Adding object to NSMutableArray changes all objects

[lynngobin]

One good thing about posting to this forum is that there seems to be a few programmers out there who are tolerant enough to tackle noobie problems like the following.

I'm trying to add objects to a mutable array. I alloc and init the object, and then set some properties:

Restaurant *restaurant = [[Restaurant alloc] init];
self.currentRestaurantObject = restaurant;
.
.
.
self.currentRestaurantObject.Company = self.currentParsedCharacterData;
.
.
.
self.currentRestaurantObject.SubCatName = self.currentParsedCharacterData;

I then add the object to the array:

[self.currentParseBatch addObject:self.currentRestaurantObject];

(You might recognize that this code is based on the Apple SeismicXML sample code.)

The problem I've having is that, not only is the object added to the array, but the properties of all the previously added objects get changed to those of the new object.

Can anyone lend a hand?

[sneaky]

The problem here is to do with pointers. When you modify the self.whatever you modify that for everything that is pointing to the same thing.

Consider the following

for (int i = 0; i < 3; i++) {
   self.stringProperty = [NSString stringWithFormat@"number %i", i];
   [self.myMutableArray addObject:self.stringProperty];
}

If we print the contents of this array it will print

number 2
number 2
number 2

Why is that? Because when you add self.stringProperty to the array you add a pointer to that object, not a copy of that object. So the next iteration of the loop you go and you change the actual value meaning that the value is changed for everyone that is pointing to the actual value.

Consider instead

for (int i = 0; i < 3; i++) {
   NSString *localVariableString = [NSString stringWithFormat@"number %i", i];
   [self.myMutableArray addObject:localVariableString];
}

If we print the contents of this array it will print

number 0
number 1
number 2

Every time we run the loop we create a brand new instance and add that instance to the array rather than re-using the pointer.

There's a fella on here - Brian Slick. He has a really good tutorial on properties in his signature, have a read of that.

[lynngobin]

Thanks much, sneaky. I was embarrassed to even ask the question as I knew that the solution was in a concept basic to Objective C. But after a few months away from programming, dang if I could figure it out. Your response pointed me in the right direction and helped solve my problem. I appreciate your help.