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copied!<p>From Modern C++ - Design Generic Programming and Design Patterns Applied - Andrei Alexandrescu</p> <pre><code>#include <iostream> class BaseVisitor { public: virtual ~BaseVisitor() {}; }; template <class T, typename R = int> class Visitor { public: virtual R visit(T &) = 0; }; template <typename R = int> class BaseVisitable { public: typedef R ReturnType; virtual ~BaseVisitable() {}; virtual ReturnType accept(BaseVisitor & ) { return ReturnType(0); } protected: template <class T> static ReturnType acceptVisitor(T &visited, BaseVisitor &visitor) { if (Visitor<T> *p = dynamic_cast< Visitor<T> *> (&visitor)) { return p->visit(visited); } return ReturnType(-1); } #define VISITABLE() \ virtual ReturnType accept(BaseVisitor &v) \ { return acceptVisitor(*this, v); } }; /** example of use */ class Visitable1 : public BaseVisitable<int> { /* Visitable accept one BaseVisitor */ public: VISITABLE(); }; class Visitable2 : public BaseVisitable<int> { /* Visitable accept one BaseVisitor */ public: VISITABLE(); }; class VisitorDerived : public BaseVisitor, public Visitor<Visitable1, int>, public Visitor<Visitable2, int> { public: int visit(Visitable1 & c) { std::cout << __PRETTY_FUNCTION__ << std::endl; } int visit(Visitable2 & c) { std::cout << __PRETTY_FUNCTION__ << std::endl; } }; int main(int argc, char **argv) { VisitorDerived visitor; Visitable1 visitable1; Visitable2 visitable2; visitable1.accept(visitor); visitable2.accept(visitor); } </code></pre> <p>Is possible to avoid dynamic_cast with CRTP pattern like:</p> <pre><code>#include <iostream> class BaseVisitor { public: virtual ~BaseVisitor() {}; }; template <class T> class Visitor { public: virtual void visit(T &) = 0; }; template <class Visitable> class BaseVisitable { public: template <typename T> void accept(T & visitor) { visitor.visit(static_cast<Visitable &>(*this)); } }; /** example of use */ class Visitable1 : public BaseVisitable<Visitable1> { }; class Visitable2 : public BaseVisitable<Visitable2> { }; class VisitorDerived : public BaseVisitor, public Visitor<Visitable1>, public Visitor<Visitable2> { public: void visit(Visitable1 & c) { std::cout << __PRETTY_FUNCTION__ << std::endl; } void visit(Visitable2 & c) { std::cout << __PRETTY_FUNCTION__ << std::endl; } }; int main(int argc, char **argv) { VisitorDerived visitor; Visitable1 visitable1; Visitable2 visitable2; visitable1.accept<VisitorDerived>(visitor); visitable2.accept<VisitorDerived>(visitor); } </code></pre>

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