Accessing base class member variable from derived class, instantiated from derived class

Answer 1

If SuffixableElement is your interface, then StemmedSentence must derive from it.

Answer 2

// --- returns n-th sentence of this snippet
SuffixableElement* ArrayStemmedSnippet::getSentence(int n) {
    // ...
    return (SuffixableElement*)sentences[ n ];
}

'sentences' is an array of objects of the StemmedSentence class. StemmedSentence is not derived from SuffixableElement, that is it does not provide that interface.

Try the following:

class StemmedSentence : public ArrayStemmedSnippet, public SuffixableElement {
    // ...
};

and

SuffixableElement* ArrayStemmedSnippet::getSentence(int n) {
    StemmedSentence* sentence = sentences[ n ];
    sentence->setParent(this);
    return sentence;
}