ARC和MRC实现单例模式

代码如下，可直接拷贝到头文件中

#define singleton_h(name) +(instancetype)shared##name
# if __has_feature(objc_arc) //ARC

#define singleton_m(name) static id _instance;+(id)allocWithZone:(struct _NSZone *)zone{    static dispatch_once_t onceToken;    dispatch_once(&onceToken, ^{        _instance = [super allocWithZone:zone];    });    return _instance;}+(instancetype)shared##name{    static dispatch_once_t onceToken;    dispatch_once(&onceToken, ^{        _instance = [[self alloc] init];    });    return _instance;}+(id)copyWithZone:(struct _NSZone *)zone{    return _instance;}
#else //非ARC
#define singleton_m(name) static id _instance;+(id)allocWithZone:(struct _NSZone *)zone{static dispatch_once_t onceToken;dispatch_once(&onceToken, ^{_instance = [super allocWithZone:zone];});return _instance;}+(instancetype)shared##name{static dispatch_once_t onceToken;dispatch_once(&onceToken, ^{_instance = [[self alloc] init];});return _instance;}+(id)copyWithZone:(struct _NSZone *)zone{return _instance;}-(oneway void)release{    }-(instancetype)autorelease{    return _instance;}-(instancetype)retain{    return _instance;}-(NSUInteger)retainCount{    return 1;}

#endif

MRC要重写四个方法：

-(oneway void)release

{

}

-(instancetype)autorelease

{

return self;

}

-(instancetype)retain{

return self;

}

-(NSUInteger)retainCount{

return 1;

}