objective-c - 示例 Cocoa 应用程序，引用计数，[池耗尽] 产生 EXC_BAD_ACCESS？

Question

我正在处理Cocoa Programming for Mac OS X (3rd ed)在第 4 章中我编写了这个应用程序:

int main (int argc, const char * argv[]) {
    NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];

    //create the date object
    NSCalendarDate *now = [[NSCalendarDate alloc] init];

    //seed random # generator
    srandom(time(NULL));

    NSMutableArray *array;
    array = [[NSMutableArray alloc] init];
    int i;
    for (i=0; i<10; i++){
        //create a date/time 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 instance of lottery entry
        LotteryEntry *entry = [[LotteryEntry alloc] init];
        [entry setEntryDate:iWeeksFromNow];

        [array addObject:entry];
        [entry release];
    }
    [now release];
    now = nil;

    for (LotteryEntry *entryToPrint in array) {
        NSLog(@"%@", entryToPrint);
    }
    [array release];
    array = nil;

    NSLog(@"about to drain the pool... (%@)", pool);
    [pool drain];
    NSLog(@"done");
    NSLog(@"GC = %@", [NSGarbageCollector defaultCollector]);
    return 0;
}

LotteryEntry 类如下所示:

@implementation LotteryEntry

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

- (NSCalendarDate *)entryDate
{
    return entryDate;
}

- (int)firstNumber
{
    return firstNumber;
}

- (int)secondNumber
{
    return secondNumber;
}

- (id)init
{
    return [self initWithDate:[NSCalendarDate calendarDate]];
}

- (id)initWithDate:(NSCalendarDate *)date
{
    if(![super init])
        return nil;

    NSAssert(date != nil, @"Argument must be non-nil");

    firstNumber = random() % 100 + 1;
    secondNumber = random() % 100 + 1;
    entryDate = [date retain];  
    return self;
}

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

- (void)dealloc
{
    NSLog(@"deallocating %@", self);
    [entryDate release];
    [super dealloc];
}

@end

如您所见，我在这里保留和释放对象。我很确定我的代码与书中的代码匹配，但是当我运行该应用程序时，在 [池排水] 处我收到此消息:

Program received signal: “EXC_BAD_ACCESS”.

我不确定是什么原因造成的。我希望我错过了一些愚蠢的事情，但我肯定会感谢其他几双眼睛的关注。提前致谢!

(旁注:我是一名 .NET 开发人员，因此引用计数对我来说非常陌生!)

Answer 1

这个方法似乎也有一个错误:

- (id)initWithDate:(NSCalendarDate *)date
{
    if(![super init])
        return nil;

    NSAssert(date != nil, @"Argument must be non-nil");

    firstNumber = random() % 100 + 1;
    secondNumber = random() % 100 + 1;
    entryDate = [date retain];  
    return self;
}

您实际上是在丢弃 [super init] 的结果，虽然在这种情况下这可能不是问题，但在其他情况下可能会导致严重的问题。您应该“始终”像这样构建 init 方法:

- (id)initWithDate:(NSCalendarDate *)date
{
    if(self = [super init]) {
        NSAssert(date != nil, @"Argument must be non-nil");

        firstNumber = random() % 100 + 1;
        secondNumber = random() % 100 + 1;
        entryDate = [date retain];
}
return self;

如果你不打算从 init 方法返回 self(例如它是一个工厂或类似的奇怪的东西)，你应该记住释放 self。已经分配了，如果不归还，就无法正常释放。示例:

- (id) init
{
    NSObject*    newSelf = [[NSObject alloc] init];

    [self release];
    return newSelf;
}