LePUS3에서의 비지터
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.

구조
--

### UML 클래스 및 시퀀스 다이어그램

비지터 디자인 패턴을 위한 샘플 UML 클래스와 시퀀스 다이어그램.
### 클래스 다이어그램

LePUS3의 비지터 (범례)

예제
--

### C++

```
#include <iostream>
#include <string>
using namespace std;

// 1. Add an accept(Visitor) method to the "element" hierarchy
class 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" type
class 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());
 }
}

```