I've go a situatiion in Objective-C where I'm trying to access an object's variable through another object. The classes (simplified):
A.h
#interface A : NSObject {
NSMutableArray *someStuff;
}
#property (nonatomic, retain) NSMutableArray *someStuff;
#end
A.m
#implementation A
#synthesize someStuff;
// blah, blah, blah
Then, because I'm doing an iPhone app, there is an app delegate that contains a variable of this object type:
AppDelegate.h
#interface AppDelegate : NSObject <UIApplicationDelegate> {
A *aPtr;
}
#property (nonatomic, retain) A *aPtr;
#end
AppDelegate.m
#implementation AppDelegate
#synthesize aPtr;
// blah, blah, blah
Then, in another class (in this a view controller), I'm trying to access 'someStuff' in this manner:
AViewController.m
AppDelegate *appDelegate = [[UIApplication sharedApplication] delegate];
NSMutableArray *someArray = appDelegate.aPtr.someStuff;
So, the problem is that this blows up in fine fashion. I think I'm too much of a Java junkie to understand why this won't work. Can anyone elighten me?
Many thanks,
Craig
You need to initialize this in this way
AppDelegate *appDelegate = (AppDelegate*)[[UIApplication sharedApplication] delegate];
NSMutableArray *someArray = appDelegate.someArray;
This will resolve your problem.....
Craig,
appDelegate.aPtr will return null. as it is not initialized yet, and when you try to access some member of a null object,(in java NULLPointerException).Its behavior is as expected.(this blows up in fine fashion).
Thanks,
Ravin is correct. The class definition defines the iVars and properties for a class so you have defined an iVar aPtr that references an object of type A. However, you have not allocated and initialized this object.
An example using the default initialization would be `aPtr = [[A alloc] init]'.
This all sounds suspicious since in ObjC you are allowed to send messages to nil without a problem and properties are just diguised methods. For example you can
view = nil;
view.hidden = NO;
and it doesn't blow up, it just does nothing.
So since appDelegate.aPtr.someStuff is just
[[appDelegate aPtr] someStuff];
and [appDelegate aPtr] does nothing and returns nil so it should be safe to call [[appDelegate aPtr] someStuff] without a problem but also without any results.
So while it is a problem with using objects that hadn't been initialized (which most often should be done in a designated constructor of the appropriate object), since you don't get results that you expect, in my undestanding of "sending message to nil" in ObjC it shouldn't blow up. If it is then either I am missing the point or something other causes the problem and not this call.
EDIT
just checked: if not initialized at all it works as I explained: ObjC allows messages to be sent to nil:
A *aPtr = appDelegate.aPtr;
NSMutableArray *someArray = aPtr.someStuff;
NSLog(#"%#", someArray);
or
NSMutableArray *someArray = appDelegate.aPtr.someStuff;
NSLog(#"%#", someArray);
both don't break and print null.
If you initialize A properly but not initialize someStuff in A it still doesn't break but print null. The problem might be that you initialize aPtr to a different class than A, in which case you get unrecognized selector exception (you should be able to see it in the error log) and program crash.
Related
in my AppDelegate I have imported the header of a class I have created and propertied and syntesized an instance of it (in AppDelegate). Now I'm trying to access a method and a variable inside this instance from two other views. I'm relatively new to objective-c, but so far I've learned that if I do this:
AppDelegate *appdelegate = [AppDelegate new];
I will just get a fresh instance from the class inside AppDelegate, so if I set a variable from one view, I can't access it from the other. I've read that if I would do it this way:
AppDelegate *ap = (AppDelegate *)[[UIApplication sharedApplication] delegate];
It would allow me to access the existing instance. But it doesn't work.
Is the way I'm trying to do this totally wrong? Thanks a lot for your help!
UPDATE:
When I do this inside AppDelegate:
myClass.string = #"test";
NSLog(#"appDelegate: %#", myClass.string);
I get this:
appDelegate: (null)
UPDATE2:
I wrote #class AppDelegate; underneath the #import lines in the viewController, but still I can't access myClass. A main problem, which may be the cause why this isn't working from the views, is that I can't even access myClass from AppDelegate.
