Example showing how casting to BOOL causes failure 6% of the time, as explained in http://www.mikeash.com/pyblog/friday-qa-2012-12-14-objective-c-pitfalls.html

Casting to bool fails 6% of time

int failsUsingCurrentMethod = 0;
    int failsUsingNotNot = 0;
    for (int i=0; i < 10000; i++) {
        NSDate *date = [NSDate date];
        NSAssert(date, @"Date should never be nil");
        BOOL booleanUsingCurrentMethod = (date);
        if (!booleanUsingCurrentMethod) {
            failsUsingCurrentMethod++;
        }
        
        BOOL booleanUsingNotNot = !!(date);
        if (!booleanUsingNotNot) {
            failsUsingNotNot++;
        }
    }
    
    NSLog(@"Fails using current method = %i, which is %g percent of cases",failsUsingCurrentMethod,(failsUsingCurrentMethod/10000.0)*100);
    NSLog(@"Fails using !! AKA convert to boolean = %i, which is %g percent of cases",failsUsingNotNot,(failsUsingNotNot/10000.0)*100);