I am in the process of learnig objective-c and programming an iPad app. One thing I keep tripping myself up on and having to re-read is memory management. I am getting there...slowly. Basic rules such as for every alloc / retain you must have a release is useful. However, one relatively basic thing eludes me and I wonder if someone could explain...
Take the following code...
NSArray *myArray = [[NSArray alloc] init];
myArray = [someNSSet allObjects];
This is relatively straight forward coding and would require a [myArray release] statement.
However, I keep seeing examples of (and indeed, I have used extensively the following 'short cut'...
NSArray *myArray = (NSArray *)[someNSSet allObjects];
How, as far as I understand when you use the (NSString *) you dont need to use a [myArray release] statement, but I dont understand why.
Could someone possible explain?
NSArray *myArray = [[NSArray alloc] init];
myArray = [someNSSet allObjects];
this code is leaking myArray because you lose the reference to NSArray that you've allocated on the first line; you don't need to alloc here, because on the second line you're assigning a new value to myArray.
NSArray *myArray = (NSArray *)[someNSSet allObjects];
and this code example is perfectly fine, you're assigning the result of [someNSSet allObjects] to myArray pointer and you don't own the returned value, so you don't need to care about releasing it.
Consider using ARC (Automatic Retain Counting) for you project. With ARC the compiler takes care of retain counts so you don't have to, in fact aren't allowed to. There is a refactoring that will convert a current project.
As you said, there is a leak in the first code you posted. so you must add a release:
NSArray *myArray = [[NSArray alloc] init];
[myArray release];
myArray = [someNSSet allObjects];
In fact, when you obtain an object through a method that starts with alloc, new or copy, you own it, and you should release it. That's why, here you should release the array you obtained using the method alloc. This convention makes it easy to know when you own objects and when you don't. So remember: alloc, new or copy.
As for the second example, you obtained the array though a method that doesn't start with one of the three words (alloc, new or copy), so you don't own the object, and you are not responsible of releasing it. In fact, the array you obtained is an autoreleased object, which means that even though its retain count is currently 1, it will be automatically released when something called the autorelease pool is drained.
Here is a reference about Memory Management Rules.
In the first line:
NSArray *myArray = [[NSArray alloc] init]
some amount of memory is allocated for an array (actually in this case it is senseless since the size of the array is 0. Keep in mind that NSArray is immutable!).
The variable myArray holds the address of the first byte of the reserved memory area.
Now in the second line you change the value of myArray which now will point to the first byte of the memory area where [someNSSet allObjects] is stored. At this moment you do not know any more where the array is stored what you've created in the first line. And so you have a leak.
The line:
NSArray *myArray = (NSArray *)[someNSSet allObjects];
is correct, since you do not reserve any memory at this point. If you are not using ARC you might call retain in order to keep GC away from the referenced block of memory. In other way you might receive a BAD_EXEC when the owner of the object releases it and you try to access it through e.g.: [myArray objectAtIndex:0]
Related
What is the right way to get an NSString object from an NSArray, keeping Memory Managment in mind.
Suppose I have an array
NSArray *myNewArray = [[NSArray alloc] initWithObjects:.......];
Now I want to get an object from this NSArray at index 2.
NSString *nameString = [myNewArray objectAtIndex:2]; // is it the right way? how to deal with "nameString"
// now regarding memory managment, should I release it ?
OR I should first alloc nameString and then assign value to it ?
Check out the Cocoa Memory Management rules (how many times have I started a post with that sentence?). In particular
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”, “new”, “copy”, or “mutableCopy” (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” (specifically: when the used autorelease pool receives a drain message—to understand when this will be, see “Autorelease Pools”).
Does the method objectAtIndex: begin with "alloc"? No. Does it begin with "new"? No. Does it begin with "copy" or "mutableCopy"? No. Have you sent retain to the returned object? No.
Therefore you do not own nameString. Therefore you must not release or auto release it.
Sorry if the above seems a bit "leading by the nose" but when I first started with Objective-C, I found it useful to pretty much go through all the above in my head in exactly that way, otherwise I tended to get it wrong. It doesn't take long for it all to become second nature.
I think you don't totally get the "pointer" concept yet. Your variable nameString is just a pointer. Not a string.
In the line :
NSString *nameString = [myNewArray objectAtIndex:2];
You just assign the pointer to the actual memory address of the second object of the array. That's all.
If you are about to keep this object alive (even if the array will be deallocted), you better retain that object.
NSArray *myNewArray = [[NSArray alloc] initWithObjects:.......];
NSString *nameString = [myNewArray objectAtIndex:2]
[myArray release];
That's it, no need to bother any more with it. NSString will be released by environment itself.
There is no need to allocate or initialize NSString object and after all if you are not allocating any memory then there is no need to release....Only release NSArray objects nothing else....
