在派生类中扩展枚举 [英] Extending enum in derived classes

问题描述

我有一个类层次结构，它的每个类都有一个异常类，在并行层次结构中派生，因此...

I have a class hierarchy, and each class in it has an exception class, derived in a parallel hierarchy, thus...

class Base
{
};

class Derived : public Base
{
};

class BaseException : public std::exception
{
   enum {THIS_REASON, THAT_REASON};
};

class DerivedException : public BaseException
{
    // er...what?
};

我想在DerivedException类中扩展枚举类型以包括一个新值THE_OTHER_REASON，所以DerivedException类可以保存这三个值中的任何一个。

I would like, in the DerivedException class, to extend the enumeration type to include a new value THE_OTHER_REASON, so that the DerivedException class could hold any of the three values.

首先，我应该这样做吗？它似乎是一个合理的做法吗？如果是这样，我该怎么办呢？如果没有，您会推荐什么替代方案？

First of all, ought I to want to do this? Does it seem a reasonable practice? If so, how do I go about it? If not, what alternatives would you recommend?

编辑：建议可能的重复项这里，但是建议的解决方案是不同的，因为这个问题是针对C＃和C ++。

A possible duplicate has been suggested here, but the suggested solutions are different because that question is for C# and this for C++.

推荐答案

从OO的角度来看，这是不合理的。因为你说 DerivedException 是-a BaseException ，其可能的原因必须是 > c> ，而不是超集。否则，您最终会违反 Liskov替换原则。

From the OO standpoint, it is not reasonable. Since you say that DerivedException is-a BaseException, its possible reasons must be a subset of that of BaseException, not a superset. Otherwise you ultimately break the Liskov Substitution Principle.

此外，因为C ++枚举不是类，所以不能扩展或继承它们。您可以在 DerivedException 中的单独枚举中定义其他原因，但最终会遇到上述相同的问题：

Moreover, since C++ enums are not classes, you can't extend or inherit them. You can define additional reasons in a separate enum within DerivedException, but then ultimately you bump into the same problem described above:

class DerivedException : public BaseException
{
  enum {
    SOME_OTHER_REASON = THAT_REASON + 256, // allow extensions in the base enum
    AND_ANOTHER_REASON
  };
  ...
};

...
try {
  ...
} catch (BaseException& ex) {
  if (ex.getReason() == BaseException::THIS_REASON)
    ...
  else if (ex.getReason() == BaseException::THAT_REASON)
    ...
  else if (ex.getReason() == ??? what to test for here ???)
    ...
}

$ b b

你可以做的是为每个不同的原因定义一个单独的异常子类。然后你可以处理它们多态（如果需要）。这是标准C ++库以及其他类库的方法。因此，您遵守约定，这使您的代码更容易理解。

What you can do instead is define a separate exception subclass for each distinct reason. Then you can handle them polymorphically (if needed). This is the approach of the standard C++ library as well as other class libraries. Thus you adhere to the conventions, which makes your code easier to understand.

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