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copied!<p>Sounds like a <a href="http://en.wikipedia.org/wiki/Double_dispatch" rel="nofollow noreferrer">double dispatch</a> problem. Perhaps this would be a good place to implement the <a href="http://en.wikipedia.org/wiki/Visitor_pattern" rel="nofollow noreferrer">visitor pattern</a>?</p> <p>For example, create a class <code>Evaluator</code>, and for each <code>T</code> a subclass <code>ConcreteEvaluator<T></code>. Give <code>A</code> and <code>B</code> methods that visit the <code>Evaluator</code>. Something like:</p> <pre><code>class Evaluator { virtual void visit_A(A* object); virtual void visit_B(B* object); }; template <typename T> class ConcreteEvaluator : public Evaluator { public: String* input_reference; T& result_reference; ConcreteEvaluator(String& input_reference_,T& result_reference_) : input_reference(input_reference_), result_reference(result_reference_) {} virtual void visit_A(A* object) { object->Func(input_reference,result_reference); } virtual void visit_B(B* object) { object->Func(input_reference,result_reference); } } class A { ... virtual void apply_evaluator(Evaluator *eval) {eval->visit_A(this);} ... } class B { ... virtual void apply_evaluator(Evaluator *eval) {eval->visit_B(this);} ... } </code></pre> <p>For each subclass of <code>A</code>, a new method must be added to <code>ConcreteEvaluator</code>, so that this technique works best if <code>A</code>'s class hierarchy is stable. And for each subclass of <code>A</code>, it must have an <code>apply_evaluator</code> function defined properly.</p> <p>On the other hand, this may be total overkill. For about the same amount of work, you could always just pay the price to update <code>M::Evaluate</code>:</p> <pre><code>class M { ... void Evaluate(const String& sInput, T& tResult) { // try to downcast to each subclass of A. Be sure to check // sub-subclasses first try { dynamic_cast<B*>(pInterface.get())->Func(sInput, tResult); return; } catch (std::bad_cast& ) { } ... // nothing worked. It must really be an A pInterface->Func(sInput,tResult); } ... }; </code></pre>

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