避免并行继承层次结构 [英] Avoiding parallel inheritance hierarchies

问题描述

我有两个平行的继承链：

I have two parallel inheritance chains:

Vehicle <- Car
        <- Truck <- etc.

VehicleXMLFormatter <- CarXMLFormatter
                    <- TruckXMLFormatter <- etc.

我的经验是，并行继承层次结构随着它们的增长而成为一种维护问题。

My experience has been that parallel inheritance hierarchies can become a maintenance headache as they grow.

ie不要将 toXML（），toSoap（），toYAML（）方法添加到我的主要类中。

i.e. NOT adding toXML(), toSoap(), toYAML() methods to my principal classes.

我如何避免并行继承层次结构而不破坏关注点分离的概念？

How do I avoid a parallel inheritance hierarchy without breaking the concept of separation of concerns?

推荐答案

我正在考虑使用访问者模式。

I am thinking of using the Visitor pattern.

public class Car : Vehicle
{
   public void Accept( IVehicleFormatter v )
   {
       v.Visit (this);
   }
}

public class Truck : Vehicle
{
   public void Accept( IVehicleFormatter v )
   {
       v.Visit (this);
   }
}

public interface IVehicleFormatter
{
   public void Visit( Car c );
   public void Visit( Truck t );
}

public class VehicleXmlFormatter : IVehicleFormatter
{
}

public class VehicleSoapFormatter : IVehicleFormatter
{
}

这样，您可以避免使用额外的继承树，并将格式化逻辑与您的车辆分开 - 类。
Offcourse，当你创建一个新的车辆时，你将不得不为Formatter接口添加另一个方法（并在formatter接口的所有实现中实现这个新方法）。

但是，我认为这比创建一个新的Vehicle类更好，并且对于你拥有的每个IVehicleFormatter，创建一个可以处理这种新型车辆的新类。

With this, you avoid an extra inheritance tree, and keep the formatting logic separated from your Vehicle-classes. Offcourse, when you create a new vehicle, you'll have to add another method to the Formatter interface (and implement this new method in all the implementations of the formatter interface).
But, I think that this is better then creating a new Vehicle class, and for every IVehicleFormatter you have, create a new class that can handle this new kind of vehicle.

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