Question

While implementing a subclass of NSArray (a class cluster), I was surprised to see that my overridden description method was not called. Can somebody explain what is happening here?

@interface MyArrayClassCluster : NSArray

@end

@implementation MyArrayClassCluster
{
    NSArray *_realArray;
}

// Implement the class cluser stuff here
- (NSUInteger)count
{
    return [_realArray count];
}

- (id)objectAtIndex:(NSUInteger)index
{
    return [_realArray objectAtIndex:index];
}

// lifeCycle

- (id)initWithItems:(NSArray *)items
{
    self = [super init];
    _realArray = [items retain];
    return self;
}

- (void)dealloc
{
    [_realArray release];
    [super dealloc];
}

- (NSString *)description
{
    return [NSString stringWithFormat:@"My Custom Array: %p, objs:%@", self, _realArray];
}

@end

int main(int argc, const char * argv[])
{
    @autoreleasepool {
        NSArray *a = @[@1, @2, @3];
        NSLog(@"a: %@", a);
        MyArrayClassCluster *clzCluster = [[MyArrayClassCluster alloc] initWithItems:a];
        NSLog(@"clzCluster: %@", clzCluster);
    }
    return 0;
}

Output

2013-01-29 18:52:38.704 ClassClusterTester[31649:303] a: (
    1,
    2,
    3
)
2013-01-29 18:52:38.707 ClassClusterTester[31649:303] clzCluster: (
    1,
    2,
    3
)

Answer 1

The link @Rob pointed to had the correct answer at the bottom. An obscure fact: if something implements descriptionWithLocale:, NSLog will call that instead. Since my class is a subclass of NSArray, and NSArray implements that, my version of description was not called.