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POModelling abstract compositions with type-safety
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  1. POModelling abstract compositions with type-safety
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    copied!<p>I have a structural problem that I could use your help with. I'll explain the abstract problem first, then an example to illustrate the problem.</p> <p>Consider an abstract class <b>A</b> holding a number of instances of an abstract class <b>B</b>, also assume the following methods in <b>A</b>: <b>void a.foo1(B val)</b> and <b>B a.foo2()</b>. The problem arises when we inherit the classes (<b>A'</b> inherits <b>A</b> and <b>B'</b> inherits <b>B</b>) and demand that the relation <b>A</b> has to <b>B</b> should be the same as <b>A'</b> has to <b>B'</b>. That is, in <b>A'</b>: <b>void a.foo1(B' val)</b> and <b>B' a.foo2()</b>. The second method will work, but not the first one (unless we do a unsafe type-cast). In other words, in <b>A'</b>: <b>a.foo1(B val)</b> should be illegal unless parameter is an instance of <b>B'</b>. I've tried to model this relationship with generics/templates with little sucess.</p> <p>This problem arises when making a graph framework. Here we have classes <b>Graph</b> and <b>GraphVertex</b>. (In my actual implementation I overload GraphVertex's delete operator.) In C++:</p> <pre><code>template&lt;class T&gt; class Graph; // Forward reference. template&lt;class T&gt; class GraphVertex { public: GraphVertex(Graph&lt;T&gt; &amp;graph) : m_graph(graph){} virtual ~GraphVertex() {} ... // Abstract methods, using T parameter. void remove() { m_graph.removeVertex(this); } protected: Graph&lt;T&gt; &amp;m_graph; } template&lt;class T&gt; class Graph { public: virtual ~Graph(){} ... virtual GraphVertex&lt;T&gt; *add(T val) = 0; virtual void removeVertex(GraphVertex&lt;T&gt; *vertex) = 0; // &lt;--- !!! } </code></pre> <p>The problem here is the <b>removeVertex</b> method. Lets say we have implemented the classes with <b>AdjacencyMatrixGraph</b> and <b>AMGVertex</b>. When removing a vertex in the <b>AdjacencyMatrixGraph</b> we need more data than is provided in the abstract base class <b>GraphVertex</b>. We know that the parameter type <i>should</i> be <b>AMGVertex</b> but sending another type as parameter would not generate an (compile-time) error.</p> <p>To solve this problem I've tried to add a new template parameter, specifying the implemented type. That is:</p> <pre><code>template&lt;class T, class G&gt; class GraphVertex { public: GraphVertex(G &amp;graph) : m_graph(graph) {} ~GraphVertex() {} ... void remove() { m_graph.removeVertex(this); } protected: G &amp;m_graph; } template&lt;class T, class V&gt; class Graph { public: virtual ~Graph() {} ... virtual V *add(T val) = 0; virtual void removeVertex(V *vertex) = 0; } template&lt;class T&gt; AdjacencyMatrixGraph; // Forward declaration. template&lt;class T&gt; AMGVertex : public GraphVertex&lt;T, AdjacencyMatrixGraph&lt;T&gt;&gt; { ... } template&lt;class T&gt; AdjacencyMatrixGraph : public Graph&lt;T, AMGVertex&lt;T&gt;&gt; { ... } </code></pre> <p>However, this will not work. It is not possible to use the base class <b>Graph</b> due to circular reference of the base classes.</p> <pre><code>Graph&lt;int&gt; *p = new AdjacencyMatrixGraph&lt;int&gt;(); // Won't work. </code></pre> <p>The example above has the same problems in Java with generics.</p> <p>So, is there anyway to model the relation in a type-safe way? Or am I stuck with casting pointers around?</p> <p>Thank you for reading!</p> <p>EDIT:</p> <p>An example usage of the above could look like:</p> <pre><code>Graph&lt;int&gt; *someGraph = getSomeGraph(); GraphVertex&lt;int&gt; *newVertex = someGraph-&gt;add(3); ... newVertex-&gt;remove(); </code></pre>
 

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