toNewbie question: Could someone help me understand if the receiver of an initWith.... method take ownership of the returned object and should manage it.
For example:
NSString *msg; //could be used to hold some data, then
[msg initWithFormat: #"some text"]; //do I need to do a retain or is the method returning an auto-released string object
Just trying to get my head wrapped around methods that don't use alloc explicitly
Methods with init in the name are generally called in conjunction with alloc, as in:
MyObject *obj = [[MyObject alloc] init];
alloc returns an object with a +1 retain count, so you shouldn't need to retain it.
Related
I am new to iOS development.
I have property as follows,
#property(nonatomic,retain)NSMutableArray *dataArray;
I am doing the following, to alloc it
self.dataArray=[[NSMutable alloc]init];
In the dealloc I am doing the following
-(void)delloc{
[dataArray release];
[super dealloc];
}
But I am getting memory leak for my array initialization.However , it doesn't create the
leak when I don't use self. But I wonder is it a write approach to initialise the array
without using self. Any help is appreciated.
You're getting a leak because the dataArray property is declared with retain, which means that when you use self (thus you use the setter), your retain count goes up to 2 and you only release it once. On the other hand, if you only use the ivar, the retain count is 1 (because of alloc) and you release it once, which is fine. To avoid the memory leak in the first situation, autorelease it like this.
self.data = [[NSMutableArray alloc] init] autorelease];
That will balance the retain count. As for access, except for inside the dealloc method, try to use self (setter and getter)
You should read the memory management docs, first thing to start with when developing for Cocoa Touch.
Also, why don't you use ARC?
If you use the self. signature you are accessing to the object via automatically generated / custom getter/setter. The setter will tipically manage the memory and you don't need to do that.
If you don't use self you access directly to the object.
The code what you presented is leaking, because the default setter of the dataArray will retain to the array, what you set with self.dataArray = [[NSMutableArray alloc] init];
The correct usage is:
self.dataArray = [[[NSMutableArray alloc] init] autorelease];
or:
_dataArray = [[NSMutableArray alloc] init];
What's happening here is that alloc is adding one to the retain count of the new object. The property reference is also retaining the object. If you want to do it this way, you only want one of those. A common method is:
self.dataArray = [[[NSMutableArray alloc]init] autorelease];
However, better still is to use ARC as #c.cam108 suggested and avoid the whole problem.
I'm currently developing an iOS application which was started by another developer.
Usually, I make a property for every instance variable (assign for int, bool etc. / retain for all classes).
So in my projects, this line causes a leak:
myVar = [[NSString alloc] init]; (alloc/init +1, retain in setter +1, release in dealloc -1 => +1)
So I use:
NSString *tmpMyVar = [[NSString alloc] init];
[self setMyVar: tmpMyVar];
[tmpMyVar release];
Or:
NSString *tmpMyVar = [[[NSString alloc] init] autorelease];
[self setMyVar: tmpMyVar];
In this new project, the previous developer didn't use #property/#synthesize so I'm wondering what will be the result of the previous line of code in this context (it doesn't call setter I guess)? Memory Leak?
The previous developer releases variable in dealloc method, just like me.
Thank you very much!
Since it directly assigns the instance variable to the allocated object it's retain count is 1 (because, like you said, a setter isn't called).
And because it's released in dealloc, it's all balanced out. So no memory leaks.
So in my projects, this line causes a leak:
myVar = [[NSString alloc] init]; (alloc/init +1, retain in setter +1, release in dealloc -1 => +1)
No,it wouldn't even in your projects, because, as you pointed out, no setter is used.
Also, when using properties, it is the recommended way to access instance variables directly in the init method, instead of using setters.
To inspect for questionable memory-leaks like your example, also use the clang static analyzer or instrument's leak tool.
You need to look at the other developer's setter implementation. Make sure they release the existing value and retain the new value; something like:
- (void)setMyString:(NSString *)string
{
[string retain];
[_string release]; // ivar
_string = string;
}
The only advantage to implementing your own setter/getter methods is to do something (other than setting the ivar) when a value is set. If the methods don't do anything like this then why not change all implementations to #property/#synthensize?
