eventPoints = [[NSMutableArray array] retain];
What does the "retain" keyword do along with the "array"?. "array" is not defined anywhere.
Also eventPoints was declared as a NSMutableArray.
I am just trying to learn. Thanks
Check out this question I asked: iPhone memory management (with specific examples/questions)
It took me a while to get a hang of this too. Hope this helps!
EDIT: As for what [NSMutableArray array] does, according to the docs on NSArray, it does this: "Creates and returns an empty array." and is used by mutable subclasses of NSArray, such as NSMutableArray. Basically, it's the same as doing: [[[NSMutableArray alloc] init] autorelease] (or something really similar). Because it's autoreleased, you need to call retain on it to keep the variable.
1) What does the "retain" keyword do along with the "array"?
As you know, objective-C uses referencing counting for memory management. "retain" increments 1 by everyPoints.
2) "array" is not defined anywhere.
"array" is defined in NSArray. NSMutableArray is a subclass of NSArray, so NSMutableArray can use functions defined in NSArray. "array" is a class method that creates and returns an empty array.
There are four ways to explicitly increment 1 in objective-c: alloc, copy, retain, attain
Because you create an empty array without using any of these, you manually increment 1 by "retain". So in the future, you might need to [everyPoints release] to decrement 1 to deallocate it.
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];
NSArray *array = [dictionary objectForKey:#"field"];
and
NSArray *array = [[NSArray alloc] initWithArray:[dictionary objectForKey:#"field"]];
I see both kind of approaches very frequently in objective C code.
When tried to understand, I found both of them used in similar situation too, which makes contradiction. I am not clear on when I should use 1st approach and when 2nd one?
Any idea?
Detailed explanation and useful references are moms welcome.
First off, those two examples are doing slightly different things. One is retrieving something from an existing dictionary and one is creating a new array by retrieving something from an existing dictionary (the value of that key is an array).
But, if you're asking the difference between getting objects by alloc vs. convenience methods. ([NSString alloc] init vs [NSString stringWith ...), by convention, you own anything that you call alloc, new copy or mutableCopy on. Anything that you call that is not those, is autoreleased.
See the memory guide here. Specifically, look at the rules.
Getting an autoreleased object means it will go away at some point in the near future. If you don't need to hold onto outside the scope of that function, then you can call autorelease on it or use one of the convenience methods that's not alloc, etc...
For example:
// my object doesn't need that formatted string - create the autoreleased version of it.
- (NSString) description {
return [NSString stringWithFormat:#"%# : %d", _title, _id];
}
// my object stuffed it away in an iVar - I need the retained version of it. release in dealloc
- (void) prepare {
_myVal = [[NSString alloc] initWithFormat:"string I need for %d", _id];
}
In the first example, I created a convenience methods for others to call, my class doesn't need that object beyond the scope of that method so I create the autoreleased version of it and return it. If the caller needs it beyond the scope of his calling method, he can retain it. If not he can use it and let it go away. Very little code.
In the second example, I'm formatting a string and assigning it to an iVar variable that I need to hold onto for the lifetime of my class so I call alloc which will retain it. I own it and releasing it eventually. Now, I could have used the first version here and just called retain on it as well.
You have a fundamental misunderstanding of allocations versus instance methods.
The first example, NSDictionary's -objectForKey method, returns id, not an instance of NSDictionary, therefore it does not allocate or initialize the variable.
The second, however is the classic retain part of the retain-release cycle.
The two methods are fundamentally equal (if we are to assume that array is alloc'd but empty in the first, and nil in the second), and both get ownership of the array object. I would go with the second, as it guarantees a reference, and it's shorter.
What I think you're confusing this with are new and convenience methods. Convenience methods (like NSNumber's +numberWithInt:, NSString's +stringWithFormat:, and NSMutableArray's +array), return an autorelease instance of the class (usually). New takes the place of alloc and init in just one word.
Can you please explain me exact difference between these two lines?
NSArray *foo = [NSArray arrayWithObjects:#"hai",#"how",#"are",#"you",nil];
NSArray *bar = [[NSArray alloc] initWithObjects:#"hai",#"how",#"are",#"you",nil];
arrayWithObjects is "convenience constructor".
It will do:
return [[[NSArray alloc] initWithObjects:#"hai",#"how",#"are",#"you",nil] autorelease]
for you.
It is just a convenience method to get an autoreleased object while improving the readability of the statement. Keep in mind that the fact that the object is autoreleased is a simple convention of the language (not a rule, so you could do differently, but I would not suggest that).
arrayWithObject returns an autoreleased array so you do not have to worry about releasing it when you don't need it anymore (but if you store it in an instance variable, you should retain it to prevent the autorelease pool from freeing it). initWithObject returns an array with a retain count of 1, i.e. you own the array and you must release it at some point to prevent memory leaks.
