なぜ私のコードを出力している* nilの説明*

Question

コード：

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; 
} 

出力：

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   

Answer 1

-descriptionメソッドは実際に何も返しません。これを伝えるコンパイラの警告が表示されます。

また、（あなたがARCで実行している場合を除いて）その方法ではalloc/initという文字列も漏れています。

代わりに以下のことを試してみてください。

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