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In
order to implement a virtual friend, we must
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implement
a virtual helper member function that
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performs
the operation that we want and then call it from
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the
non-member function.
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We
must be careful to declare the object for which we
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want
polymorphic behavior to be a pointer or a reference
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in
the helper function.
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If
the object is passed by value to the non-member
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function
then dynamic binding cannot be used because
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we
have an object and not a pointer or reference.
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It is
best to make the helper functions protected or
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private
in the base class and in all derived classes and
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then
the non-member function a friend of the class.
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