今天读cocoa design patterns 引发的问题 alloc和allocwithzone的存在解决了很多问题，但是还是很疑惑。（随便说句 此书中文版翻译不靠谱。。。）
下面是截自http://stackoverflow.com/的一段分析，虽然还没有体会，但是还是保留下来。
When one object creates another, it’s sometimes a good idea to make sure they’re both allocated from the same region of memory. The zone method (declared in the NSObject protocol) can be used for this purpose; it returns the zone where the receiver is located.
This suggests to me that your ivars, and any objects your classes "create" themselves could make use of+allocWithZone: in this way, to make the instances they create in the same zone.
-(id)init { if (self = [super init]) { someIvar = [[SomeOtherClass allocWithZone:[self zone]] init]; } return self;}
