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 (Click here for a personal note from Marshall Cline.) |
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Section 14:
| 14.1 | What is a friend? |
| 14.2 | Do friends violate encapsulation? |
| 14.3 | Pros/const of usign friend functions? |
| 14.4 | Meaning of "friendship isn't inherited, transitive, or reciprocal"? |
| 14.5 | Declaring a member function or a friend function? |
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[14.4] What does it mean that "friendship isn't inherited, transitive, or reciprocal"?
Just because I grant you friendship access to me doesn't automatically grant your kids access to me, doesn't automatically grant your friends access to me, and doesn't automatically grant me access to you.
- I don't necessarily trust the kids of my friends. The privileges of friendship aren't inherited. Derived classes of a friend aren't necessarily friends. If class Fred declares that class Base is a friend, classes derived from Base don't have any automatic special access rights to Fred objects.
- I don't necessarily trust the friends of my friends. The privileges of friendship aren't transitive. A friend of a friend isn't necessarily a friend. If class Fred declares class Wilma as a friend, and class Wilma declares class Betty as a friend, class Betty doesn't necessarily have any special access rights to Fred objects.
- You don't necessarily trust me simply because I declare you my friend. The privileges of friendship aren't reciprocal. If class Fred declares that class Wilma is a friend, Wilma objects have special access to Fred objects but Fred objects do not automatically have special access to Wilma objects.