iphone assigning instance variables

Question

I just spent the last 3 hours trying to figure out this error. I would like someone to explain it to me so I don't do it again.

I assigned an NSString instance variable without using "self". When the class ("self") released, I received a "bad access" error. I have done this exact same thing in another class with the same variable declarations and do not have this error. Below is my code. I commented out the line that broke and the line below fixes it. But I don't understand why... Notice that there are other instance variables that do not cause this problem. Should I always use the "self" reserved word when assigning instance variables? Please let me know.

declarations

@property (nonatomic, readonly, assign) int IID;
@property (nonatomic, assign) int ProfileIID;
@property (nonatomic, retain) NSDate *NoteDate;
@property (nonatomic, copy) NSString *NoteText;

code snippet

    // the default date format is Year-Month-Day 
NSDateFormatter *df = [[[NSDateFormatter alloc] init] autorelease];
[df setDateFormat:kDateFormat];

IID = sqlite3_column_int(selectstmt, 0);
ProfileIID = sqlite3_column_int(selectstmt, 1);

// notice this does not cause a memory error
NoteDate = [[df dateFromString: [NSString stringWithUTF8String:(char *)sqlite3_column_text(selectstmt, 2)]] retain]; 

// the following may be NULL.  Checking using the sqlite3_column_text method
const char *columnText = (const char *)sqlite3_column_text(selectstmt, 3);
if(columnText != NULL)
{
 // this causes a memory error
 //NoteText = [NSString stringWithUTF8String: columnText ];
 // this does not cause memory error
 self.NoteText = [NSString stringWithUTF8String: columnText ];
}

Answer 1

Assigning NoteText from within a class method doesn't invoke the synthesized setter method, it sets the instance variable directly. This means your string isn't being retained (or copied, in the case of your setter), and the crash is when trying to release an object which has already been released. Do this:

self.NoteText = [NSString stringWithUTF8String: columnText ];

This will call your setter, and all will be well.

Edit: Just to be clear, this is true for all ivars.

myVariable = someValue;  // Sets the ivar directly.
self.myVariable = someValue;  // calls [self setMyVariable].

Answer 2

The reason that

NoteDate = [[df dateFromString: [NSString stringWithUTF8String:(char *)sqlite3_column_text(selectstmt, 2)]] retain];

is fine is because you retain the variable. Since you do not allocate the string, but call stringWithUTF8String on NSString, you do not take ownership of the variable, and so the string returned to you is autoreleased. However, since you retain it, this does not cause problems.

If variables are returned autoreleased, then they are released when the autorelease pool is drained, which occurs at the end of each event (see more on autorelease pools). This is no good with an instance variable, because it needs to stick around after the current event.

When you assign the variable by:

NoteText = [NSString stringWithUTF8String: columnText];

Your setter method is not invoked, so the returned string (which, again, is autoreleased) is not retained, and so is released by the autorelease pool at the end of the event.

Calling it as:

self.NoteText = [NSString stringWithUTF8String: columnText];

does retain the string, since the line is another way of writing:

[self setNoteText:[NSString stringWithUTF8String: columnText]];

which invokes your setter method and retains the variable, preventing it from being released at the end of the current event.

Answer 3

This confusion between ivar and setter methods is why I never name my setters and my ivars the same.

Apple typically names its ivars starting with an underscore (_), for example NoteText. In my case, I've taken to having a prefix of i for ivars. For example:

NSString* i_name;

@property (nonatomic, copy) NSString* name;

@synthesize name = i_name;

That way you can easily tell the difference between an ivar assignment:

i_name = [[whatever title] retain];

and a setter method call

self.name = [whatever title]; // Equivalent to [self setName:[whatever title]

The setter, since it is defined with copy (or similarly for retain) will take ownership of the variable passed in and release ownership of the old value. The ivar assignment does none of that.

Note also that your propery names should start with a lower case letter or they will not be KVO compliant.