Visitor in C++
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Watch Visitor's video tutorialVisitor design pattern
- Add an
accept(Visitor)method to the "element" hierarchy - Create a "visitor" base class w/ a
visit()method for every "element" type - Create a "visitor" derived class for each "operation" to do on "elements"
- Client creates "visitor" objects and passes each to
accept()calls
#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 thiss()
{
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->thiss() << '\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->thiss() << '\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);
}
do Up on This do Down on This
do Up on That do Down on That
do Up on TheOther do Down on TheOther
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This work is licensed under a Creative Commons Attribution-Noncommercial-No Derivative Works 3.0 Unported License |