http://blog.sina.com.cn/s/blog_6531b9b80101c6cr.html
很早以前比较弱，网上不知道哪里看了篇博文，留下了ARC属性中不能使用retain关键字的印象，今天再LLVM官网上看了下，发现以前MRC中的这些关键字还是可以使用（自己没学好，很容易误人子弟，以后我要慎重去写每篇博文，争取不出现这样的现象），他们在ARC中的效用如下：
------------------------------在属性中使用以下关键字的效用-------------------------
- assign implies __unsafe_unretained ownership.
- copy implies __strong ownership, as well as the usual behavior of copy semantics on the setter.
- retain implies __strong ownership.
- strong implies __strong ownership.
- unsafe_unretained implies __unsafe_unretained ownership.
- weak implies __weak ownership.
assign 等同unsafe_unretained
retain 等同 strong
copy的效用和MRC一样，同时又有strong的效果。