Question

I’m still slogging through learning memory allocation and Objective-C (my first run with Leaks was unpretty, at best). Here is a current issue, I hope everyone can understand it, I’ve used some pseudo-code, but I don’t believe I’ve left anything out.

I have a Row Class set for some database stuff, we’ll use this:

@interface Row : NSObject {
    int rowID;
NSString *firstName;
NSString *lastName;
}

I have a getRow function in my database class that looks something like this:

-(Row *) getRow 
{
  rowPlaceholder = [[Row alloc] init];    
       ...
  rowPlaceholder.rowID = column1;
  rowPlaceholder.firstName = column2;
  rowPlaceholder.lastName = column3;
       ...
    return [rowPlaceholder autorelease]; //<-----Problem is happening here
}

Now in my ViewController I have 2 functions of importance to this situation:

-(void) viewWillAppear:(BOOL)animated{
currentRow = [[Row alloc] init];
currentRow = getRow;
firstNamelabel.text = currentRow.firstName;
}

The second function is a button the gets the second word:

-(IBAction)btnLastName:(id)sender{
lastNamelabel.text = currentRow.lastName; //<---Crashes the program
}

So, as you can see, using currentRow against causes the program to crash, and what I can’t understand is that if I don’t release rowPlaceholder the program works fine. At this point why does currentRow care about the data from another function, since it already has what it want (a value from getRow()).

Answer 1

autorelease means that the object in question will be automatically released. When this occurs is tied to when the autorelease pool is emptied, but for the default case it’s when the UI “comes up for air” — when your call chain returns back to the UI system code. If you’re going to hold something “across” such a return to UI system code, you need to have it explicitly retained. In general, making it a retained property is the best way to do this (though of course you need to clear/release the property in your dealloc method).

[And why do you do this:]

currentRow = [[Row alloc] init];
currentRow = getRow;