You are going through right way...
There is no need to release the nameString.You only release when you are using(alloc”, “new”, “copy”, or “mutableCopy”) .
NSArray *myNewArray = [[NSArray alloc] initWithObjects:.......];
and getting value in NSString from any of index wont affect memory ...no need to allocate here.
The collected NSString need not required to be retained until you are passing its ownership to another controller/object.
Short Question with a code example:
NSLog(#"%i", [[[NSArray alloc] init] retainCount]);
NSLog(#"%i", [[[NSMutableArray alloc] init] retainCount]);
Output:
2
1
Why is the retainCount from the NSArray and NSMutableArray different?
Nobody outside of apple knows for sure (but I'm sure soon there will be somebody that claims he knows exactly why that happened).
Maybe this happens because iOS is smart and it reuses empty NSArrays. And obviously [[NSArray alloc] init] creates an empty array that is of no real use. And since it`s not mutable (ie you can't add objects later, and it will be empty forever) all empty NSArrays pointers can reference the same object.
The mutable one can't be reused because you can add objects to it.
Do not use retainCount!
From the apple documentation:
Important: This method is typically of no value in debugging memory management issues. Because any number of framework objects may have retained an object in order to hold references to it, while at the same time autorelease pools may be holding any number of deferred releases on an object, it is very unlikely that you can get useful information from this method.
To understand the fundamental rules of memory management that you must abide by, read “Memory Management Rules”. To diagnose memory management problems, use a suitable tool:
The LLVM/Clang Static analyzer can typically find memory management problems even before you run your program.
The Object Alloc instrument in the Instruments application (see Instruments User Guide) can track object allocation and destruction.
Shark (see Shark User Guide) also profiles memory allocations (amongst numerous other aspects of your program).
The reason is that [[NSArray alloc] init] returns the same object no matter how many times you call it. Look at this code:
NSArray *array1 = [[NSArray alloc] init];
NSArray *array2 = [[NSArray alloc] init];
NSArray *array3 = [[NSArray alloc] init];
NSLog(#"\narray1: %p\narray2: %p\narray3: %p",
array1, array2, array3);
The output is:
array1: 0x10010cae0
array2: 0x10010cae0
array3: 0x10010cae0
This makes sense, because NSArray is immutable, and all empty arrays are identical. It looks like NSArray keeps an empty array handy for this purpose since the retain count for the array pointed to by array1, array2, and array3 is 4.
I don't disagree with #fluchtpunkt's answer, but I think it's safe to say that we know exactly why this happens. I suppose you could say that nobody knows exactly why Apple chose to implement it this way, but it does seem like a good idea.
This is probably a dumb question, but in the introduction to TableViews, the author has a property of NSArray *listData to fill the table with dummy data. In the viewDidLoad, he basically does this:
- (void)viewDidLoad {
NSArray *array = [[NSArray alloc] initWithObjects#"1", #"2", #"3", more stuff, nil];
self.listData = array;
[array release];
...
}
Why does he create another array and set it to the property as opposed to doing something like
- (void)viewDidLoad {
listData = [[[NSArray alloc] initWithObjects#"1", #"2", #"3", more stuff, nil]autorelease];
Is it to manage memory better by using alloc/init vs the autorelease pool? Or is the second way just not going to work? Thanks.
Your code is wrong and is likely to crash. In his code, he calls alloc, meaning the retain count is 1. He then assigns it to a property. I'm assuming this property is declared to be retain, in which case the retain count would go up to 2. He then called release, which drops the retain count back to 1.
In your code, you call alloc, meaning the retain count is 1, then you call autorelease, which means that the retain count will drop to 0 and the object's memory will be deallocated soon. You assign the object to an instance variable - not a retained property like he does - so you won't increase the retain count any more. This means that you'll be left with a dangling pointer to memory that could be overwritten with anything else at any time. When you try to access listData, you'll crash because it is likely to have been overwritten.
Please read Memory Management Programming Guide if you do not understand what is happening here.
Having said that, the core of your question is valid. There's nothing stopping him from doing the same as you, except assigning to a retained property instead of an instance variable.
I've seen in (Apple) sample code two types of ways of allocation memory, and am not sure I understand the difference and resulting behavior.
// FAILS
NSMutableArray *anArray = [NSMutableArray array];
[anArray release];
// WORKS
NSMutableArray *anArray1 = [[NSMutableArray alloc] init];
[anArray release];
By "FAILS" I mean I get crashes/runtime warnings etc., and not always as soon as I call the release...
Any explanation appreciated.
Thanks
Please keep in mind that
NSMutableArray *anArray = [NSMutableArray array];
acts like:
NSMutableArray *anArray1 = [[[NSMutableArray alloc] init] autorelease];
So doing a release again will cause the crash as you are trying to release an autoreleased object.
