访客模式+开放/封闭原则 [英] Visitor Pattern + Open/Closed Principle

问题描述

可以实施访问者模式，遵守打开/关闭原则，但是仍然能够添加新的可访问类？

Is it possible to implement the Visitor Pattern respecting the Open/Closed Principle, but still be able to add new visitable classes?

struct ConcreteVisitable1;
struct ConcreteVisitable2;

struct AbstractVisitor
{
   virtual void visit(ConcreteVisitable1& concrete1) = 0;
   virtual void visit(ConcreteVisitable2& concrete2) = 0;
};

struct AbstractVisitable
{
   virtual void accept(AbstractVisitor& visitor) = 0;
};

struct ConcreteVisitable1 : AbstractVisitable
{
   virtual void accept(AbstractVisitor& visitor)
   {
      visitor.visit(*this);
   }
};

struct ConcreteVisitable2 : AbstractVisitable
{
   virtual void accept(AbstractVisitor& visitor)
   {
      visitor.visit(*this);
   }
};

您可以实现派生自AbstractVisitor的任何数量的类：它是开放的扩展。您不能添加新的可访问类，因为派生自AbstractVisitor的类将不会编译：它关闭以进行修改。

You can implement any number of classes which derives from AbstractVisitor: It is open for extension. You cannot add a new visitable class as the classes derived from AbstractVisitor will not compile: It closed for modification.

AbstractVisitor类树遵循开放/封闭原则。
AbstractVisitable类树不遵守开放/封闭原则，因为它不能被扩展。

The AbstractVisitor class tree respects the Open/Closed Principle. The AbstractVisitable class tree does not respect the Open/Closed Principle, as it cannot be extended.

有没有任何其他解决方案，而不是扩展AbstractVisitor和AbstractVisitable如下？

Is there any other solution than to extend the AbstractVisitor and AbstractVisitable as below?

struct ConcreteVisitable3;

struct AbstractVisitor2 : AbstractVisitor
{
   virtual void visit(ConcreteVisitable3& concrete3) = 0;
};

struct AbstractVisitable2 : AbstractVisitable
{
   virtual void accept(AbstractVisitor2& visitor) = 0;
};

struct ConcreteVisitable3 : AbstractVisitable2
{
   virtual void accept(AbstractVisitor2& visitor)
   {
      visitor.visit(*this);
   }
};

推荐答案

在C ++中，环聊访客（pdf）可以为您提供您想要的东西。

In C++, Acyclic Visitor (pdf) gets you what you want.

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