Question

I have two simple NSMutableArray that consists of few objects. Some of these objects can be common but need to be stored in both of the arrays as uses of both arrays are defined for totally different purpose.

However, the problem is that after adding same objects to both array, on changing the value of one of the common object, does not reflect in 2nd array.

For example,

Let’s say we have two mutable NSArray:

NSMutableArray *mutableArrayOne;
NSMutableArray *mutableArrayTwo;

Now let’s create the object definition that these array needs to contain.

@interface: DummyObject : NSObject
{
     int objectValue;
}

@property (nonatomic) int objectValue;

-(void) printObjectValue;

@end

Now let’s create the base class to store the arrays.

Base Class Definition

@interface: BaseClass : NSObject
{
    NSMutableArray *mutableArrayOne;
    NSMutableArray *mutableArrayTwo;
}
-(void) init;
-(void) printBothArrays;

@end

Base Class Implementation

@implementation BaseClass

-(void) init
{
     // initialize the mutable array.
     mutableArrayOne = [[NSMutableArray alloc] initWithCapicity:5];
     mutableArrayTwo = [[NSMutableArray alloc] initWithCapicity:5];

     DummyObject *dummyObject = [DummyObject alloc];
     [dummyObject setObjectValue:5];

     DummyObject *dummyObjectTwo = [DummyObject alloc];
     [dummyObjectTwo setObjectValue:2];

     [mutableArrayOne addObject:dummyObject];
     [mutableArrayOne addObject:dummyObjectTwo];

     [mutableArrayTwo addObject:dummyObjectTwo];

}

@end

Now let me the modify the DummyObject in array One:

for (DummyObject* dummyObject in mutableArrayOne)
{
    [dummyObject setValue:100]; 
}

Problem
Now here starts the problem when I am printing the values for both array objects:-

Printing First Array

for (DummyObject* dummyObject in mutableArrayOne)
{
    [dummyObject printObjectValue]; 
}

*Output Log (from first array) *

100
100

Printing second Array

for (DummyObject* dummyObject in mutableArrayTwo)
{
    [dummyObject printObjectValue]; 
}

*Output Log (from second array) *

2

So here we can see that MutableArray is keeping the copy of the object, however, I want to store only the reference. That means, on changing the value of the object in 1st array should reflect in 2nd array.

How can we do that?
Do we any other alternative?

Thanks,
Paras Mendiratta

Answer 1

This will ideally work, seems like some issue with setting object.
For e.g. you can check like this –

NSMutableString *firstString = [[NSMutableString alloc] initWithString:@"first"];
NSMutableString *secondString = [[NSMutableString alloc] initWithString:@"second"];

NSMutableArray *originalArray = [[NSMutableArray alloc] initWithObjects: firstString, secondString, nil];
NSMutableArray *copyArray = [[NSMutableArray alloc] initWithObjects: firstString, secondString, nil];

[[copyArray objectAtIndex:0] appendString:@"add some text"];

for (int index = 0; index < [originalArray count]; index++) {
    NSLog(@"Original:%@ --- copy:%@", [originalArray objectAtIndex:index], [copyArray objectAtIndex:index]);
}

And output is –

2012-05-09 10:28:40.382 Demo[5237:f803] Original:firstadd some text --- copy:firstadd some text
2012-05-09 10:28:40.384 Demo[5237:f803] Original:second --- copy:second

EDIT – (Not adding object at the time of initialization)

NSMutableString *firstString = [[NSMutableString alloc] initWithString:@"first"];
NSMutableString *secondString = [[NSMutableString alloc] initWithString:@"second"];

NSMutableArray *originalArray = [[NSMutableArray alloc] init];
NSMutableArray *copyArray = [[NSMutableArray alloc] init];

[originalArray addObject:firstString];
[copyArray addObject:firstString];

[originalArray addObject:secondString];
[copyArray addObject:secondString];

[[copyArray objectAtIndex:0] appendString:@"add some text"];

for (int index = 0; index < [originalArray count]; index++) {
    NSLog(@"Original:%@ --- copy:%@", [originalArray objectAtIndex:index], [copyArray objectAtIndex:index]);
}

Output –

2012-05-09 11:11:18.275 Demo[5433:f803] Original:firstadd some text --- copy:firstadd some text
2012-05-09 11:11:18.277 Demo[5433:f803] Original:second --- copy:second