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Is it possible to implement the Visitor Pattern respecting the Open/Closed Principle , but still be able to add new visitable classes? The Open/Closed Principle states that "software entities (classes, modules, functions, etc.) should be open for extension, but closed for modification". struct ConcreteVisitable1; struct ConcreteVisitable2; struct AbstractVisitor { virtual void visit(ConcreteVisitable1& concrete1) = 0; virtual void visit(ConcreteVisitable2& concrete2) = 0; }; struct AbstractVisitable { virtual void accept(AbstractVisitor& visitor) = 0; }; struct ConcreteVisitable1 : AbstractVisitable { virtual void accept(AbstractVisitor& visitor) { visitor.visit(*this); } }; struct ConcreteVisitable2 : AbstractVisitable { virtual void accept(AbstractVisitor& visitor) { visitor.visit(*this); } }; You can implement any number of classes which derives from AbstractVisitor: It is open for extension. You cannot add a new visitable class as the classes derived from AbstractVisitor will not compile: It closed for modification. The AbstractVisitor class tree respects the Open/Closed Principle. The AbstractVisitable class tree does not respect the Open/Closed Principle, as it cannot be extended. Is there any other solution than to extend the AbstractVisitor and AbstractVisitable as below? struct ConcreteVisitable3; struct AbstractVisitor2 : AbstractVisitor { virtual void visit(ConcreteVisitable3& concrete3) = 0; }; struct AbstractVisitable2 : AbstractVisitable { virtual void accept(AbstractVisitor2& visitor) = 0; }; struct ConcreteVisitable3 : AbstractVisitable2 { virtual void accept(AbstractVisitor2& visitor) { visitor.visit(*this); } }; </

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