友元使用的一个总结

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这个总结来自C++ FAQ Lite：

14.4 友元不能被传递，不能被继承，也不是互利的
14.4 friendship isn't inherited, transitive, or reciprocal

1. I don't necessarily trust the kids of my friends.
2. I don't necessarily trust the friends of my friends.
3. You don't necessarily trust me simply because I declare you my friend.

另外应该还有一个：
你是我的朋友，但不代表你能访问我小孩的专用成员
My children doesn't necessarily trust you regarding their own properties.