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 1; }
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
原文：http://www.cnblogs.com/syios/p/4797783.html