On 7 Jun 2013, at 07:29, Maxthon Chan <[email protected]> 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
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