作用:
表示一個作用于某對象結(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模式把對結(jié)點的訪問封裝成一個抽象基類,通過派生出不同的類生成新的訪問方式.在實現(xiàn)的時候,在visitor抽象基類中聲明了對所有不同結(jié)點進行訪問的接口函數(shù),如圖中的VisitConcreateElementA函數(shù)等,這樣也造成了Visitor模式的一個缺陷--新加入一個結(jié)點的時候都要添加Visitor中的對其進行訪問接口函數(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模式一個缺點就是添加新的結(jié)點十分困難.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,要對某一個結(jié)點進行訪問,首先需要產(chǎn)生一個Element的派生類對象,其次要傳入一個Visitor類派生類對象來調(diào)用對應(yīng)的Accept函數(shù),也就是說,到底對哪種Element采用哪種Visitor訪問,需要兩次動態(tài)綁定才可以確定下來,具體的實現(xiàn)可以參考下面實現(xiàn)代碼中的Main.cpp部分是如何調(diào)用這些類的.
實現(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;
}