[CFDictionary objectForKey:]：发送到已释放实例的消息

Question

在没有任何见解的情况下在网络上搜索后，我决定在这里发布我的问题，希望有人能够解释以下代码有什么问题。我只是无法在 Objective-C 中实现单例设计模式。我在互联网上找到了一些实现该模式的代码，并对它进行了一些修改以添加从 plist 文件启动的字典（其中属性列表中的每个条目本身就是一个字典）：

@interface UnitManager : NSObject {
NSString *someProperty;
NSDictionary *factors;
}

@property (nonatomic, retain) NSString *someProperty;
@property (nonatomic, retain) NSDictionary *factors;

+ (id) sharedUnitManager;

@end

// implementation file
#import "Unit.h"

#define MARGIN 20

static UnitManager *unitManager = nil;

@implementation UnitManager

@synthesize someProperty;
@synthesize factors;

#pragma mark Singleton Methods

+ (id)sharedUnitManager {
    @synchronized(self) {
        if(unitManager == nil)
            unitManager = [[super allocWithZone:NULL] init];
    }
    return unitManager;
}
+ (id)allocWithZone:(NSZone *)zone {
    return [[self sharedUnitManager] retain];
}
- (id)copyWithZone:(NSZone *)zone {
    return self;
}
- (id)retain {
   return self;
}
- (unsigned)retainCount {
    return UINT_MAX; //denotes an object that cannot be released
}
- (void)release {
    // never release
}
- (id)autorelease {
    return self;
}
- (id)init {
    if (self = [super init]) {
         someProperty = [[NSString alloc] initWithString:@"Default Property Value"];

        // get path to file that stores category order
        NSString *path = [[NSBundle mainBundle] pathForResource:@"factors" ofType:@"plist"];

        factors = [NSDictionary dictionaryWithContentsOfFile:path];


    }
    return self;
}
- (void)dealloc {
    // Should never be called, but just here for clarity really.
    [someProperty release];
    NSLog(@"should NEVER GET HERE!");
    [factors release];
    [super dealloc];
}

@end

然后，稍后我尝试一些代码访问单例对象中的字典，第一次调用它工作正常，但第二次它访问已释放的内存！

- (void) someFunction {
...

    NSString *str = [[[[UnitManager sharedUnitManager] factors] objectForKey:category] objectForKey:name];
...
...



}

我收到的消息是

2011-09-12 17:39:44.567 Test[7837:b603] -[CFDictionary objectForKey:]: message sent to deallocated instance 0x4d565e0*

以及内存跟踪：

(gdb) info malloc-history 0x5c73300 分配：块地址：0x05c73300 长度：48 堆栈 - pthread: 0xac2bd2c0 帧数: 44

0. 0x92573993 in malloc_zone_malloc
1. 0xebe87d in _CFRuntimeCreateInstance
2. 0xebe47a in CFBasicHashCreate
3. 0xf81e44 in __CFDictionaryCreateTransfer
4. 0xeed3ff in __CFBinaryPlistCreateObject2
5. 0xee2009 in __CFTryParseBinaryPlist
6. 0xee1a48 in _CFPropertyListCreateWithData
7. 0xee19ba in CFPropertyListCreateWithData
8. 0xee194f in CFPropertyListCreateFromXMLData
9. 0x791e34 in _NSParseObjectFromASCIIPropertyListOrSerialization
10. 0x7913d5 in +[NSDictionary(NSDictionary) newWithContentsOf:immutable:]
11. 0xf067bf in -[__NSPlaceholderDictionary initWithContentsOfFile:]
12. 0x791308 in +[NSDictionary(NSDictionary) dictionaryWithContentsOfFile:]
13. 0xcb38 in -[UnitManager init] at Unit.m:54
14. 0xc833 in +[UnitManager sharedUnitManager] at Unit.m:25
15. 0xe03a in -[Unit factor] at Unit.m:226
16. 0x39c9 in -[MainViewController updateTextFields] at MainViewController.m:113
17. 0x4037 in -[MainViewController refresh] at MainViewController.m:229
18. 0x79c669 in _nsnote_callback
19. 0xf879f9 in __CFXNotificationPost_old
20. 0xf0693a in _CFXNotificationPostNotification
21. 0x79220e in -[NSNotificationCenter postNotificationName:object:userInfo:]
22. 0x79e551 in -[NSNotificationCenter postNotificationName:object:]
23. 0x62f7 in -[MainViewController changeUnit:] at MainViewController.m:447
24. 0x2a4fd in -[UIApplication sendAction:to:from:forEvent:]
25. 0xba799 in -[UIControl sendAction:to:forEvent:]
26. 0xbcc2b in -[UIControl(Internal) _sendActionsForEvents:withEvent:]
27. 0xba750 in -[UIControl sendActionsForControlEvents:]
28. 0xfa59b in -[UISegmentedControl setSelectedSegmentIndex:]
29. 0xff39d in -[UISegmentedControl touchesBegan:withEvent:]
30. 0x4ed41 in -[UIWindow _sendTouchesForEvent:]
31. 0x2fc37 in -[UIApplication sendEvent:]
32. 0x34f2e in _UIApplicationHandleEvent
33. 0x11e8992 in PurpleEventCallback
34. 0xf90944 in __CFRUNLOOP_IS_CALLING_OUT_TO_A_SOURCE1_PERFORM_FUNCTION__
35. 0xef0cf7 in __CFRunLoopDoSource1
36. 0xeedf83 in __CFRunLoopRun
37. 0xeed840 in CFRunLoopRunSpecific
38. 0xeed761 in CFRunLoopRunInMode
39. 0x11e71c4 in GSEventRunModal
40. 0x11e7289 in GSEventRun
41. 0x38c93 in UIApplicationMain
42. 0x2639 in main at main.m:14
43. 0x25b5 in start

有人能解释一下这里发生了什么吗？我实在不明白为什么第二次就不能查字典了。我没有看到任何东西会在第二次使用字典时释放它。

谢谢大家啊

aa

Answer 1

    factors = [NSDictionary dictionaryWithContentsOfFile:path];

您正在将实例变量设置为自动释放的对象。一旦池耗尽，发送给 factors 的下一条消息可能会BOOM。

该方法遵循框架 API 中每个方法的标准规则。请参阅 政策指南。

该文档竭尽全力不重复每个方法的通用规则；也就是说，如果您不阅读指南，您将无法理解每种方法的工作原理。

这就是“授人以鱼，可食一日……授人以鱼，可食终生”的道理。