In AppDelegate.h I wrote:
#property (strong, nonatomic) testClass *myClass;
In AppDelegate.m:
#import "testClass.h"
#synthesize myClass;
This should be right, right?
myClass.h
#property (strong, nonatomic) NSString *string;
myClass.m
#synthesize string;
When I then try to access myClass from appDelegate, I write:
self.myClass.string = #"test";
NSLog(#"appDelegate: %#", self.myClass.string);
The result is:
appDelegate: (null)
I think you have to allocate and initialize the myClass
Write myClass = [MyClass alloc] init] in AppDelegate.m file
#import "AppDelegate"
and also write
#class AppDelegate;
Unless you haven't shown it, you're not allocating testClass and not assigning it to myClass. Objective-C is not like C++ or Java where you can simply declare a variable of a particular class type and have it instantiated on the stack. Each class you use must be instantiated, whether manually or through InterfaceBuilder. The exception is there are some classes provided by the various frameworks which have a single shared instance. Rather than allocating those classes, you simply ask for the shared instance. However, that's not the case here. It's your own class, so you need to allocate it.
It would look like:
myClass = [[testClass alloc] init];
What I am doing is
//ClassB.h
#property (strong, nonatomic) NSString *name;
and
//ClassA.h
#interface ClassA : NSObject
+(ClassA*)methodA:(NSData*)data;
-(id)initWithData:(NSData*)data;
#property (nonatomic, strong) NSMutableArray *arr;
#property (nonatomic, strong) RXMLElement *rxmlRoot;
#end
//ClassA.m
#implementation ClassA
#synthesize arr;
#synthesize rxmlRoot;
+(ClassA*)methodA:(NSData*)data {
return [[ClassA alloc] initWithData:data];
}
-(id)initWithData:(NSData*)data {
self = [super init];
if (self) {
arr = [NSMutableArray array];
rxmlRoot = [RXMLElement elementFromXMLData:data];
/*****edit : just been added to make codes clear*****/
NSString *node = #"players.player";
[rxmlRoot iterate:node with:^(RXMLElement *e){
ClassB *classB = [[[ClassB alloc] init] autorelease];
[classB setName: [e attribute:#"name"]];
// adding ClassB into arr
[arr addObject:classB];
}];
}
return self;
}
#end
So now I am having ClassA object whose arr contains ClassB
Question : later on, when I try to access an particular property of ClassB like
((ClassB*)[classA.arr objectAtIndex:0]).name
and I am getting EXC_BAD_ACCESS at above line..
Please advice me on this issue and how to correct the error. Any comments are welcomed here
Thanks
This line
[arr addObject:ClassB]
makes no sense. Is your intention to put an instance of ClassB into that array, or the class itself (i.e. [ClassB class])? Presumably you must intend to put an instance of ClassB in there, otherwise trying to access its properties later on (e.g. firstName) would make no sense. Also, does your ClassB even have a firstName property, because the piece of ClassB's interface that you show us only mentions a name property.
Update:
Since you are using manual memory management, you need to retain the objects (arr, rxmlRoot) you create in your initializer using convenience constructors, which return autoreleased objects. For example, the code should be
arr = [[NSMutableArray array] retain];
Post your ClassB.m .
Are you making the #synthesize name?
Also make the Alloc for arr.
This line is so wrong:
((ClassB*)[classA.arr objectAtIndex:0]).firstName
Your string is called name , not firstName. It should be :
((ClassB*)[classA.arr objectAtIndex:0]).name
The code in the question has changed substantially, so my previous answer now makes no sense and I have removed it. Given revised code, the first thing to do is to log what's going on.
NSLog(#"classA: %#", classA);
NSLog(#"classA.arr: %#", classA.arr);
((ClassB*)[classA.arr objectAtIndex:0]).name
If it blows up on the first log statement, things are really bad. But then at least you know that classA is pointing to something rotten and you can work back from there.
You can achieve the same thing in the debugger, by setting a break point ahead of the line and inspecting. Given that you are getting an EXC_BAD_ACCESS one of the pointers is pointing to a dodgy object, e.g. one that has been released. It looks as if you are using ARC (because you have strong in your property), which should help manage the memory - but then again, you have an autorelease in there, so maybe not.
I have a class myClass and would like to access its properties, a NSArray *currentOptions (specifically to get the size of currentOptions and access the NSStrings which I've put in it.)
I have a method called generate options which assigns an filled array to *currentOptions. Generate options is called before I try to access *currentOptions. An instance of myClass has also been added to the ViewController via the App delegate. However when buttonOnePressed is called, I keep getting this error:
[myClass currentOptions]: unrecognized selector sent to instance 0x9b10490
Here is the parts of my code:
//TClass.h
#interface TClass : NSObject {
NSArray *currentOptions;
}
#property (nonatomic, retain) NSArray *currentOptions;
#end
//viewController
- (IBAction) buttonOnePressed:(id)sender {
NSLog(#"button1 pressed");
NSLog(#"int: %d",[myClass.currentOptions count]);
//myClass here is the instance of TClass
}
One thing that sometimes causes that error is failing to properly retain myClass. (Aside: "myClass" is a really bad name for a pointer because the thing being pointed to is almost certainly not a class but an object, i.e. an instance of a class.) If you don't retain the object that myClass points to, it will be deallocated. Sometimes, a different object happens to be created at that some location, and you end up sending a message meant for the original object to the new one, which is a different type and doesn't understand the message.
