
# interface MyClass : NSObject {
UILabel *mTextLabel;
}
#property (nonatomic, retain) UILabel *mTextLabel;
and Declare #synthesize mTextLabel in the MyClass.m;
and release the object like this.
[self setMTextLabel:nil];
[mTextLabel release];
NSLog (#"%d",[mTextLabel retainCount]);
This result is 0. and I have not found any error or interrupt.
But. When I release mTextLabel like this. I have just got EXC_BAD_ACCESS
[mTextLabel release];
[self setMTextLabel:nil];
I don't understand why it happen. Plz help me.
When you have a synthesized property with the retain attribute, the synthesized setter calls release on the old ivar before it sets the new value.
Here is an expanded view of what is happening in the first example:
[mTextLabel release];
mTextLabel = nil;
[mTextLabel release];
Since calling a method on a nil pointer does nothing, there is no problem.
In the second example, here is what is happening:
[mTextLabel release];
[mTextLabel release];
mTextLabel = nil;
See the problem?
Edit: it is also worth noting that inspecting the retain count of an object is rarely useful, as any number of Cocoa classes may retain it for their own purposes. You just need to be sure that every time you call retain, alloc, copy or new on an object, there is a matching release or autorelease somewhere in your code.
The problem is you are calling release then you are setting the property to nil which also sends a release to mTextLabel before setting it to nil. This is what happens when the property is defined as copy or retain. All you need is the following code.
[mTextLabel release];
mTextLabel = nil;
Edit:
I would like to add that in your code outside of init and dealloc it is completely fine to call self.mTextLabel = nil to properly release if necessary and nil the value of the property. It is however recommended to NOT use the property in the init/dealloc calls. In those cases you will want to create / release the objects directly to avoid the side effects of the accessor.
The value is already released when you do [self setMTextLabel:nil]. You don't need to release the value explicitly (unless you created the value using an init or copy method, in which case you should release it as soon as you've assigned to self.mTextLabel).
Note that retainCount has a return type of NSUInteger, so cannot ever be negative. So checking to make sure the retain count is zero and not -1 doesn't work.
Interface
#property (nonatomic, retain) PracticalSignsMainViewController *practicalVC;
Implementation
if (self.practicalVC == nil) {
PracticalSignsMainViewController *vc = [[PracticalSignsMainViewController alloc] init];
self.practicalVC = vc;
[vc release];
}
I only want to make one viewcontroller object in this case and change the data that it acts upon. However is it correct to retain or assign this viewcontroller (and why so)? It is used in a navigation-hierachy.
Thanks.
Retain. Typically you want object properties to be retained, and primitive properties to be assigned. The answer here is really good: Use of properties for primitive types
If you're planning to cache a view controller that's used in a navigation, you would need to retain it. The reason is that while the navigation controller retains it temporarily, once the user hits the back button the navigation controller will send the view controller a release message. If you haven't retained the view controller anywhere else at that point it will be deallocated.
If you're creating a new instance each time, that would be fine, but obviously that would destroy a cached instance if the property uses assign semantics.
Firstly, your code is right, but it can be more simple.
If you did the following synthesize,
#synthesize practicalVC; // synthesize release the existing reference if has
the following code is same as your code.
self.practicalVC = [[[PracticalSignsMainViewController alloc] init] autorelease];
As you mentioned, if your app does not need many access to the view controller, you need not to have view controller's instance separately.
============== Modified ====================
And I modified my answer after seeing answers of #6NSString #Abizern.
Regarding autorelease and coding style.
1. autorelease,
#6NSString said that "it uses autorelease which many avoid unless returning an newly alloced object."
But even if you use "autorelease" in return statement, the retain count of the returned object is not immediately decreased. It will be decreased at an appropriate time.
So If we want to decrease the retaincount explicitly and immediately, we should use paired "release".
-(NSObject*)testAuto {
return [[[NSObject alloc] init] autorelease];
}
...
self.anObj = [self testAuto];
// the retainCount of anObj is 2
..
-(void)testAuto {
self.anObj = [[[NSObject alloc] init] autorelease];
}
...
[self testAuto];
// the retainCount of anObj is also 2
..
-(void)testAuto {
self.anObj = [[[NSObject alloc] init] autorelease];
[self.anObj release]; // Yes, yes, you may say this code does not look good. :)
}
...
[self testAuto]
// the retainCount of anObj is 1
...
As you see, (I naturally tested above code again) "autorelease"s both in return statement and in alloc statement are almost same. And if you want to manage retain count harder, we should use "release".
I think "autorelease" in alloc statement is better than one in return statement because "autorelease" in return can be omitted. :)
2. Coding style,
As the end of "1. autorelease", I think, autorelease in alloc statement would be more safe to avoid missing release.
I'm new to memory-management, and am reading different things about how to best release properties.
If I have:
in .h:
#property(retain) NSString *myStr;
and in .m:
#synthesize myStr = _iVarStr;
Should my dealloc have:
[_iVarStr release];
or
self.myStr = nil;
or something else?
Thanks!
Both self.myStr = nil and [myStr release] ultimately do the same thing.
Calling [myStr release] is obvious and just releases it.
Meanwhile, the setter method for myStr looks roughly like this:
- (void)setMyStr:(NSString *)newMyStr
{
[newMyStr retain];
[myStr release];
myStr = newMyStr;
}
So when we do self.myStr = nil, we're first retaining a nil object, which does nothing. Then we release the old variable, which is what we want. Finally, we set the pointer to nil.
What's the difference? The latter sets the pointer to nil. This is better because if we (accidentally) send a message to the released object, we crash if the pointer isn't nil (EXC_BAD_ACCESS). Now honestly, since you're in -dealloc, the object is being destroyed anyways, so it wouldn't really matter what you use.
When a property is set to retain then
self.ivar = nil;
will properly manage the memory allocation. For other property types check the at the official documentation page. It also has a bunch of sample code so you can understand what happens "under the hood" for all the options.
Your dealloc should be this:
- (void)dealloc {
[_iVarStr release];
[super dealloc];
}
Although setting the property to nil is possible, I worry about unintended side effects or KVO actions triggered by the change that may not realize the object is currently being deallocated.
I recommend you use self.ivar=nil(the code ivar=nil previously I wrote was wrong) way in dealloc method. Because, if the ivar's property change from retain to assign (or from assign to retain), you don't have to change your code.