Hope this helps you.
Thanks,
Madhup
In the first instance you are getting an autoreleased object, which you don't need to release
The second instance is where you are manually allocating the memory yourself, so you a responsible for releasing it.
Read this documentation for help:-
http://developer.apple.com/mac/library/documentation/cocoa/Conceptual/MemoryMgmt/MemoryMgmt.html
To clarify on djhworlds response:
alloc, copy, mutableCopy and new make you the owner of the new object, retain makes you an owner of an existing object, and you become responsible for -[(auto)release]ing it. Other methods return an object that has been -[autoreleased], and thus you don't have any responsibility for it, but beware: It will disappear on the next iteration of the run loop (usually), as that is generally when the autorelease pool drains.
The practical upshot of this is that the //FAILS version works perfectly in the context of that particular piece of code, but once the run loop rolls around and the pool is drained, your object, being already released and gone, causes things to go boom.
I'm new to memory managed code but I get the idea pretty well.
On taking my app through the leaks tool in XCode, I noticed I only had to clean up my custom objects, but not dynamically created arrays for example, so I figured those data types are autoreleased - makes sense since I only had to release the arrays I used as properties that had a (retain) on them.
Then I noticed something peculiar : I was getting a leak on a certain array initialized like this :
NSMutableArray *removals = [NSMutableArray new];
but not a similar one
NSMutableArray *removals = [NSMutableArray arrayWithCapacity:9];
Now, the reason one was set up with "new" is that it could have 0-99 items in it, whereas the other one I knew was going to always be 9. Since both arrays are passed to the same method later based on user interaction, I was either getting a leak if I did not release at the end of the method, or an exception if I did!
I changed the first array to
NSMutableArray *removals = [NSMutableArray arrayWithCapacity:99];
and I get no leaks and don't have to release anything. Can anyone explain?
As noted in the memory management rules, whenever you have an object that you have created with +alloc, +new, -copy, or -mutableCopy, you own it and are responsible for releasing it at some point. (In fact, +new is just shorthand for [[MyClass alloc] init].) As you noted, creating an array via [NSArray new] without releasing it is a memory leak. However, if you handle this object properly, it is usually possible to release it at some point. For example:
If the method that uses the array is called from within the method that creates the array, then you should be able to release the array after it has been used. If the inner method needs to keep a more permanent reference to the array around, then that method is responsible for sending -retain and, eventually, -release to the object. For example:
- (void)myMethod {
NSArray *removals = [NSArray new];
// ...
[someObject someOtherMethod:removals];
[removals release];
}
If you created the array in an -init method for an object, then the -dealloc method can release it when the object is destroyed.
If you need to create the array and then return it from the method, you've discovered the reason that autoreleasing was invented. The caller of your method isn't responsible for releasing the object, since it isn't an +alloc, +new, -copy, or -mutableCopy method, but you need to ensure it is released eventually. In this case, you manually call -autorelease on the object before you return it. For example:
- (NSArray *)myMethod {
NSArray *removals = [NSArray new];
// ...
return [removals autorelease];
}
When you create the array via +arrayWithCapacity:, you aren't calling one of the "special" methods, so you do not have to release the result. This is probably implemented with -autorelease, much like the last example above, but not necessarily. (Incidentally, you can also create an empty autoreleased NSMutableArray with [NSMutableArray array]; the method is found in NSArray, so it won't show up in the documentation under NSMutableArray, but it will create a mutable array when sent to the NSMutableArray class.) If you're going to be returning the array from your method, you can use this as shorthand for [[[NSMutableArray alloc] init] autorelease]—but it is just a shortcut. In many situations, though, you can create an object with -init or +new and manually release it at the appropriate time.
This is how things implemented behind the scene:
+(NSMutableArray*) new
{
return [[NSMutableArray alloc] init];
}
and
+(NSMutableArray*) arrayWithCapacity:(NSNumber)capacity
{
return [[NSMutableArray alloc] initWithCapacity:capacity] **autorelease**];
}
In first case the array is allocated only and you're responsible for de-allocating it. In contrary the arrayWithCapacity has autoreleased for you and won't cause leak even you forget to deallocate.
Cocoa uses certain naming conventions. Anything that starts with alloc, new, or copy returns something with a retainCount of 1 and you are required to release. Anything else that a function returns has a balanced retainCount (it might be held by something else, or it might be retained and out released).
So:
NSMutableArray *removals = [NSMutableArray new];
Has a retainCount of 1, and:
NSMutableArray *removals = [NSMutableArray arrayWithCapacity:99];
or
NSMutableArray *removals = [NSMutableArray array];
Don't since the methods are not prefixed with alloc, new or copy. This is all spelled out in the memory management documentation. In particular:
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.