Objective-C で継承可能な Singleton Class を作る ref: http://qiita.com/items/b4e51c33e7c9d29964ab

file3.txt

SingletonA *s1 = [SingletonA sharedInstance];
[s1 printA]; // 'NSInvalidArgumentException', reason: '-[Singleton printA]: unrecognized selector sent to instance が発生

file5.txt

SingletonA *s1 = [SingletonA sharedInstance];
[s1 printA];
SingletonB *s2 = [SingletonB sharedInstance];
[s2 printB];

Singleton.m

@implementation Singleton
static NSMutableDictionary *_instances;

+ (id) sharedInstance {
    __block Singleton *obj;
    @synchronized(self) {
        if ([_instances objectForKey:NSStringFromClass(self)] == nil) {
            obj = [[self alloc] initSharedInstance];
        }
    }
    obj = [_instances objectForKey:NSStringFromClass(self)];
    return obj;
}

+ (id)allocWithZone:(NSZone *)zone {
    @synchronized(self) {
        if ([_instances objectForKey:NSStringFromClass(self)] == nil) {
            id instance = [super allocWithZone:zone];
            if ([_instances count] == 0) {
                _instances = [[NSMutableDictionary alloc] initWithCapacity:0];
            }
            [_instances setObject:instance forKey:NSStringFromClass(self)];
            return instance;
        }
    }
    return nil;
}
- (id)initSharedInstance {
    self = [super init];
    if (self) {
        // do something
    }
    return self;
}

- (id)init {
    [self doesNotRecognizeSelector:_cmd]; // init を直接呼ぼうとしたらエラーを発生させる
    return nil;
}


- (void)print{
    NSLog(@"hoge");
}

@end

SingletonA.h

@interface SingletonA : Singleton
-(void)printA;
@end

SingletonB.h

@interface SingletonB : Singleton
-(void)printB;
@end