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But...
A member function cannot be declared a friend
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before
it is first declared as a member of its own class.
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Unlike
a friend class, where we can have an incomplete
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declaration,
friend member function declarations (in our
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declaring
class) cannot take place until after the member
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functions
have been declared (in their own class).
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If
two or more classes share the same friend class or
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function,
those classes share a mutual friend. This means
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that
more than one class gives a single friend class or
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function
permission to access their members.
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