NSMutableArray addObject getting set to nil after release of object - ios

I have declared an NSMutableArray *arrAllRecordsM and am trying to add NSMutableDictionary *dicRecordM to it using addObject. The dicRecordM gets added to arrAllRecordsM but on doing [dicRecordM removeAllObjects] it sets to nil in arrAllRecordsM. Below is the code, please help me fix it.
self.arrAllRecordsM = [NSMutableArray array];
self.dicRecordM = [NSMutableDictionary dictionary];
// Some Method
[self.dicRecordM setObject:#"Test" forKey:#"ADDRESS"];
[self.arrAllRecordsM addObject:self.dicRecordM];
// Value: Test
NSLog(#"Value: %#", [[self.arrAllRecordsM objectAtIndex:0] valueForKey:#"ADDRESS"]);
[self.dicRecordM removeAllObjects];
// Value: null
NSLog(#"Value: %#", [[self.arrAllRecordsM objectAtIndex:0] valueForKey:#"ADDRESS"]);

Adding an object to an NSMutableArray just stores a pointer (or "strong reference")
to the object into the array. Therefore
[self.arrAllRecordsM objectAtIndex:0]
and
self.dicRecordM
are two pointers to the same object. If your remove all key/value pairs from self.dicRecordM then [self.arrAllRecordsM objectAtIndex:0] still points to the same
(now empty) dictionary. That is the reason why
[[self.arrAllRecordsM objectAtIndex:0] valueForKey:#"ADDRESS"]
returns nil.
If you want an independent copy of the dictionary in the array, use
[self.arrAllRecordsM addObject:[self.dicRecordM copy]];
copy can be used on many classes, such as NSDictionary, NSArray and NSString
and their mutable variants, to get a "functionally independent object". Formally, it is available for all classes conforming to the NSCopying protocol.

This is expected behavior.
You removed all the objects from the dictionary by calling removeAllObjects, then tried to retrieve an object from it and rightfully getting nil, since it doesn't exist in the dictionay anymore (you removed it).
What's maybe unclear to you is that NSArray doesn't copy the element you add to it, instead it just holds a strong reference.
So both dicRecordM and arrAllRecordsM are holding a reference to the same object, hence any modification to it (in this case removeAllObjects) will affect the same dictionary.
Incidentally, you shouldn't use valueForKey: for accessing the dictionary's entries. Use objectForKey: or the shorter subscripted syntax. For instance
self.arrAllRecordsM[0][#"ADDRESS"]
You can read this answer Difference between objectForKey and valueForKey? as a reference, but the main problem is that valueForKey: can behave very differently from objectForKey: in case the key contains special KVC characters, such as # or ..

Related

Managing arrays - mutating method sent to immutable object

I try to populate existing array with other array:
if (!self.photos){
self.photos = [responseDictionary valueForKeyPath:#"data"];
} else {
NSArray *newPhotosFromFeed = [responseDictionary valueForKeyPath:#"data"];
[self.photos addObjectsFromArray:newPhotosFromFeed];
}
Compiler throw me an error - mutating method sent to immutable object .
Photos declared like this:
#property (nonatomic) NSMutableArray *photos;
Method declaration (from Apple.developer) is following:
- (void)addObjectsFromArray:(NSArray *)otherArray
So, how am i violate the rules?
I add NON-mutable array NSArray *newPhotosFromFeed to MUTABLE array self.photos.
So why am i get that error??
If this line is called first:
self.photos = [responseDictionary valueForKeyPath:#"data"];
then you will be storing an immutable array in a mutable property. The compiler can't tell it's wrong and the runtime doesn't check so you don't know it's happening. Then, later, when you call:
[self.photos addObjectsFromArray:newPhotosFromFeed];
you get the crash.
So, you should always ensure that the object being set to photos is mutable, the easiest way it by taking the mutableCopy as that will work on both mutable and immutable source objects:
self.photos = [[responseDictionary valueForKeyPath:#"data"] mutableCopy];
Here's the fix :
self.photos = [responseDictionary valueForKeyPath:#"data"].mutableCopy;
and
NSArray *newPhotosFromFeed = [responseDictionary valueForKeyPath:#"data"].mutableCopy;
You're attempting to set an NSMutableArray to a NSArray. That doesn't work without using mutableCopy. Use mutableCopy to create a mutable copy of the NSArray and that object becomes an NSMutableArray.

