IOS实现单例模式的几种实现方法

static CUser * gUser = nil;
+(id)currentUser
{
if(gUser == nil)
gUser = [[self.class alloc]init];
return gUser;
}

+ (id)currentUser
{
static CUser *gUser = nil;
static dispatch_once_t once;
dispatch_once(&once, ^{
gUser = [self.class alloc]init];
});
return gUser;
}