Question

Is it possible to use fast enumeration with an NSArray that contains an NSDictionary?

I’m running through some Objective C tutorials, and the following code kicks the console into GDB mode

NSMutableArray *myObjects = [NSMutableArray array];
NSArray *theObjects = [NSArray arrayWithObjects:@"easy as 1",@"easy as two", @"Easy as Three"];
NSArray *theKeys    = [NSArray arrayWithObjects:@"A",@"B",@"C"];    
NSDictionary *theDict = [NSDictionary dictionaryWithObjects:theObjects forKeys:theKeys];
[myObjects addObject:theDict];

for(id item in myObjects)
{
    NSLog(@"Found an Item: %@",item);
}

If I replace the fast enumeration loop with a traditional counting loop

int count = [myObjects count];
for(int i=0;i<count;i++)
{
    id item;
    item = [myObjects objectAtIndex:i];
    NSLog(@"Found an Item: %@",item);
}

The application runs without a crash, and the dictionary is output to the console window.

Is this a limitation of Fast Enumeration, or am I missing some subtly of the language? Are there other gotchas when nesting collections like this?

For bonus points, how could I used GDB to debug this myself?

Answer 1

Oops! arrayWithObjects: needs to be nil-terminated. The following code runs just fine:

NSMutableArray *myObjects = [NSMutableArray array];
NSArray *theObjects = [NSArray arrayWithObjects:@"easy as 1",@"easy as two", @"Easy as Three",nil];
NSArray *theKeys    = [NSArray arrayWithObjects:@"A",@"B",@"C",nil];    
NSDictionary *theDict = [NSDictionary dictionaryWithObjects:theObjects forKeys:theKeys];
[myObjects addObject:theDict];

for(id item in myObjects)
{
    NSLog(@"Found an Item: %@",item);
}

I’m not sure why using a traditional loop hid this error.