访问者模式可以将算法与其操作的对象类进行解耦。这种模式通过构建一个新的Visitor类，以防止新功能被整合进已有的类中（至少以更小的代价被整合进已有类中）。

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class Shape {
  public:
    virtual void move(int x, int y) = 0;
    virtual void draw() = 0;
    virtual void accept(Visitor* v) = 0;
    virtual ~Shape() {}
};

class Dot: public Shape {
  // ...
  void accept(Visitor* v) override {
    v->visitDot(this);
  }
};

class Circle: public Shape {
  // ...
  void accept(Visitor* v) override {
    v->visitCircle(this);
  }
};

class Rectangle: public Shape {
  // ...
  void accept(Visitor* v) override {
    v->visitRectangle(this);
  }
};

class CompoundShape: public Shape {
  // ...
  void accept(Visitor* v) override {
    v->visitCompoundShape(this);
  }
};

class Visitor {
  public:
    virtual std::string visitDot(Dot* d) = 0;
    virtual std::string visitCircle(Circle* c) = 0;
    virtual std::string visitRectangle(Rectangle* r) = 0;
    virtual std::string visitCompoundShape(CompoundShape* cs) = 0;
}

class XMLExportVisitor: public Visitor {
  std::string visitDot(Dot* d) override {
    // export the dot coords in xml format
  }
  std::string visitCircle(Circle* c) override {
    // ...
  }
  // ...
};

int main(int argc, char* argv[]) {
  std::vector<Shape*> shapes;
  // ...
  XMLExportVisitor exportVisitor = new XMLExportVisitor();
  for (Shape* s: shapes) {
    s->accept(exportVisitor);
  }
  delete exportVisitor;
  return 0;
}