why is my code outputting *nil description*

问题

Code:

LotteryEntry.h

#import <Foundation/Foundation.h>

@interface LotteryEntry : NSObject
{
    NSCalendarDate *entryDate;
    int firstNumber;
    int secondNumber;
}

- (void) prepareRandomNumbers;
- (void) setEntryDate:(NSCalendarDate *)date;
- (NSCalendarDate*)entryDate;
- (int) firstNumber;
- (int) secondNumber;

@end

LotteryEntry.m

#import "LotteryEntry.h"

@implementation LotteryEntry

- (void) prepareRandomNumbers
{
    firstNumber = random() % 100 + 1;
    secondNumber = random() % 100 + 1;
}

- (void) setEntryDate:(NSCalendarDate *)date
{
    entryDate = date;
}

- (NSCalendarDate *) entryDate
{
    return entryDate;
}

- (int)firstNumber
{
    return firstNumber;
}

- (int)secondNumber
{
    return secondNumber;
}

- (NSString *)description
{
    NSString *result;
    result = [[NSString alloc] initWithFormat:@"%@ = %d and %d", 
              [entryDate descriptionWithCalendarFormat:@"%b %d %Y"],
              firstNumber, secondNumber];
}
@end

lottery.m

#import  <Foundation/Foundation.h>
#import "LotteryEntry.h";

int main (int argc, const char * argv[])
{        
    @autoreleasepool
    {      
        //create the date object
        NSCalendarDate *now = [[NSCalendarDate alloc] init];

        //see the random number generator
        srandom(time(NULL));
        NSMutableArray *array;
        array = [[NSMutableArray alloc] init];

        int i;
        for( i = 0; i < 10; i++) 
        {
            //create date time object 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 isntand of lottery entry
            LotteryEntry *newEntry = [[LotteryEntry alloc] init];
            [newEntry prepareRandomNumbers];
            [newEntry setEntryDate:iWeeksFromNow];

            //add the lottery entry object to the array
            [array addObject:newEntry];

            /*
            NSNumber *newNumber;
            newNumber = [[NSNumber alloc] initWithInt:(i * 3)];
            //[array addObject:newNumber];

            @try 
            {

                [array addObject:newNumber];
            } 
            @catch (NSException *exception) 
            {
                NSLog(@"catched error: %@", exception.description);
            }
             */
        }


        for (LotteryEntry *entryToPrint in array)
        {
            //display entry
            NSLog(@"%@", entryToPrint);
        }


    }
    return 0;
}

Output:

GNU gdb 6.3.50-20050815 (Apple version gdb-1708) (Mon Aug  8 20:32:45 UTC 2011)
Copyright 2004 Free Software Foundation, Inc.
GDB is free software, covered by the GNU General Public License, and you are
welcome to change it and/or distribute copies of it under certain conditions.
Type "show copying" to see the conditions.
There is absolutely no warranty for GDB.  Type "show warranty" for details.
This GDB was configured as "x86_64-apple-darwin".tty /dev/ttys000
[Switching to process 898 thread 0x0]
2011-11-08 11:16:21.193 lottery[898:707] *nil description*
2011-11-08 11:16:21.196 lottery[898:707] *nil description*
2011-11-08 11:16:21.198 lottery[898:707] *nil description*
2011-11-08 11:16:21.199 lottery[898:707] *nil description*
2011-11-08 11:16:21.200 lottery[898:707] *nil description*
2011-11-08 11:16:21.201 lottery[898:707] *nil description*
2011-11-08 11:16:21.202 lottery[898:707] *nil description*
2011-11-08 11:16:21.204 lottery[898:707] *nil description*
2011-11-08 11:16:21.205 lottery[898:707] *nil description*
2011-11-08 11:16:21.206 lottery[898:707] *nil description*
Program ended with exit code: 0           

回答1:

Your -description method isn't actually returning anything. You should be getting a compiler warning telling you this.

It's also leaking the string that you're alloc/initing in that method (unless you're running under ARC).

Try the following instead:

- (NSString *)description
{
    return [NSString stringWithFormat:@"%@ = %d and %d", 
              [entryDate descriptionWithCalendarFormat:@"%b %d %Y"],
              firstNumber, secondNumber];
}

来源：https://stackoverflow.com/questions/8057240/why-is-my-code-outputting-nil-description