How does inheritance work in Objective-C?

What is the difference between
NSArray *arr1 = [[NSMutableArray alloc] init];
NSDictionary *dict1 = [[NSMutableDictionary alloc] init];
and
NSMutableArray *arr2 = [[NSMutableArray alloc] init];
NSMutableDictionary *dict2 = [[NSMutableDictionary alloc] init];
I know both (arr1, dict1, arr2, dict2) are going to create NSMutableArray and NSMutableDictionary object respectively, arr1 and dict1 would have access to all the functions that could be accessed by arr2 and dict2.
My Question to you guys is:
What is the difference between arr1 and arr2 or dict1 or dict2? Why would any one make object like arr1 and dict1 when we could do same things by making objects like arr2 and dict2? Are there any benefits of initializing objects like arr1 and dict1 over arr2 and dict2? If none, what could be the use of initializing objects like this?
arr1 is a reference to NSArray which gets an instance of a specialization of NSArray assigned, that is NSMutableArray. (NSMutableArray class inherits from NSArray and adds some methods to modify the array)
The compiler will prevent you from modifying arr1. (However, in Obj-C you could still call the NSMutableArray methods on arr1 during runtime when the object actually is of type NSMutableArray, but that is a different topic and not a proper way of doing it. See below for a better way of modifying arr1 if it really is mutable.)
The same applies for dic1.
arr2 and dict2 on the contrary are references to NSMutableSomething. Therefore those methods that belong to the mutable classes but not to their immutable base class can be accessed in code.
Some addition: Later in your code you could assign (and cast) arr1 to arr2 and then access the methods of NSMutableArray. Before assigning and casting a reference to a base class to a reference to a specialzation, you should always check the current class by isKindOfClass: method of NSObject.
Sampe:
if ([arr1 isKindOfClass:[NSMutableArray class]]) {
arr2 = (NSMutableArray*) arr1;
// Do something with arr2 that requires its mutability.
}
Mutable types have some special privileges like runtime update/insert/delete object from collection.
But they must have to be initialise before use and they occupies more memory than immutable.
Where else immutable type collection can not be alter at runtime. They are static containers as and contains elements which can't be alter from collection.
Immutable does not need to be alloc init and they consume less memory than mutable.
In short NSArray is not flexible. You can't add or remove object into it. Same goes for NSDictionary. You can initialize but dynamic cast will occur and you will have in arr1 simply NSArray.
Difference is simple
1-Mutable classes are those which could be changed after initialization but other can't be changed using SetObject: or SetValue: etc.
2-Mutable classes dynamically create on heap other are created on stack.
I prefer immutable on mutable because if mutable is exposed then other can
change its data using setobject or setvalue and may be we don't know about it.
but if immutable is exposed then we can't change its data so data remain protected.

NSDictionary objectForkey and accessing the key directly

I have a simple question about accessing different ways of accessing values stored in NSDictionary. so if i have a dictionary like
NSDictionary *dict =#{#"Mammal" : #"Cow", #"Bird" : #"Eagle"};
Now what is the difference if i access the elements like dict[#"Mammal"] and [dict objectForKey:#"Mammal"] what if there is no object with key #"Mammal" in this case. Does it crash with dict[#"Mammal"] or dict[#"Mammal"] calls [dict objectForKey:#"Mammal"] under the hood.
Is it the similar case with NSArray, NSMutableArray & NSMutableDictionary
Using dict[#"Mammal"] actually calls - (id)objectForKeyedSubscript:(id)keyunder the hood.
The behaviour is the same as calling [dict objectForKey:#"Mammal"] according to the documentation:
Return Value
The value associated with aKey, or nil if no value is associated with aKey.
Discussion
This method behaves the same as objectForKey:.

