why are concrete types rarely useful as bases for further derivation
<p>I read "Concrete types are rarely useful as bases for further derivation"------------ Stroustrup p. 768</p>

<p>How to interpret this? Does it mean that we should get derived class from the base class with pure virtual function? However, I see a lot of code with derived class derived from concrete types.</p>

<p>Can anyone help me out?</p>
