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Virtual
functions cannot be static member functions.
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Second,
the signature and return type must be the same for
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all
implementations of the virtual function.
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Third, while the function must be defined
as a virtual
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function
within a direct or indirect base class, it need not
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be
defined in those derived classes where the inherited
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behavior
does not need to differ.
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And
finally, the keyword virtual is only required within
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the
base class itself; derived class implementations of the
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overridden
function do not need to repeat the use of that
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keyword.
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Once
a member function is declared to be virtual, it
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remains
virtual for all derived classes.
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