Why can you sometimes cast a NSArray to NSMutableArray, and sometimes you can't?

Specifically:
self.words = (NSMutableArray*)[self.text componentsSeparatedByString:#" "];
works fine so long as there are separators. I see that the method returns the original string wrapped in an NSArray if there isn't, though. This single element NSArray stubbornly refuses to be cast as an NSMutableArray.
But, when I do:
self.words = [NSMutableArray arrayWithArray:self.words];
It works just fine.
Is there something I'm missing here? Is it bad practice to cast from NSArray to NSMutableArray?
You are confused.
This code:
self.words = (NSMutableArray*)[self.text componentsSeparatedByString:#" "];
...is wrong, and is setting you up for a crash later on. Casting just tells the compiler "trust me, I know what I'm doing." It does not change the type of the underlying object. The method componentsSeparatedByString returns an NSArray, not a mutable array. If you then try to mutate the resulting array, you will crash with an unrecognized selector message. With the cast, the compiler trusts you that your object will really be a mutable array at runtime, but it will not be.
This would crash:
self.words = (NSMutableArray*)[self.text componentsSeparatedByString:#" "];
[self.words addObject: #"new"];
However, this code:
self.words = [[self.text componentsSeparatedByString:#" "] mutableCopy];
[self.words addObject: #"new"];
...does the right thing. It doesn't cast a pointer, it is a method call to a method that takes an NSArray as input and returns a mutable array with the same contents. Thus the second line will work because the first line takes the immutable array it gets back from componentsSeparatedByString and uses it to create a mutable array.
It is bad practice to cast from NSArray to NSMutableArray. It may works if you are lucky because the array are constructed using NSMutableArray, but you can't rely on it.
If you want NSMutableArray from NSArray, use mutableCopy
self.words = [[self.text componentsSeparatedByString:#" "] mutableCopy];
Is it bad practice to cast from NSArray to NSMutableArray?
Yes. Bordering on nonsensical, really.
Typecasting does not change the object in any way at all, it just tells the compiler that it should regard the object as if it were an instance of the new type. At run time though, the object stays exactly the same. You may cast an NSArray to an NSMutableArray but it doesn't make the object transform into an NSMutableArray. It's still the same object of the same type.
Your misconception seems to be about the nature of casting. Casting doesn't change the object, it just tells the compiler to pretend that it's an object of that type. If the object really isn't an NSMutableArray, casting is not expected to make it become one.
NSString* string1 = #"this thing";
NSString* string2 = #"this";
NSMutableArray* array1;
NSMutableArray* array2;
array1 = (NSMutableArray*)[string1 componentsSeparatedByString:#" "];
array2 = (NSMutableArray*)[string2 componentsSeparatedByString:#" "];
[array1 addObject:string1]; //(A)
[array2 addObject:string1]; //(B)
This will break at (B) the last line with :
-[__NSArrayI addObject:]: unrecognized selector sent to instance 0x1702257a0
But will not break at (A)1
An object that is publicly declared as immutable may have been privately created as mutable. In this case, if you cast from the immutable public type to the private mutable type, everything will work fine. Where the object was not created as mutable, such a cast will not get you what you want.
In the case of componentsSerparatedByString, this suggests that the method implementation creates a mutable array only if it needs to - i.e. if it has to add more than one object to the array. If it only finds one object, you get an NSArray, if it finds more than one, you get an NSMutableArray. This is an implementation detail that is deliberately hidden from you as the user.
The interface tells you to expect an NSArray in all cases, and in all cases this will work.
You should not rely on such details to get you what you want. Stick to the public API, that's what it is for.
1 rather, as Bryan Chen points out, it may not break now but could well do so in the future
When you do that:
self.words = (NSMutableArray*)[self.text componentsSeparatedByString:#" "];
the array still remains immutable and if you send it a message from NSMutableArray class it will crash. The trick (NSMutableArray*) is only good to make the compiler happy.
In the second case:
self.words = [NSMutableArray arrayWithArray:self.words];
you do not CAST, you CREATE a new array object.
And of course you don't need to cast the arrays this way. NSMutableArray object IS already NSArray just like any object of a derived class is an object of a base class at the same time
Use second variant in all cases because it is right solution and more clearly for users who will support your code:
NSArray *array = [self.text componentsSeparatedByString:#" "];
self.words = [NSMutableArray arrayWithArray:array];
Never do this one:
self.words = [NSMutableArray arrayWithArray:self.words];
It will totally confuse all devs =)
In the first case, the componentsSeparatedByString: method is specifically returning an NSArray, which can't just be cast to the mutable type. If you wanted that to be mutable, you would have to do this:
self.words = [[self.text componentsSeparatedByString:#" "] mutableCopy];
The second one is calling the arrayWithArray: method on the NSMutableArray class, meaning it is making an instance of NSMutableArray. That's why it works. You can cast an NSMutableArray to an NSArray, but not the other way around, because an NSMutableArray is a subclass of NSArray.
Casting does nothing with an object. example:
NSString *mouse = (id)#[#"mouse"];
it will compile, but variable mouse is not NSString. it is NSArray. you can check it simply by writing
po mouse
in console.
The only way to create mutable copy of an object is to call 'mutableCopy' method on it:
NSArray *array = #[#"a"];
NSMutableArray *mutableCopy = [array mutableCopy];

