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I do some iOS programming stuff and I have a UIViewController with a NSMutableArray:

@property (nonatomic, retain) NSMutableArray* mutableTestArray;
...
@synthesize mutableTestArray;

In viewDidLoad I want to call a method which is inside the implementation of this UIViewController:

//- (void)aTestMethod:(NSMutableArray *)myMutableTestArray;
[self aTestMethod:self.mutableTestArray];

So I call the method with a NSMutableArray which is an instance variable of the UIViewController. Inside this method, I do this:

myMutableTestArray = [NSMutableArray arrayWithCapacity:100];
//... looping & generating some objects and adding them to the array:
[myMutableTestArray adObject:myObject];

Now, I debug it and inside the method myMutableTestArray is fine. There are all objects inside the array. But leaving this method the instance variable mutableTestArray is empty.

So, where is the problem? Anyone an idea? Note: I know I can access the instance variable with self.mutableTestArray and then everything will be okay, instead using it as a parameter, but I want to know what's wrong with my code.

Thank you in advance & Best Regards.

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3 Answers 3

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3

Parameters are passed by value in Objective-C. Thus, you are creating a copy of a pointer to the object and passing that into the method. When you do myParam = ... new object ...; that resets the copy to point to a new location, but has no effect on the original copy in the caller.

(To reiterate -- you are copying the pointer, not copying the object.)

To solve, declare your test method as returning an object:

- (NSMutableArray *)aTestMethod;

Then, you can simply:

self.mutableTestArray = [whateverObject aTestMethod];

(Since you aren't actually using the value passed in in the first place, there is no need for a parameter at all).

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2 Comments

I can not do this, because in my real code, I have to pass several mutable and immutable arrays, I only did an example, otherwise the overview could be lost.
You'll need to add a more complete example. There are a number of possible solutions. Personally, anytime I see "multiple related return values" it begs for a simple class whose instances can encapsulate said related values.
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By your command

myMutableTestArray = [NSMutableArray arrayWithCapacity:100];

you are creating new allocation of mutableTestArray. So passing mutableTestArray as parameter to aTestMethod:

[self aTestMethod:self.mutableTestArray];

is useless because you override its value immediately when create this array inside your aTestMethod.

Try to create your array before passing it to your method (where you will fill it with data).

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0

Remember that parameters are transmitted by value, so in aTestMethod: you are modifying not the original pointer but a copy of it! For this to work, you should pass the address of the pointer as in

self aTestMethod:&(self.mutableArray)

then the prototype of the method should be 

-(void)aTestMethod:(NSMutableArray **)myArray

and in the code of it you should used *myArray as in

*myArray = [[NSMutableArray alloc] init];

Yours, JB

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7 Comments

Pass-by-reference should never be used in Objective-C save for the most exceptionally rare cases. This isn't one. By definition using pass-by-reference is unnecessary when the method returns (void).
There is absolutely no reason not to use pass-by-reference. I agree that in that case I may not be considered as necessary, but in a more general context, it could be used
I get in the line self aTestMethod:&(self.mutableArray) the error "address of property expression requested" if I put a & in front of it.
Pass-by-reference is not used across the frameworks; it is not a part of the standard API patterns of the system (and making your APIs look/feel like the system's is a boon to consistency and maintainability). It is error prone and pass-by-reference of an instance variable breaks encapsulation. The only two uses of pass-by-reference are the NSError** pattern and multiple return values (which are really the same thing). In the multiple return value case, it almost always indicates that your API design should be revisited.
Not to mention that your code is wrong for exactly the reason @Tim pointed out; self.mutableArray is a method call and cannot be used in a pass-by-reference context.
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