问题
I'm studying Visitor Pattern advantages, and quoting Design Patterns:
But an iteratorcan't work across object structures with different types of elements. Forexample, the Iterator interface defined on page 295 can access only objects of type Item:
template <class Item>
clas Iterator { // ... Item CurrentItem() const; };
This implies that all elements the iterator can visit have a common parentclass Item. Visitor does not have this restriction...
class Visitor {
public:
// ...
void VisitMyType(MyType*);
void VisitYourType(YourType*);
};
MyType and YourType do not have to be related throughinheritance at all.
I agree about this quote, but I can't figure out an example where the Visitor Pattern could explore a structure (like a List) where objects collected in it are not related by a super class.
In other words, can you show me an example where the features above is true please?
回答1:
First, you should know what these patterns are for.
The Iterator Pattern is used to access an aggregate sequentially without exposing its underlying representation. So you could Hide a List or array or similar aggregates behind an Iterator.
Visitor Pattern is used to perform an action on a structure of elements without changing the implementation of the elements themselves.
So you use the patterns in two different situations and not as alternatives to each other.
In the Visitor Pattern you implement an Interface IAcceptor in each element you want to visit. So the Visitor Pattern doesn't rely on a superclass but on Interfaces
public interface IAcceptor
{
public void Accept(IVisitor visitor);
}
So if you have a List of objects you can iterate over it and visit the objects implementing IAcceptor
public VisitorExample()
{
MyVisitorImplementation visitor = new MyVisitorImplementation();
List<object> objects = GetList();
foreach(IAcceptor item in objects)
item.Accept(visitor);
}
public interface IVisitor
{
public void Visit(MyAcceptorImplementation item);
public void Visit(AnotherAcceptorImplementation item);
}
public class MyAcceptorImplementation : IAcceptor
{
//Some Code ...
public void Accept(IVisitor visitor)
{
visitor.Visit(this);
}
}
To complete the code here is Visitor to write to Console if it visits my or another implementation of an acceptor.
public class MyVisitorImplementation : IVisitor
{
public void Visit(MyAcceptorImplementation item)
{
Console.WriteLine("Mine");
}
public void Visit(AnotherAcceptorImplementation item)
{
Console.WriteLine("Another");
}
}
For more useful examples and better explanation have a look at Visitor Pattern and Iterator Pattern
EDIT: Here an example using both, the visitor and Iterator. The iterator is just the logic how to move through your aggregate. It would make more sense with a hierarchical structure.
public VisitorExample2()
{
MyVisitorImplementation visitor = new MyVisitorImplementation();
List<object> myListToHide = GetList();
//Here you hide that the aggregate is a List<object>
ConcreteIterator i = new ConcreteIterator(myListToHide);
IAcceptor item = i.First();
while(item != null)
{
item.Accept(visitor);
item = i.Next();
}
//... do something with the result
}
回答2:
There are two good examples I know of where visitor is clearly preferable to iterator.
The first is interacting with some unknown set of class members, in particular in C++. For example, here's a visitor that prints out all the members of other classes. Imagine you're the author of Printer and someone you're unacquainted with is the author of Heterogeneous3Tuple.
#include <iostream>
template<class ElemType1, class ElemType2, class ElemType3>
class Heterogeneous3Tuple
{
public:
Heterogeneous3Tuple(ElemType1 elem1, ElemType2 elem2, ElemType3 elem3)
: elem1_(std::move(elem1)), elem2_(std::move(elem2)), elem3_(std::move(elem3))
{}
template<class Visitor>
void accept(const Visitor& visitor)
{
visitor(elem1_);
visitor(elem2_);
visitor(elem3_);
}
private:
ElemType1 elem1_;
ElemType2 elem2_;
ElemType3 elem3_;
};
class Printer
{
public:
template<class VisitedElemType>
void operator()(const VisitedElemType& visitee) const
{
std::cout << visitee << std::endl;
}
private:
};
int main() {
Heterogeneous3Tuple<char, int, double> h3t('a', 0, 3.14);
Printer p;
h3t.accept(p);
}
a
0
3.14
coliru
There's no sensible way to get an iterator to work here. Without even knowing what types our Printer class might interact with this works so long as the visitor is accept()ed and the elements all interact in a similar way with operator << and a stream.
The other good example I know of shows up in abstract syntax tree manipulations. CPython and LLVM both use visitors. Using a visitor here prevents code that manipulates certain AST nodes from needing to know how to iterate over all the various AST nodes that might branch in complicated ways. The LLVM source code goes into more detail. Here's the highlight:
/// Instruction visitors are used when you want to perform different actions
/// for different kinds of instructions without having to use lots of casts
/// and a big switch statement (in your code, that is).
///
/// To define your own visitor, inherit from this class, specifying your
/// new type for the 'SubClass' template parameter, and "override" visitXXX
/// functions in your class. I say "override" because this class is defined
/// in terms of statically resolved overloading, not virtual functions.
///
/// For example, here is a visitor that counts the number of malloc
/// instructions processed:
///
/// /// Declare the class. Note that we derive from InstVisitor instantiated
/// /// with _our new subclasses_ type.
/// ///
/// struct CountAllocaVisitor : public InstVisitor<CountAllocaVisitor> {
/// unsigned Count;
/// CountAllocaVisitor() : Count(0) {}
///
/// void visitAllocaInst(AllocaInst &AI) { ++Count; }
/// };
///
/// And this class would be used like this:
/// CountAllocaVisitor CAV;
/// CAV.visit(function);
/// NumAllocas = CAV.Count;
来源：https://stackoverflow.com/questions/28319129/visitor-pattern-vs-iterator-pattern-visiting-across-hierarchy-class