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    <p>When you are overriding virtual methods, you have to match the function signatures <em>precisely</em>. The return type may vary in accordance with covariance rules, but the parameter types must be exactly the same.</p> <p>In the base class <code>Data</code> function <code>compareTo</code> is declared as</p> <pre><code>virtual int compareTo(Data * other) const </code></pre> <p>In the derived class <code>Term</code> it is declared as</p> <pre><code>int compareTo(Term * term) </code></pre> <p>Firstly, the parameter type is different. Secondly, the <code>const</code> is missing.</p> <p>This means that you wrote a completely unrelated function in the derived class. It does not override the base class's pure virtual function. Since the base pure virtual function remain non-overriden, class <code>Term</code> is still abstract.</p> <p>In <code>Term</code> you have to declare your function precisely as</p> <pre><code>int compareTo(Data * other) const </code></pre> <p>I assume that you expect to use <code>compareTo</code> in <code>Term</code> only for <code>Term</code>-to-<code>Term</code> comparisons. But in this design you'll have to either receive <code>Data</code> as an argument and then cast it to <code>Term</code>, or use the double-dispatch technique.</p> <p>P.S. On top of that you declare a <code>Term</code> object as a member <code>head</code> of your <code>Polynomial</code> class and then later use it as if it is a <em>pointer</em> </p> <pre><code>Term * current = head; </code></pre> <p>This makes no sense at all. If you want your <code>head</code> to be a pointer, declare it as a pointer. If you want it to be an object, then stop using it as a pointer. Either this or that.</p>
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    1. COMight be worth mentioning the [C++11 `override` identifier](http://en.wikipedia.org/wiki/C%2B%2B11#Explicit_overrides_and_final).
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    2. CO@AndreyT Thank you for your help! I would upvote, but I don't have reputation for it yet. I didn't understand why the pointer was doing that. That alone removed a chunk of the errors I was getting.
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    3. CO@user1527482: Turning the `head` declaration into a pointer will also *hide* the original error (about abstract class). However, it will resurface anyway in some other place.
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