How can I make an implementation of pure virtual function in derived class be another pure virtual function? : <p>Consider</p>

<pre><code>class A
{
protected:
 virtual void function() =0;
}

class B: public class A 
{
protected:
 virtual void function() { blah blah blah };
}
</code></pre>

<p>I want to make function() in class B also a pure virtual function. That is, derived classes of B must make another implementation of function(). How can I perform this task?</p>
