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