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copied!<p>After some experimenting I have concluded that the general design cannot be changed to be 100% type-safe. However, there are some situations where you can help ensure that the offended method is only called with the correct sub-type.</p> <p>Taking the graph example, what we need to do is to insure that a sub-class of <b>GraphVertex</b> (in this case <b>AMGVertex</b>) only can access the related sub-class of <b>Graph</b> (that is for <b>AMGVertex</b>, class <b>AdjacencyMatrixGraph</b>). Hence insuring that the correct type is inserted at the <b>removeVertex(GraphVertex *v)</b> method. To do this, a non-public visibility can be used for methods that are accessed by the abstract class <b>GraphVertex</b>. Also the visibility of the constructors of all the sub-types of <b>GraphVertex</b> must be non-public. Of course the constructor of <b>AMGVertex</b> must be visible from <b>AdjacencyMatrixGraph</b> and the <b>removeVertex</b> must be visible from <b>AMGVertex</b>. In C++ this can be done with <i>friends</i>.</p> <pre><code>template<class T> class GraphVertex { public: virtual ~GraphVertex() {} void remove() { m_graph.removeVertex(this); } protected: GraphVertex(Graph<T> &graph) : m_graph(graph) {} Graph<T> &m_graph; } template<class T> class Graph { friend class GraphVertex<T>; public: virtual GraphVertex<T> add(T value) = 0; protected: // Or private? virtual void removeVertex(GraphVertex<T> *v) = 0; } template<class T> class AMGVertex : public GraphVertex<T> { friend class AdjacencyMatrixGraph<T>; protected: // Or private? AMGVertex(AdjacencyMatrixGraph<T> &graph) : GraphVertex<T>(graph) {} } template<class T> class AdjacencyMatrixGraph : public Graph<T> { public: AMGVertex *add(T value) { ... } // Calls AMGVertex's constructor. protected: // Or private? void removeVertex(GraphVertex<T> *v) // Only visible from this class and through base class. { ... } } </code></pre> <p>So, <b>AMGVertex</b> can only be created from <b>AdjacencyMatrixGraph</b>, and hence a call to <b>AMGVertex::remove()</b> will always call <b>AdjacencyMatrixGraph::removeVertex(GraphVertex *v)</b> with the correct sub-class as parameter.</p> <p>It is however still possible to circumvent this, simply by creating a new vertex type with a <b>AdjacencyMatrixGraph</b> as <b>m_graph</b>. This is because the <i>friendship</i> from vertex to graph is in the abstract base-class.</p> <p>So a 100% type-safe solution is (at least from what I've gathered) not possible.</p> <p>In Java I believe similar results can be achieved with nested classes (to overcome visibility limitations).</p> <p>Thank you for the replies! If someone has a better solution, let us know. </p>

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