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POUsing the Visitor Pattern with template derived classes
 

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    <p>I try to implement the Visitor pattern with templated derived classes</p> <h1><strong>I work with gcc 4.5</strong></h1> <p>here is the VisitorTemplate.hpp, I specialized Derived in the class Visitor, but I'd like to be able to handle any type:</p> <p>edit : thanks to the suggestions of interjay, the code compiles and runs without errors now</p> <pre class="lang-cpp prettyprint-override"><code>#ifndef VISITORTEMPLATE_HPP_ #define VISITORTEMPLATE_HPP_ #include &lt;iostream&gt; #include &lt;string&gt; using namespace std; template&lt;class T&gt; Derived; class Visitor { public: virtual void visit(Derived&lt;string&gt; *e) = 0; }; class Base { public: virtual void accept(class Visitor *v) = 0; }; template&lt;class T&gt; Derived: public Base { public: virtual void accept(Visitor *v) { v-&gt;visit(this); } string display(T arg) { string s = "This is : " + to_string(arg); return s; } }; class UpVisitor: public Visitor { virtual void visit(Derived&lt;string&gt; *e) { cout &lt;&lt; "do Up on " + e-&gt;display("test") &lt;&lt; '\n'; } }; class DownVisitor: public Visitor { virtual void visit(Derived&lt;string&gt; *e) { cout &lt;&lt; "do Down on " + e-&gt;display("test") &lt;&lt; '\n'; } }; #endif /* VISITORTEMPLATE_HPP_ */ </code></pre> <p>main.cpp</p> <pre class="lang-cpp prettyprint-override"><code>Base* base = new Derived&lt;string&gt;(); Visitor* up = new UpVisitor(); Visitor* down = new DownVisitor(); base-&gt;accept(up); base-&gt;accept(down); </code></pre> <p>Now my goal is to use Derived in visit without specializing; unfortunately, visit is a virtual method so I can't template it</p>
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    1. COWhat compiler are you using? I just fed your example code to Visual C++ 2010 and it compiled quite happily, apart from complaining that std::to_string can't handle the template parameter argument type. I can't see any reason in your example code why it would complain about an incomplete type, as you didn't miss out providing implementations for the single pure virtual function.
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    2. CO@MatthewWalton: "I work with gcc 4.5." However, it's always nice to indicate the lines compiler messages refer to. I don't want to count 31 lines in my browser.
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    3. CO@sbi doh, I must've forgotten that when I was feeding my compiler the code. Still, I wouldn't think that particular kind of error is the kind that GCC and VC++ would disagree on.
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