How does this NSMutableArray alloc&init work?

I see a new kind of alloc&init NSMutableArray way in one project. It's like this A
NSMutableArray *array = [#[] mutableCopy]; and this works well, and i want to try whether its possible to use BNSMutableArray *array = [NSMutableArray mutableCopy]; it build succeeded, but got this error when used: +[NSMutableArray addObject:]: unrecognized selector sent to class 0x38bedc2c
Now i want to know how does A work? and why B is wrong? A is better than normal alloc&init?
Any help will be appreciated.
mutableCopy is an instance method declared in NSObject class. It is called on any instance to create a mutable copy of it.
In first case #[] will create an autoreleased NSArray instance on which calling mutableCopy will create NSMutableArray instance.
In second case calling mutableCopy on the class is incorrect because it is not meant to be called that way. This will get compiled but will cause exception at runtime.
Hope that helps!
In the first case, you're first initializing an empty NSArray instance; think of #[] as equivalent to [[NSArray alloc] init]. Therefore you're sending mutableCopy to a correct instance, so it works fine.
In the second case, you're sending the message to a class (as opposed to an instance of it), which doesn't make much sense, because the addObject message can only be sent to an instance, not the class itself.
#[] means an NSArray with no object. It returns an NSArray, and then its mutableCopy is copied to array.
+[NSMutableArray addObject:] is invalid as addObject is an instance method and you are trying to use it as class method.
Even NSMutableArray *array = [NSMutableArray mutableCopy]; is incorrect!!! As nothing is created, it is not been allocated and inited. If you log the array, it will only print the string NSMutableArray. Also you can't use array to addObject and other operations.
You should use NSMutableArray *array = [NSMutableArray array];
The first one is lazy typing.
You should avoid it.
It creates an empty NSArray from the array literal syntax and the creates a mutable copy.
That's saving a little typing by creating an unnecessary array.
You should just use
[NSMutableArray new]
Or
[[NSMutableArray alloc] init]
Or if possible because you know the initial capacity in advance
[[NSMutableArray alloc] initWithCapacity:someNSUIntegerValue]
Anything else above is laziness.
Only use mutableCopy when you are actually copying some content.

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