Sample Cocoa app, ref counting, [pool drain] yields EXC_BAD_ACCESS? - objective-c

Question

I'm working through Cocoa Programming for Mac OS X (3rd ed) and in chapter 4 I wrote this app:
int main (int argc, const char * argv[]) {
NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];
//create the date object
NSCalendarDate *now = [[NSCalendarDate alloc] init];
//seed random # generator
srandom(time(NULL));
NSMutableArray *array;
array = [[NSMutableArray alloc] init];
int i;
for (i=0; i<10; i++){
//create a date/time that is 'i' weeks from now
NSCalendarDate *iWeeksFromNow;
iWeeksFromNow = [now dateByAddingYears:0
months:0
days:(i*7)
hours:0
minutes:0
seconds:0];
//create a new instance of lottery entry
LotteryEntry *entry = [[LotteryEntry alloc] init];
[entry setEntryDate:iWeeksFromNow];
[array addObject:entry];
[entry release];
}
[now release];
now = nil;
for (LotteryEntry *entryToPrint in array) {
NSLog(#"%#", entryToPrint);
}
[array release];
array = nil;
NSLog(#"about to drain the pool... (%#)", pool);
[pool drain];
NSLog(#"done");
NSLog(#"GC = %#", [NSGarbageCollector defaultCollector]);
return 0;
}
The LotteryEntry class looks like this:
#implementation LotteryEntry
- (void)setEntryDate:(NSCalendarDate *)date
{
entryDate = date;
}
- (NSCalendarDate *)entryDate
{
return entryDate;
}
- (int)firstNumber
{
return firstNumber;
}
- (int)secondNumber
{
return secondNumber;
}
- (id)init
{
return [self initWithDate:[NSCalendarDate calendarDate]];
}
- (id)initWithDate:(NSCalendarDate *)date
{
if(![super init])
return nil;
NSAssert(date != nil, #"Argument must be non-nil");
firstNumber = random() % 100 + 1;
secondNumber = random() % 100 + 1;
entryDate = [date retain];
return self;
}
- (NSString *)description
{
NSString *result;
result = [[NSString alloc] initWithFormat:#"%# = %d and %d",
[entryDate descriptionWithCalendarFormat:#"%b %d %Y"],
firstNumber,
secondNumber];
return result;
}
- (void)dealloc
{
NSLog(#"deallocating %#", self);
[entryDate release];
[super dealloc];
}
#end
As you can see I'm retaining and releasing the objects here. I'm pretty sure my code matches the book's, however when I run the app, at the [pool drain] I get this message:
Program received signal:
“EXC_BAD_ACCESS”.
I'm not sure what's causing this. I'm hoping it's something stupid that I missed, but I'd sure appreciate a few other pairs of eyes on it. Thanks in advance!
(side note: I'm a .NET developer, so ref counting is pretty foreign to me!)

Answer 1

It also looks like you have a bug in this method:
- (id)initWithDate:(NSCalendarDate *)date
{
if(![super init])
return nil;
NSAssert(date != nil, #"Argument must be non-nil");
firstNumber = random() % 100 + 1;
secondNumber = random() % 100 + 1;
entryDate = [date retain];
return self;
}
You are essentially discarding the results from [super init], while it may not be a problem in this instance, it could cause serious problems in others. You should 'always' structure you init methods like this:
- (id)initWithDate:(NSCalendarDate *)date
{
if(self = [super init]) {
NSAssert(date != nil, #"Argument must be non-nil");
firstNumber = random() % 100 + 1;
secondNumber = random() % 100 + 1;
entryDate = [date retain];
}
return self;
If you are not going to return self from an init method (for instance it is a factory or something odd like that), you should remember to release self. It has been alloc'ed, and if you don't return it, it cannot be released properly. Example:
- (id) init
{
NSObject* newSelf = [[NSObject alloc] init];
[self release];
return newSelf;
}

Answer 2

doh! Just typing the code made me realize my problem. Doncha love it when that happens?
I am retaining the date in my init, but I still had that extra setEntryDate method that was not calling retain. Removing this and calling the initWithDate method instead seemed to fix the problem.

Answer 3

It's not related to your issue at hand but you should avoid using NSCalendarDate, it's been deprecated for a while now and will probably be removed from the API entirely soon.