You might want to read this guide for more clarification.
You own the second array but not the first.
Sorry for the newbie question, but I need a NSMutableArray with some NSNumber inside, created dynamically in a for cycle. My code looks like this:
for (...){
NSNumber *temp_number = [[NSNumber alloc] initWithInteger:someNSInteger];
[target_array addObject:[temp_number copy]];
[temp_number release];
}
Is this a correct way to do it? Does it leak?
Thanks! Miguel
Yep, that leaks. You want:
NSNumber *temp_number = [[NSNumber alloc] initWithInteger:someNSInteger];
[target_array addObject:temp_number];
[temp_number release];
So, no copy. The logic is that because you use alloc, you end up owning temp_number. You then add it to the array and the array does whatever it needs to. You've used temp_number for its intended purpose, so you no longer want to own it and release it.
If you were to take a copy, that would create another instance of NSNumber, which you also own, and therefore which you should also release when you're finished with.
In practice, the array (if it's allocated and exists, rather than being nil), will retain the object for itself, but that's an implementation detail specific to that class and not something you should depend upon or even be particularly interested in beyond the contract that says that the objects you add can later be found in the array.
I'm getting lost in pointer land, I believe. I've got this (code syntax might be a little off, I am not looking at the machine with this code on it...but all the pertinent details are correct):
NSMutableArray *tmp = [[NSMutableArray alloc] init];
I them pass that to a routine in another class
- (BOOL)myRoutine: (NSMutableArray *)inArray
{
// Adds items to the array -- if I break at the end of this function, the inArray variable has a count of 10
}
But when the code comes back into the calling routine, [tmp count] is 0.
I must be missing something very simple and yet very fundamental, but for the life of me I can't see it. Can anyone point out what I'm doing wrong?
EDIT: www.stray-bits.com asked if I have retained a reference to it, and I said "maybe...we tried this: NSMutableArray *tmp = [[[NSMutableArray alloc] init] retain]; not sure if that is what you mean, or if I did it right.
EDIT2: Mike McMaster and Andy -- you guys are probably right, then. I don't have the code here (it's on a colleague's machine and they have left for the day), but to fill the array with values we were doing something along the lines of using a decoder(?) object.
The purpose of this function is to open a file from the iPhone, read that file into an array (it's an array of objects that we saved in a previous run of the program). That "decoder" thing has a method that puts data into the array.
Man, I've totally butchered this. I really hope you all can follow, and thanks for the advice. We'll look more closely at it.
You don't need to call retain in this case. [[NSMutableArray alloc] init] creates the object with a retain count of 1, so it won't get released until you specifically release it.
It would be good to see more of the code. I don't think the error is in the very small amount you've posted so far..
I agree with Mike - based on the code you've posted, it looks correct. In addition to posting the code used to call the function and add items to the array, you could try checking the memory addresses of the pointer at the end of the function (when it has all of the objects), and also once it has returned (when it has no objects). I'm not sure why it would be different, but then again the items should stick in the array as well.
You need to show us a bit more of how you're adding objects to the array for us to really help.
I've seen a lot of people write code like this:
NSMutableArray *array = [[NSMutableArray alloc] initWithCapacity:0];
array = [foo bar];
People doing this think it "creates and then sets" a mutable array, but that's not at all what it does. Instead, it creates a mutable array, assigns it to the variable named array, and then assigns a different mutable array to that variable.
So be sure you're not confusing the variable for the object to which it is a reference. The object isn't the variable, it's interacted with through the variable.
NSMutableArray retains objects added to it, but have you retained the array itself?
The code you posted should work. You must be doing something funny in the decoder function.
You should not retain that array. It's automatically retained with init. If you retain it, you'll leak memory. If you are just starting with objective c, take time and read "Introduction to Memory Management Programming Guide for Cocoa". It will spare you lots of headache.
Why are you writing so much code to read an array from a file? It's already supported by the framework:
+ arrayWithContentsOfFile:
Returns an array initialized from the contents of a specified file.
The specified file can be a full or
relative pathname; the file that it
names must contain a string
representation of an array, such as
that produced by the
writeToFile:atomically: method.
So you can do this:
NSMuatableArray *myArray = [NSMutableArray arrayWithContentsOfFile:#"path/to/my/file"];
This is a convenience method, so the object will autorelease. Make sure to retain this one if you want to keep it around.