To all who have been following, the problem has been resolved by making the following changes:
1) Synthesized current options TClass.m
#implementation TClass
#synthesize currentOptions;
#end
2) I made currentOptions a NSMutableArray instead of a NSArray. This is because I need to reassign values to current options. Somehow it crashes with NSArray and everything goes smoothly with NSMutable array like such
#implementation TutorialClass
if ([currentOptions count] > 0) {
[currentOptions removeAllObjects];
}
[currentOptions addObject:[options objectAtIndex:0]];
[currentOptions addObject:[options objectAtIndex:1]];
[currentOptions addObject:[options objectAtIndex:2]];
[currentOptions addObject:[options objectAtIndex:3]];
3) And of course, I'll also have to do the following in the init method of TClass.m
currentOptions = [[NSMutableArray alloc] init];
Now its time to get some food. Thanks Caleb :D
I'm having some trouble with a NSMutableArray. I'm sure i'm doing something wrong with the allocation of the NSMutableArray but it's not obvious to me being an iPhone newbie. When i run the code below i can add the object MyObject to the array objects_ and set the name etc. The NSLog displays the correct data.
But when i try to access the objects_ member from the function printObject i get a SIGABRT. Looks like the memory has been deallocated or something?
Any help appreciated.
#interface MyObject : NSObject {
NSString *name;
}
-(void) SetName:(NSString*) name_str;
-(NSString*) GetName;
#end
#interface ObjectListViewController : UITableViewController {
NSMutableArray* objects_;
}
-(void) initTableData;
#end
#implementation ObjectListViewController
- (void)initTableData {
objects_ = [[NSMutableArray alloc] initWithCapacity:10];
MyObject *obj = [MyObject alloc];
[obj SetName:#"Test"];
[objects_ addObject:obj];
MyObject* testObj = (MyObject*)[objects_ objectAtIndex:0];
NSLog([testObj GetName]);
}
- (void)printObject {
MyObject* testObj = (MyObject*)[objects_ objectAtIndex:0];
NSLog([testObj GetName]);
}
We can eliminate the lack of an init call on MyObject as the cause of the crash as in this case it will be benign. Calling init on NSObject will just return self, so calling it in this case won't change the behaviour. So I don't think the first two answers here will make any difference:
An object isn’t ready to be used until it has been initialized. The init method defined in the NSObject class does no initialization; it simply returns self.
Chuck correctly points out that init is a fundamental step in object allocation and initialization and you should be calling it when you allocate MyObject.
I am not sure the third answer is correct either. I don't really see how adding synthesise on the objects_ array will make any difference. You haven't defined it as a property, and I don't really see why you would need to, given it is just data internal to the class.
The comment on the question Well, for starters, you never define printObject in the #interface. from eykanal doesn't really help you either, because you must be calling printObject internally, otherwise you wouldn't be hitting the crash.
Reading the through the code, I can't see an obvious error. The retain count on objects_ after initTableData finishes should be one, the retain count on the instance of MyObject should also be one. So I think there must be some other code that is releasing objects_ elsewhere?
I am assuming it is crashing on the objectAtIndex call? Is there any info in the console? What does the call stack look like?
MyObject *obj = [MyObject alloc];
should be:
MyObject *obj = [[MyObject alloc] init];
#interface ObjectListViewController : UITableViewController {
NSMutableArray* objects_;
}
#property (nonatomic, retain) NSMutableArray *objects_;
-(void) initTableData;
-(void) printObject;
#end
add the synthesize in the implementation
#implementation ObjectListViewController
#synthesize objects_;
Here are some issues in your code:
You never initialise your MyObj object. Although it inherits directly from NSObject and NSObject is documented to do nothing except return self, you never know if other stuff happens behind the scenes, so put it in just to eliminate the posssibility.
Your methods don't follow the normal naming conventions. method names should begin with a lower case letter and "get" should only be used when passing back data by reference through the parameters as in e.g. NSData -getBytes:length:. Your getter and setter should be -name and -setName: respectively. This may seem like a minor nitpick, but it'll help you later on if you start to use KVO and KVC.
