비지터 패턴

객체 지향 프로그래밍과 소프트웨어 공학에서 비지터 패턴(visitor pattern; 방문자 패턴)은 알고리즘을 객체 구조에서 분리시키는 디자인 패턴이다. 이렇게 분리를 하면 구조를 수정하지 않고도 실질적으로 새로운 동작을 기존의 객체 구조에 추가할 수 있게 된다. 개방-폐쇄 원칙을 적용하는 방법의 하나이다.

C++

#include <iostream>#include <string>using namespace std;// 1. Add an accept(Visitor) method to the "element" hierarchyclass Element{
  public:
    virtual void accept(class Visitor &v) = 0;};class This: public Element{
  public:
     /*virtual*/void accept(Visitor &v);
    string this()
    {
        return "This";
    }};class That: public Element{
  public:
     /*virtual*/void accept(Visitor &v);
    string that()
    {
        return "That";
    }};class TheOther: public Element{
  public:
     /*virtual*/void accept(Visitor &v);
    string theOther()
    {
        return "TheOther";
    }};// 2. Create a "visitor" base class w/ a visit() method for every "element" typeclass Visitor{
  public:
    virtual void visit(This *e) = 0;
    virtual void visit(That *e) = 0;
    virtual void visit(TheOther *e) = 0;};

 /*virtual*/void This::accept(Visitor &v){
  v.visit(this);}

 /*virtual*/void That::accept(Visitor &v){
  v.visit(this);}

 /*virtual*/void TheOther::accept(Visitor &v){
  v.visit(this);}// 3. Create a "visitor" derived class for each "operation" to do on "elements"class UpVisitor: public Visitor{
     /*virtual*/void visit(This *e)
    {
        cout << "do Up on " + e->this() << '\n';
    }
     /*virtual*/void visit(That *e)
    {
        cout << "do Up on " + e->that() << '\n';
    }
     /*virtual*/void visit(TheOther *e)
    {
        cout << "do Up on " + e->theOther() << '\n';
    }};class DownVisitor: public Visitor{
     /*virtual*/void visit(This *e)
    {
        cout << "do Down on " + e->this() << '\n';
    }
     /*virtual*/void visit(That *e)
    {
        cout << "do Down on " + e->that() << '\n';
    }
     /*virtual*/void visit(TheOther *e)
    {
        cout << "do Down on " + e->theOther() << '\n';
    }};int main(){
  Element *list[] = 
  {
    new This(), new That(), new TheOther()
  };
  UpVisitor up; // 4. Client creates
  DownVisitor down; //    "visitor" objects
  for (int i = 0; i < 3; i++)
  //    and passes each
    list[i]->accept(up);
  //    to accept() calls
  for (i = 0; i < 3; i++)
    list[i]->accept(down);}// OUTPUT//// do Up on This// do Up on That// do Up on TheOther// do Down on This// do Down on That// do Down on TheOther

자바

interface CarElementVisitor {
    void visit(Wheel wheel);
    void visit(Engine engine);
    void visit(Body body);
    void visit(Car car);}interface CarElement {
    void accept(CarElementVisitor visitor); // CarElements have to provide accept().}class Wheel implements CarElement {
    private String name;

    public Wheel(String name) {
        this.name = name;
    }

    public String getName() {
        return this.name;
    }

    public void accept(CarElementVisitor visitor) {
        visitor.visit(this);
    }}class Engine implements CarElement {
    public void accept(CarElementVisitor visitor) {
        visitor.visit(this);
    }}class Body implements CarElement {
    public void accept(CarElementVisitor visitor) {
        visitor.visit(this);
    }}class Car implements CarElement{
    CarElement[] elements;

    public CarElement[] getElements() {
        return elements.clone(); // Return a copy of the array of references.
    }

    public Car() {
        this.elements = new CarElement[]
          { new Wheel("front left"), new Wheel("front right"),
            new Wheel("back left") , new Wheel("back right"),
            new Body(), new Engine() };
    }

    public void accept(CarElementVisitor visitor) {
        for(CarElement element : this.getElements()) {
            element.accept(visitor);
        }
        visitor.visit(this);
    }}class CarElementPrintVisitor implements CarElementVisitor {
    public void visit(Wheel wheel) {
        System.out.println("Visiting "+ wheel.getName()
                            + " wheel");
    }

    public void visit(Engine engine) {
        System.out.println("Visiting engine");
    }

    public void visit(Body body) {
        System.out.println("Visiting body");
    }

    public void visit(Car car) {
        System.out.println("Visiting car");
    }}class CarElementDoVisitor implements CarElementVisitor {
    public void visit(Wheel wheel) {
        System.out.println("Kicking my "+ wheel.getName() + " wheel");
    }

    public void visit(Engine engine) {
        System.out.println("Starting my engine");
    }

    public void visit(Body body) {
        System.out.println("Moving my body");
    }

    public void visit(Car car) {
        System.out.println("Starting my car");
    }}public class VisitorDemo {
    static public void main(String[] args){
        Car car = new Car();
        car.accept(new CarElementPrintVisitor());
        car.accept(new CarElementDoVisitor());
    }}