作用:
表示一個(gè)作用于某對(duì)象結(jié)構(gòu)中的各元素的操作.它使你可以在不改變各元素的類的前提下定義作用于這些元素的新操作.
UML結(jié)構(gòu)圖:
解析:
Visitor模式把對(duì)結(jié)點(diǎn)的訪問封裝成一個(gè)抽象基類,通過派生出不同的類生成新的訪問方式.在實(shí)現(xiàn)的時(shí)候,在visitor抽象基類中聲明了對(duì)所有不同結(jié)點(diǎn)進(jìn)行訪問的接口函數(shù),如圖中的VisitConcreateElementA函數(shù)等,這樣也造成了Visitor模式的一個(gè)缺陷--新加入一個(gè)結(jié)點(diǎn)的時(shí)候都要添加Visitor中的對(duì)其進(jìn)行訪問接口函數(shù),這樣使得所有的Visitor及其派生類都要重新編譯了,也就是說Visitor模式一個(gè)缺點(diǎn)就是添加新的結(jié)點(diǎn)十分困難.另外,還需要指出的是Visitor模式采用了所謂的"雙重分派"的技術(shù),拿上圖來作為例子,要對(duì)某一個(gè)結(jié)點(diǎn)進(jìn)行訪問,首先需要產(chǎn)生一個(gè)Element的派生類對(duì)象,其次要傳入一個(gè)Visitor類派生類對(duì)象來調(diào)用對(duì)應(yīng)的Accept函數(shù),也就是說,到底對(duì)哪種Element采用哪種Visitor訪問,需要兩次動(dòng)態(tài)綁定才可以確定下來,具體的實(shí)現(xiàn)可以參考下面實(shí)現(xiàn)代碼中的Main.cpp部分是如何調(diào)用這些類的.
實(shí)現(xiàn):
1)Visitor.h
/**//********************************************************************
????created:????2006/08/09
????filename:?????Visitor.h
????author:????????李創(chuàng)
????????????????http://www.shnenglu.com/converse/
????purpose:????Visitor模式的演示代碼
*********************************************************************/
#ifndef?VISITOR_H
#define?VISITOR_H
class?Visitor;
class?Element
{
public:
????virtual?~Element(){}
????virtual?void?Accept(Visitor?&rVisitor)?=?0;
protected:
????Element(){}
};
class?ConcreateElementA
????:?public?Element
{
public:
????virtual?~ConcreateElementA()?{}
????virtual?void?Accept(Visitor?&rVisitor);
};
class?ConcreateElementB
????:?public?Element
{
public:
????virtual?~ConcreateElementB()?{}
????virtual?void?Accept(Visitor?&rVisitor);
};
class?Visitor
{
public:
????virtual?~Visitor(){}
????virtual?void?VisitConcreateElementA(ConcreateElementA?*pConcreateElementA)?=?0;
????virtual?void?VisitConcreateElementB(ConcreateElementB?*pConcreateElementB)?=?0;
protected:
????Visitor(){}
};
class?ConcreateVisitorA
????:?public?Visitor
{
public:
????virtual?~ConcreateVisitorA(){}
????virtual?void?VisitConcreateElementA(ConcreateElementA?*pConcreateElementA);
????virtual?void?VisitConcreateElementB(ConcreateElementB?*pConcreateElementB);
};
class?ConcreateVisitorB
????:?public?Visitor
{
public:
????virtual?~ConcreateVisitorB(){}
????virtual?void?VisitConcreateElementA(ConcreateElementA?*pConcreateElementA);
????virtual?void?VisitConcreateElementB(ConcreateElementB?*pConcreateElementB);
};
#endif
?2)Visitor.cpp
/**//********************************************************************
????created:????2006/08/09
????filename:?????Visitor.cpp
????author:????????李創(chuàng)
????????????????http://www.shnenglu.com/converse/
????purpose:????Visitor模式的演示代碼
*********************************************************************/
#include?"Visitor.h"
#include?<iostream>
void?ConcreateElementA::Accept(Visitor?&rVisitor)
{
????rVisitor.VisitConcreateElementA(this);
}
void?ConcreateElementB::Accept(Visitor?&rVisitor)
{
????rVisitor.VisitConcreateElementB(this);
}
void?ConcreateVisitorA::VisitConcreateElementA(ConcreateElementA?*pConcreateElementA)
{
????std::cout?<<?"VisitConcreateElementA?By?ConcreateVisitorA\n";
}
void?ConcreateVisitorA::VisitConcreateElementB(ConcreateElementB?*pConcreateElementA)
{
????std::cout?<<?"VisitConcreateElementB?By?ConcreateVisitorA\n";
}
void?ConcreateVisitorB::VisitConcreateElementA(ConcreateElementA?*pConcreateElementA)
{
????std::cout?<<?"VisitConcreateElementA?By?ConcreateVisitorB\n";
}
void?ConcreateVisitorB::VisitConcreateElementB(ConcreateElementB?*pConcreateElementA)
{
????std::cout?<<?"VisitConcreateElementB?By?ConcreateVisitorB\n";
}
3)Main.cpp
/**//********************************************************************
????created:????2006/08/09
????filename:?????Main.cpp
????author:????????李創(chuàng)
????????????????http://www.shnenglu.com/converse/
????purpose:????Visitor模式的測(cè)試代碼
*********************************************************************/
#include?"Visitor.h"
int?main()
{
????Visitor?*pVisitorA?=?new?ConcreateVisitorA();
????Element?*pElement??=?new?ConcreateElementA();
????pElement->Accept(*pVisitorA);
????delete?pElement;
????delete?pVisitorA;
????return?0;
}