Never do NSLog(someStringVariable) always NSLog(#"%#", someStringVariable). As you have it now, if the object's name contains a percent formatting sequence e.g. %#, %d, %s etc, your program will crash on the NSLog. However, this is not the cause of your current problem - it would be crashing on the NSLog in -initTableData
you don't need to cast the result of -objectAtIndex:
Having said all that, I can't see anything that would cause the particular issue you have. It may be that the getter or setter for the name in MyObject is incorrect. Please post them.
What i'm trying to accomplish is to have an NSMutableArray defined in the AppDelegate. I then have two UIViewControllers. One view is responsible for displaying the array from the AppDelegate. The other view is used to add items to the array. So the array starts out to be empty. View1 doesn't display anything because the array is empty. The User goes to View2 and adds an item to the array in AppDelegate. Then when the user goes back to View1 it now displays one item.
Here is how I'm trying to accomplish this
#interface CalcAppDelegate : NSObject <UIApplicationDelegate> {
UIWindow *window;
UITabBarController *tabBarController;
NSMutableArray *globalClasses;
}
#property (nonatomic,retain) NSMutableArray *globalClasses;
My other view
In the viewDidload I set the array in my View to be the one in the AppDelegate. In an effort to retain values.
allCourses = [[NSMutableArray alloc]init];
CalcAppDelegate *appDelegate = (CalcAppDelegate *)[[UIApplication sharedApplication] delegate];
allCourses = appDelegate.globalClasses;
Then I would update my allCourses array by adding a new item. Then try to set the array in the AppDelegate to be equal to the modified one.
CalcAppDelegate *appDel = (CalcAppDelegate *)[[UIApplication sharedApplication] delegate];
NSLog(#"Size before reset %d",[appDel.globalClasses count]);
appDel.globalClasses = allCourses;
NSLog(#"Size after reset %d",[appDel.globalClasses count]);
What I'm seeing that's returned is 2 in the before, and 2 after. So it doesn't appear to be getting updated properly. Any suggestions?
A few things:
First, in your app delegate you need to make sure that you intialize the array before any object tries to access it. A customer getter is good for this.
-(void) getGlobalClasses{
if (globalClasses!=nil) {
return globalClasses;
}
NSMutableArray *newArray=[[NSMutableArray alloc] initWithCapacity:1]; //yes, I'm old school
self.globalClasses=newArray;
[newArray release];
return globalClasses;
}
Now any call to the property is guaranteed to return an array.
In your view controllers, you need to define properties to hold the array. Since the array is held by the app delegate and will always be there, it is best to assign the array instead of retaining it. That way you always know you are writing to the exact same array and the app delegate has complete control over its life cycle.
In the view controllers:
#property(nonatomic,assign) NSMutableArray *globalClasses;
then every time you reference it make sure to use the self notation:
self.globalClasses=//...whatever
Having said all this, it is extremely bad practice to stick an array or any other dumb data object out their buck naked in your app. You have no control over what each piece of code will do to the array. You will have to duplicate all your validation code every place you add or remove data to the array.
It would be better to wrap the array in a custom class and make it protected so it can only be altered by the classes methods.
Like so:
#interface MyData : NSObject {
#protected
NSMutableArray *myDataArray;
}
-(void) addObject:(id) anObject;
-(void) removeObjectAtIndex;(NSInteger) anIndex;
#end
scratch.m
#interface scratch ()
#property(nonatomic, retain) NSMutableArray *myDataArray;
#end
#implementation scratch
#synthesize myDataArray;
-(void) addObject:(id) anObject{
//...code to check if anObject is a valid one to add to the array
[self.myDataArray addObject:anObject];
}//------------------------------------addObject:------------------------------------
-(void) removeObjectAtIndex;(NSInteger) anIndex{
//... do bounds checking and other testing to ensure no problems
// will result from removing the object at the given idex
[self.myDataArray removeObjectAtIndex:anIndex];
}//-------------------------------------(void) removeObjectAtIndex;(NSInteger) anIndex------------------------------------
Then add the custom class an a property of the app delegate as shown above. This will keep your data clean and modular so you can safely use it in a wide range of app reobjects without having to micromanage the array in every object.
There are a couple problems here:
allCourses = [[NSMutableArray alloc] init];
CalcAppDelegate *appDelegate = (CalcAppDelegate *)[[UIApplication sharedApplication] delegate];
allCourses = appDelegate.globalClasses;
You do not need to allocate a new array since you want to retain an existing array (the one in the app delegate)
If you're using properties, you need to use the self declaration to retain the app delegate array
Instead, try:
CalcAppDelegate *appDelegate = (CalcAppDelegate *)[[UIApplication sharedApplication] delegate];
self.allCourses = appDelegate.globalClasses;