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Visitor Design Pattern in C++: Recovering lost type information

Motivation. "My Component classes do not know that Composites exist. They provide no help for navigating Composites, nor any help for altering the contents of a Composite. This is because I would like the base class (and all its derivatives) to be reusable in contexts that do not require Composites. When given a base class pointer, if I absolutely need to know whether or not it is a Composite, I will use dynamic_cast() to figure this out. In those cases where dynamic_cast() is too expensive, I will use a Visitor." 

 #include <iostream> #include <vector> using namespace std; class Visitor { public: virtual void visit(class Primitive *, class Component*) = 0; virtual void visit(class Composite *, Component*) = 0; }; class Component { int value; public: Component(int val) { value = val; } virtual void traverse() { cout << value << " "; } // Having add() here sacrifices safety, but it supports transparency // virtual void add( Component* ) { } virtual void accept(Visitor &, Component*) = 0; }; class Primitive: public Component { public: Primitive(int val): Component(val){} /*virtual*/void accept(Visitor &v, Component *c) { v.visit(this, c); } }; class Composite: public Component { vector < Component * > children; public: Composite(int val): Component(val){} void add(Component *ele) { children.push_back(ele); } /*virtual*/void accept(Visitor &v, Component *c) { v.visit(this, c); } /*virtual*/void traverse() { Component::traverse(); for (int i = 0; i < children.size(); i++) children[i]->traverse(); } }; class AddVisitor: public Visitor { public: /*virtual*/void visit(Primitive *, Component*) { /* does not make sense */ } /*virtual*/void visit(Composite *node, Component *c) { node->add(c); } }; int main() { Component *nodes[3]; // The type of Composite* is "lost" when the object is assigned to a // Component* nodes[0] = new Composite(1); nodes[1] = new Composite(2); nodes[2] = new Composite(3); // If add() were in class Component, this would work // nodes[0]->add( nodes[1] ); // If it is NOT in Component, and only in Composite, you get the error - // no member function `Component::add(Component *)' defined // Instead of sacrificing safety, we use a Visitor to support add() AddVisitor addVisitor; nodes[0]->accept(addVisitor, nodes[1]); nodes[0]->accept(addVisitor, nodes[2]); nodes[0]->accept(addVisitor, new Primitive(4)); nodes[1]->accept(addVisitor, new Primitive(5)); nodes[1]->accept(addVisitor, new Primitive(6)); nodes[2]->accept(addVisitor, new Primitive(7)); for (int i = 0; i < 3; i++) { nodes[i]->traverse(); cout << endl; } }

Output

 1 2 5 6 3 7 4 2 5 6 3 7

Code examples

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