macOS How do we pass NSArray as reference

[anni.saini]

Hi,

can we pass NSArray as reference, if yes then how?

I had tried as

NSMutableArray *temp;
...
[self set:temp];
...
NSLog(@"%@", temp);

- (void) setNSArray*) t
{
t = [NSArray arrayWithObject"Test"];
}

getting output as null.

 

[lee1210]

Every object in objective-C is a pointer, so every pass of an object is by reference. Have you tried the code in-line without the method call to see if it works?

In any event, that method is dangerous. If there is an object being pointed to by the parameter, the reference will be lost. This will mean a memory leak unless you're using GC.

You may also wish to print the pointer with the %p specifier a few places to see what's going on, and/or use the debugger.

-Lee

EDIT: Also, you define an NSMutableArray (which is an NSArray), but in your method you assign an NSArray pointer to it. If you ever try to use NSMutableArray methods on this, they will fail (as an NSArray is not an NSMutableArray).

 

[Detrius]

You're both a little confused. Try this:

NSMutableArray *temp;
...
[self set: [COLOR="Blue"]&[/COLOR]temp];
...
NSLog(@"%@", temp);

- (void) set:(NSArray[COLOR="Blue"]*[/COLOR]*) t
{
     [COLOR="Blue"]*[/COLOR]t = [NSArray arrayWithObject:@"Test"];
}

Your problem is that you are passing the pointer value of "t" by value. Normally, this isn't a problem, but you're expecting to change the actual value of the original pointer itself, which means you need to pass the address of that pointer variable to the function, be it by reference or by pointer.

It's been a long time since I've coded in Objective-C, so I don't know if you can pass "by reference" like you can in C++. You can definitely do a double-pointer, though, which is logically the same thing as passing a reference to a pointer.


EDIT: lee is not wrong about the rest of the stuff he pointed out.

 

[lee1210]

Ah, good call. Not at a mac, and didn't think that all the way through.

-Lee

 

[Sydde]

No actual memory leak here because the class method arrayWithObject: creates an autoreleased object. In this case, if you want to keep the object, you would need to retain it.

What would make the most sense would be for the set method to return an array

- (NSArray *)set {
return [NSArray arrayWithObject"Test"];
}

then, above,


temp = [self set];

 

[Detrius]

What would make the most sense would be for the set method to return an array

Agreed, but then he wouldn't have learned why his original code didn't work.

 

[anni.saini]

Thanx for your valuable reply...

could you please tell me why the solution suggested by "Detrius", not for mac. If so, so can i achieved it by any other way...

and i don't want to change method signature means it should take one argument and return void.

- (void) set<NSArray>) t;

 

[Detrius]

Thanx for your valuable reply...

could you please tell me why the solution suggested by "Detrius", not for mac. If so, so can i achieved it by any other way...

Other than the fact that the language is Objective-C, there's nothing about it that's Mac-specific. You can do double-pointers in C and C++.

 

[sammich]

could you please tell me why the solution suggested by "Detrius", not for mac. If so, so can i achieved it by any other way...

He said that he was not in front of his mac so he can't test out his theory.

 

[anni.saini]

ohhhhh... thats working fine then...
thanx

 

[HiRez]

Just for future reference, I think it's a really bad idea to name an Objective-C mehod simply "set". The Cocoa runtime uses setXXX for Bindings and KVC. You could call it setArray or better yet something more descriptive like setOrders or setEmployees.

 

[Sydde]

Indeed. One of the difficult things to get used to in Objective C/Cocoa is that method names are visible outside the bounds of the source code. When subclassing, one should take care not to accidentally override a superclass method by using an obvious name.

 

[anni.saini]

I got it, please correct me if I'm wrong...

NSMutableArray *temp = [[NSMutableArray alloc] init];

...
[self setArray:temp];
...
NSLog(@"%@", temp);

- (void) setArrayNSMutableArray*) t
{
[t addObjectsFromArray:[NSArray arrayWithObjects"Test", @"test1", nil]];
}

 

[HiRez]

I got it, please correct me if I'm wrong...

NSMutableArray *temp = [[NSMutableArray alloc] init];

...
[self setArray:temp];
...
NSLog(@"%@", temp);

- (void) setArrayNSMutableArray*) t
{
[t addObjectsFromArray:[NSArray arrayWithObjects"Test", @"test1", nil]];
}

Not quite. What you probably want is an instance variable, declared in your interface (.h) file, that will keep track of the array. Let's say that variable is called myArray.

@interface MyObject : NSObject {
    NSArray	*myArray;
}

- (void)setArray:(NSArray*)newArray;

@end

So now, back in your implementation (.m) file, you can pass any array to the setter method, from anywhere, and it will assign it to the myArray variable. In general, you don't want to modify the object passed into the setter method, just assign it. For example:

NSMutableArray *temp = [NSArray array];
// Add objects to array here, objects added are each retained by the array object
[self setArray:temp];

- (void)setArray:(NSArray*)newArray {
	[newArray retain];
	[myArray release];
	myArray = newArray;
}

That's if your instance variable is a mutable array. If you want your instance variable to be immutable, but still want to pass mutable arrays into your setter, you'll have to copy the array using something like arrayWithArray:

Also, you should wrap your code blocks in code tags when posting so it's readable.

 

[Sydde]

NSMutableArray already has a -setArray: method. What I think you a trying to do would look like this

temp = [NSMutableArray new];

[temp setArray:[NSArray arrayWithObjects:@"Test", @"test1", nil]];

-setArray: replaces the contents of a NSMutableArray with the contents of another array, removing any existing contents. -addObjectsFromArray: does not remove the existing content.

You need to think in terms of messages. -setArray: is a message that you send to the NSMutableArray. This English syntax, in the form of subject command.