Friend Function和Friend Class（小客户和大客户）授权的局限性 | 非可继承性

http://msdn.microsoft.com/library/default.asp?url=/library/en-us/vclang/html/_pluslang_friend_functions.asp


http://msdn.microsoft.com/library/default.asp?url=/library/en-us/vclang/html/_pluslang_class_member_functions_and_classes_as_friends.asp