I have a memory leak problem that just can not understand! Watch this initialization method:
- (id)initWithNomeCompositore:(NSString *)nomeCompositore nomeOpera:(NSString *)nomeOpera {
if (self = [super init]) {
NSString *pathOpere = [[NSBundle mainBundle] pathForResource:kNomeFilePlistOpere ofType:kTipoFilePlist];
NSDictionary *dicOpera = [NSDictionary dictionaryWithDictionary:
[[[NSDictionary dictionaryWithContentsOfFile:pathOpere]
objectForKey:nomeCompositore]
objectForKey:nomeOpera]];
self.nomeCompleto = [[NSString alloc] initWithString:nomeOpera];
self.compositore = [[NSString alloc] initWithString:nomeCompositore];
self.tipologia = [[NSString alloc] initWithString:[dicOpera objectForKey:kKeyTipologia]];
}
return self;}
Then this little variation (note self.tipologia):
- (id)initWithNomeCompositore:(NSString *)nomeCompositore nomeOpera:(NSString *)nomeOpera {
if (self = [super init]) {
NSString *pathOpere = [[NSBundle mainBundle] pathForResource:kNomeFilePlistOpere ofType:kTipoFilePlist];
NSDictionary *dicOpera = [NSDictionary dictionaryWithDictionary:
[[[NSDictionary dictionaryWithContentsOfFile:pathOpere]
objectForKey:nomeCompositore]
objectForKey:nomeOpera]];
self.nomeCompleto = [[NSString alloc] initWithString:nomeOpera];
self.compositore = [[NSString alloc] initWithString:nomeCompositore];
self.tipologia = [[NSString alloc] initWithString:#"Test"];
}
return self;}
In the first variant is generated a memory leak, the second is not! And I just can not understand why! The memory leak is evidenced by Instruments, highlighted the line:
[NSDictionary dictionaryWithContentsOfFile:pathOpere]
This is the dealloc method:
- (void)dealloc {
[tipologia release];
[compositore release];
[nomeCompleto release];
[super dealloc];}
Remember that alloc returns an object that you own.
If you declared your three string properties as retain, assigning those objects to your properties means you now own each one twice—once because you allocked it, and again because you assigned it to your property. The objects remain alive because nothing releases their second ownerships.
If you declared the properties as copy (which is the correct way to declare an NSString property), assigning the object there stores a copy as the value of the property. You do nothing further with the original objects, which remain alive because nothing releases them.
Either way, that is your leak.
The property should be declared as copy; if it already is, don't try to fix the leak by changing that.
You should not use property access here. Remember that assigning to a property is a set<PropertyName>: message, and that your object is not fully initialized yet. Sending a message to an incompletely-initialized or incompletely-deallocated object is asking for trouble, particularly when subclasses are involved, since they may override the accessor methods in ways the superclass doesn't expect.
So, in init only, assign directly to the instance variables. In dealloc only, send release messages directly to the objects in the instance variables. Everywhere else, use property accesses.
You also should not use alloc and initWithString: here. It'll work, but the convention is to send copy messages to the objects you already have, the same as the properties would do. Send copy messages to your input string objects, then assign the copies to your instance variables.
When you do use property accesses, use the convenience constructors (stringWith…:, for example), as these return objects that you do not own. When you assign these objects to your copy-declared properties, you will actually be storing copies that you do own.
The other way would be to use alloc and initWithWhatever:, then immediately autorelease that object before assigning it to the property; this way creates an object that you own, then immediately gives up ownership before assigning it to the property.
Try
nomeCompleto = [[NSString alloc] initWithString:nomeOpera];
compositore = [[NSString alloc] initWithString:nomeCompositore];
tipologia = [[NSString alloc] initWithString:[dicOpera objectForKey:kKeyTipologia]];
or
self.nomeCompleto = nomeOpera;
self.compositore = nomeCompositore;
self.tipologia = [dicOpera objectForKey:kKeyTipologia];
instead of self.xxx = [[yyy alloc] init...].
In the original code, the RHS of the assignment returns an object of retain count +1, and if you make the #property having (retain) or (copy), the final retain count would be +2. Therefore, even if you release these in -dealloc, the net retain count is +1, causing a memory leak.
BTW, there's no point calling +dictionaryWithDictionary:. Just use
NSDictionary* dicOpera = [[[NSDictionary dictionaryWithContentsOfFile:pathOpere]
objectForKey:nomeCompositore]
objectForKey:nomeOpera];
Ok, I know the answer to this question should be obvious, but I need a little push in the right direction.
I find myself writing a fair number of methods that follow the following pattern:
-(NSThing*)myMethod{
NSThing *thing = [[NSthing alloc] init];
// do some stuff with the thing
return thing;
}
My question is, how do I handle the release of this object? Clearly I can't release it within the method.
usually you would autorelease it
-(NSThing*)myMethod{
NSThing *thing = [[NSthing alloc] init];
// do some stuff with the thing
return [thing autorelease];
}
Autoreleasing is the easy way to get out of this, as newacct said. However, you should take into consideration the "ownership" of the object you're returning.
In Objective-C, the general rule of thumb is that any method with alloc, new, or copy in its name returns an object that is not autoreleased, while other methods (like class methods) pre-autorelease the object to be returned. So these three are not equivalent in terms of autorelease (although the compiler may do some magic and reduce them all to string constants anyway):
// Autoreleased
NSString *string1 = [NSString stringWithString:#"aString"];
NSString *string2 = [[[NSString alloc] initWithString:#"aString"] autorelease];
// NOT autoreleased
NSString *string3 = [[NSString alloc] initWithString:#"aString"];
Your code can take a similar approach, where you consider who owns the object you're returning. In the specific example you provided, the method is the one allocing and initing the object, so in general you're responsible for autoreleaseing [sic] it within your own method. However, if you were to write a method that takes a preexisting object and modifies it in some way, you would not own that object and would not be responsible for autoreleasing the returned object. (Doing so could actually cause problems down the road when the autorelease pool to which the object belongs gets drained.)
