1 static Config * instance = nil; 2 +(Config *) Instance 3 { 4     @synchronized(self) 5     { 6         if(nil == instance) 7         { 8             [self new]; 9         }10     }11     return instance;12 }13 14 +(id)allocWithZone:(NSZone *)zone15 {16     @synchronized(self)17     {18         if(instance == nil)19         {20             instance = [super allocWithZone:zone];21             return instance;22         }23     }24     return nil;25 }