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POIs the Visitor Pattern the fastest way to differentiate parameter types in C++?
 

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    <p>Is the Visitor Pattern the fastest way to accomplish method parameter type identification (effectively single dispatch on a parameter, not a member's class) in C++? I might know the exact method(s) I want to invoke on elements of not-yet-know subtype, so invariably making an additional virtual method call like <code>V::visit(A *)</code> in <code>A::accept(V &amp;v) { v.visit(this); }</code> is undesirable.</p> <pre><code>// Is the Visitor pattern recommended here? (E inherits D inherits B.) class Foo { public: virtual void visit(B *) { result = 3; } virtual void visit(D *) { result = 4; } virtual void visit(E *) { result = 5; } private: int result; }; // class Foo // Need to add generic interface to B and its children ... class B { public: virtual void accept(class Foo &amp;f) { f.visit(this); } }; // class B </code></pre> <p>I'd like something functionally equivalent the following but with O(1) cost, which is AFAIK not possible with dynamic_cast&lt;> or typeid() ladders, since <code>std::type_info</code> can't be a constexpr/switchable.</p> <pre><code>// O(n) search cost might get nasty with bigger hierarchies. int foo(B *b) { if (typeid(b) == typeid(B *)) { return 1; } if (typeid(b) == typeid(D *)) { return 2; } if (typeid(b) == typeid(E *)) { return 3; } return -1; } </code></pre> <p>What are my options here? Thanks for the advice!</p> <p><strong>Edit:</strong> Changed sample code to feed results through field, such that multiple signatures aren't needed for different method types. Thanks, Maurice!</p> <p><strong>Final decision:</strong> In addition to not liking the mandatory double dispatch cost of the Visitor Pattern, I also wanted to avoid the interface bloat of overloading <code>foo()</code>, but I don't think that there is a known clean pattern to do this. I ended up just doing straight static overloads and called it a day. Anyway, my wanting to encapsulate overloading inside a function is probably a questionable goal at best. Thanks, Maurice for the response.</p>
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