See also (thanks to Peter and Quinn in the comments!):
Memory Management Rules
Here is code I am referring to.
// Person.h
#interface Person : NSObject {
NSString *firstName;
NSString *lastName;
}
#end
// Person.m
#implementation Person
- (id)init {
if (![super init]) return nil;
firstName = #"John";
lastName = #"Doe";
}
#end
// MyClass.m
#implementation MyClass
.....
- (NSArray *)getPeople {
NSMutableArray *array = [[NSMutableArray alloc] init];
int i;
for (i = 0; i < 10; i++) {
Person *p = [[Person alloc] init];
[array addObject:p];
}
return array;
}
.....
#end
Now, I know there is no memory-management going on in this sample code. What would be required?
In the getPeople loop, I am alloc'ing a Person (retainCount 1), then adding it to array. The retain count is now 2, right? If it is two, should I be [p release]'ing after adding it to the array, bringing the retainCount back down to 1?
Am I right in that it is the caller's responsibility to release the array returned by the method? (Which would also free the memory of the Person's, and their instance variables, assuming their counts are at 1).
I have read Apple's memory management document, but I guess what I am most unclear about, is what increases an objects retain count? I think I grasp the idea of who's responsibility it is to release, though. This is the fundamental rule, according to Apple:
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 are responsible for relinquishing ownership of objects you own using release or autorelease. Any other time you receive an object, you must not release it.
bobDevil's sentence "only worry about the retain counts you add to the item explicitly" made it click for me. After reading the Ownership policy at Apple, essentially, the object/method that created the new object, is the one responsible for releasing /it's/ interest in it. Is this correct?
Now, let's say I a method, that receives an object, and assigns it to a instance variable. I need to retain the received object correct, as I still have an interest in it?
If any of this is incorrect, let me know.
You are correct that the retain count is 2 after adding it to an array. However, you should only worry about the retain counts you add to the item explicitly.
Retaining an object is a contract that says "I'm not done with you, don't go away." A basic rule of thumb (there are exceptions, but they are usually documented) is that you own the object when you alloc an object, or create a copy. This means you're given the object with a retain count of 1(not autoreleased). In those two cases, you should release it when you are done. Additionally, if you ever explicitly retain an object, you must release it.
So, to be specific to your example, when you create the Person, you have one retain count on it. You add it to an array (which does whatever with it, you don't care) and then you're done with the Person, so you release it:
Person *p = [[Person alloc] init]; //retain 1, for you
[array addObject:p]; //array deals with p however it wants
[p release]; //you're done, so release it
Also, as I said above, you only own the object during alloc or copy generally, so to be consistent with that on the other side of things, you should return the array autoreleased, so that the caller of the getPeople method does not own it.
return [array autorelease];
Edit:
Correct, if you create it, you must release it. If you invest interest in it (through retain) you must release it.
Retain counts are increased when you call alloc specifically, so you'll need to release that explicitly.
factory methods usually give you an autoreleased object (such as [NSMutableArray array] -- you would have to specifically retain this to keep it around for any length of time.).
As far as NSArray and NSMutableArray addObject:, someone else will have to comment. I believe that you treat a classes as black boxes in terms of how they handle their own memory management as a design pattern, so you would never explicitly release something that you have passed into NSArray. When it gets destroyed, its supposed to handle decrementing the retain count itself.
You can also get a somewhat implicit retain if you declare your ivars as properties like #property (retain) suchAndSuchIvar, and use #synthesize in your implementation. Synthesize basically creates setters and getters for you, and if you call out (retain) specifically, the setter is going to retain the object passed in to it. Its not always immediately obvious, because the setters can be structured like this:
Person fart = [[Person alloc] init];
fart.firstName = #"Josh"; // this is actually a setter, not accessing the ivar
// equivalent to [fart setFirstName: #"Josh"], such that
// retainCount++
Edit:
And as far as the memory management, as soon as you add the object to the array, you're done with it... so:
for (i = 0; i < 10; i++) {
Person *p = [[Person alloc] init];
[array addObject:p];
[p release];
}
Josh
You should generally /not/ be worried about the retain count. That's internally implemented. You should only care about whether you want to "own" an object by retaining it. In the code above, the array should own the object, not you (outside of the loop you don't even have reference to it except through the array). Because you own [[Person alloc] init], you then have to release it.
Thus
Person *p = [[Person alloc] init];
[array addObject:p];
[p release];
Also, the caller of "getPeople" should not own the array. This is the convention. You should autorelease it first.
NSMutableArray *array = [[[NSMutableArray alloc] init] autorelease];
You'll want to read Apple's documentation on memory management: http://developer.apple.com/documentation/Cocoa/Conceptual/MemoryMgmt/MemoryMgmt.html