单例模式ARC和非ARC

ARC环境下的单例模式：

static id _instance = nil;
+ (id)allocWithZone:(struct _NSZone *)zone
{
    if (_instance == nil) { 

        static dispatch_once_t onceToken; 

        dispatch_once(&onceToken, ^{
            _instance = [super allocWithZone:zone];
        }); 

    } 

    return _instance; 

} 

- (id)init
{ 

    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        _instance = [super init];
    });
    return _instance;
} 

+ (instancetype)sharedMethodName
{
    return [[self alloc] init];
}

非ARC模式下的单例模式：

static id _instance = nil;
+ (id)allocWithZone:(struct _NSZone *)zone
{
    if (_instance == nil) { 

        static dispatch_once_t onceToken; 

        dispatch_once(&onceToken, ^{
            _instance = [super allocWithZone:zone];
        }); 

    } 

    return _instance; 

} 

- (id)init
{ 

    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        _instance = [super init];
    });
    return _instance;
} 

+ (instancetype)sharedMethodName
{
    return [[self alloc] init];
} 

- (oneway void)release
{ 

} 

- (id)retain
{
    return self;
} 

- (NSUInteger)retainCount
{
    return ;
}

如何判断当前的环境是arc还是非arc  可以这样写：

#if __has_feature(objc_arc)  //是arc

//这里写arc下的代码

#else           //非arc

//这里写非acr下的代码

#endif