Re: Question about memory management

[Maxthon Chan]

I don't want an exclusive singleton - that is, there is not only one shared 
singleton instance, the user can also set up one for their own, like recent 
versions of NSFileManager on OS X.

Can I do this:

// Singleton.m
static Singleton *__singleton

@implementation Singleton
+ (instancetype)sharedSingleton
{
        @synchronized (__singleton)
        {
                if (!__singleton)
                        __singleton = [[self alloc] init];
        }
        return __singleton;
}
// …
@end

在 2013-6-7，下午4:26，David Chisnall <address@hidden> 写道：

> On 7 Jun 2013, at 07:29, Maxthon Chan <address@hidden> wrote:
> 
>> Just asking, with ARC, is this a good choice on implementing singleton?
> 
> No, it's not thread-safe.
> 
>> //Singleton.h
>> #import <Foundation/Foundation.h>
>> @interface Singleton : NSObject
>> + (instancetype)defaultSingleton;
>> // …
>> @end
>> extern Singleton *DefaultSingleton // Of course this is optional
>> 
>> // Singleton.m
>> Singleton *DefaultSingleton
> 
> This should be declared static, as the variable should not be exposed outside 
> of the Singleton.m compilation unit.
> 
>> @implementation Singleton
>> + (instancetype)defaultSingleton
>> {
>>      if (!DefaultSingleton)
>>              DefaultSingleton = [[self alloc] init];
> 
> If two threads call this method at once, then both will enter the body of 
> this if statement, both will allocate instances of the singleton and one 
> version will leak.
> 
> The correct way of implementing a singleton is to create it in the 
> +initialize method, which is guaranteed to be thread-safe.  My preferred 
> pattern is:
> 
> + (void)initialize
> {
>       [[self alloc] init];
> }
> + allocWithZone: (NSZone*)aZone
> {
>       if (DefaultSingleton != nil)
>       {
>               [NSException raise: NSInvalidArgumentException format: 
> @"Attempted to create multiple instances of singleton %@", self];
>       }
>       DefaultSingleton = [super allocWithZone: aZone];
>       NSAssert(nil != DefaultSingleton, @"Allocation of singleton instance of 
> %@ failed!", self);
>       return DefaultSingleton;
> }
> 
> Calling the constructor in +initialize means that the first time ANY message 
> is sent to this class, the singleton will be created and no other thread will 
> be allowed to send messages to this class until the singleton is fully 
> initialised.
> 
> The nil check in +allocWithZone: is safe, because the variable will be set 
> with a lock held and can then be safely queried because it will never 
> transition from non-nil to nil.  This exception prevents people from calling 
> +alloc on the object.
> 
> Assigning the value to DefaultSingleton in +allocWithZone: instead of 
> +initialize means that calls to things from the -init method don't have to 
> have special cases to check the presence of the singleton.  
> 
> For completeness, you may also include NSCoding methods that ensure that the 
> singleton is correctly created when serialising / deserialising.  It would be 
> nice if there were an NSSingleton protocol that included a +sharedInstance 
> method so that the serialiser code could know to always replace any 
> references to the class with the shared instance, but there isn't.  
> 
> David
>