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  1. PO
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    <p>Your sample code contains many errors, including slicing problems.</p> <p>I'm not 100% sure, but I seem to remember you can use <code>const type_info*</code> as a key for a map. </p> <p>If so, you could use something like following. It is not free from RTTI (<code>type_info</code>), but since Evaluator already checks the typeids, you can use a <code>static_cast</code> instead of a <code>dynamic_cast</code> (but it isn't that important now that the code doesn't blindly search for the right operator to apply).</p> <p>Of course, the following is completely broken in terms of memory management. Reimplement with smart pointers of your choice.</p> <pre><code>#include &lt;map&gt; #include &lt;typeinfo&gt; #include &lt;cassert&gt; #include &lt;iostream&gt; struct CompareTypeinfo { bool operator()(const std::type_info* a, const std::type_info* b) const { return a-&gt;before(*b); } }; class Var { public: virtual ~Var() {} virtual const std::type_info&amp; getType() const = 0; virtual void print() const = 0; }; template&lt;typename T&gt; class VarT : public Var { T value; public: VarT(const T&amp; v): value(v) {} const T&amp; getValue() const { return value; } virtual const std::type_info&amp; getType() const { return typeid(T); } virtual void print() const { std::cout &lt;&lt; value &lt;&lt; '\n'; } }; class Operator { public: virtual ~Operator() {} virtual Var* evaluate(const Var&amp; a, const Var&amp; b) const = 0; virtual const std::type_info&amp; getType() const = 0; }; template&lt;typename T&gt; class AddOperator : public Operator { public: typedef T type; virtual const std::type_info&amp; getType() const { return typeid(T); } virtual Var* evaluate(const Var&amp; a, const Var&amp; b) const { //it is the responsibility of Evaluator to make sure that the types match the operator const VarT&lt;T&gt;* varA = static_cast&lt;const VarT&lt;T&gt;*&gt;(&amp;a); const VarT&lt;T&gt;* varB = static_cast&lt;const VarT&lt;T&gt;*&gt;(&amp;b); return new VarT&lt;T&gt;(varA-&gt;getValue() + varB-&gt;getValue()); } }; class Evaluator { private: typedef std::map&lt;const std::type_info*, Operator*, CompareTypeinfo&gt; TypedOpMap; typedef std::map&lt;int, TypedOpMap&gt; OpMap; OpMap operatorMap; public: template &lt;class Op&gt; void registerOperator(int opId) { operatorMap[opId].insert(std::make_pair(&amp;typeid(typename Op::type), new Op)); } Var* evaluate(const Var&amp; a, const Var&amp; b,int opId) { OpMap::const_iterator op = operatorMap.find(opId); if (op != operatorMap.end() &amp;&amp; a.getType() == b.getType()) { TypedOpMap::const_iterator typed_op = op-&gt;second.find(&amp;a.getType()); if (typed_op != op-&gt;second.end()) { //double-checked assert(typed_op-&gt;second-&gt;getType() == a.getType()); return typed_op-&gt;second-&gt;evaluate(a, b); } } return 0; } }; int main() { Evaluator e; e.registerOperator&lt;AddOperator&lt;int&gt; &gt;(0); e.registerOperator&lt;AddOperator&lt;double&gt; &gt;(0); VarT&lt;int&gt; i1(10), i2(20); VarT&lt;double&gt; d1(2.5), d2(1.5); VarT&lt;float&gt; f1(1.0), f2(2.0); Var* i_result = e.evaluate(i1, i2, 0); Var* d_result = e.evaluate(d1, d2, 0); Var* f_result = e.evaluate(f1, f2, 0); Var* mixed_result = e.evaluate(i1, d2, 0); assert(i_result != 0); assert(d_result != 0); assert(f_result == 0); //addition not defined for floats in Evaluator assert(mixed_result == 0); //and never for mixed types i_result-&gt;print(); //30 d_result-&gt;print(); //4.0 } </code></pre>
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    1. POAny suggestion for doing an arbitrary operation using given arguments of arbitrary types?
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    1. USvisitor
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    1. COyeah the code is just for illustration, its not the actual code, i just write it on notepad, thanks for your suggestion, I'll read your codes for a while...
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