通过继承扩展C ++标准库？ [英] Extending the C++ Standard Library by inheritance?

问题描述

这是一个普遍认为的C ++标准库通常不打算使用继承扩展。当然，我（和其他人）批评那些建议从类如 std :: vector 派生的人。不过，这个问题： c ++异常，可以what（）是NULL？我意识到，至少有一部分标准库的意图是这样扩展 - std :: exception 。

It is a commonly held belief that the the C++ Standard library is not generally intended to be extended using inheritance. Certainly, I (and others) have criticised people who suggest deriving from classes such as std::vector. However, this question: c++ exceptions, can what() be NULL? made me realise that there is at least one part of the Standard Library that is intended to be so extended - std::exception.

所以，我的问题有两部分：

So, my question has two parts:

1. 是否有任何其他标准库类从

1. Are there any other Standard Library classes which are intended to be derived from?

如果从标准库类派生，如 std :: exception 一个由ISO标准中描述的接口绑定？例如，使用 what（）成员函数的异常类的程序不返回NTBS（假设它返回一个空指针）是否符合标准？

If one does derive from a Standard Library class such as std::exception, is one bound by the interface described in the ISO Standard? For example, would a program which used an exception class who's what() member function did not return a NTBS (say it returned a null pointer) be standard conforming?

推荐答案

我真的希望标准对于预期的使用是什么更清楚一点。也许应该有一个C ++原理文档与语言标准一起。在任何情况下，这里是我使用的方法：

Good nice question. I really wish that the Standard was a little more explicit about what the intended usage is. Maybe there should be a C++ Rationale document that sits alongside the language standard. In any case, here is the approach that I use:

（a）我不知道任何这样的列表的存在。相反，我使用以下列表来确定标准库类型是否可能被设计为继承自：

(a) I'm not aware of the existence of any such list. Instead, I use the following list to determine whether a Standard Library type is likely to be designed to be inherited from:

• t有任何 virtual 方法，那么你不应该使用它作为基础。这排除了 std :: vector 等。


• 如果它有 virtual 方法，那么它是用作基类的候选。


• 如果有很多 friend


• 如果它是一个模板，那么在继承它之前要仔细一些，因为你可以使用专门化来定制它。


• 存在基于策略的机制（例如， std :: char_traits ）是一个很好的线索，你不应该使用它a base。

• If it doesn't have any virtual methods, then you shouldn't be using it as a base. This rules out std::vector and the like.
• If it does have virtual methods, then it is a candidate for usage as a base class.
• If there are lots of friend statements floating around, then steer clear since there is probably an encapsulation problem.
• If it is a template, then look closer before you inherit from it since you can probably customize it with specializations instead.
• The presence of policy-based mechanism (e.g., std::char_traits) is a pretty good clue that you shouldn't be using it as a base.

不幸的是，我不知道一个不错的全面或黑白名单。

Unfortunately I don't know of a nice comprehensive or black and white list. I usually go by gut feel.

（b）我会使用 LSP 。如果有人在你的异常上调用 what（），那么它的observable行为应该匹配 std :: exception 。我不认为这是一个标准一致性问题，而是一个正确性问题。该标准不要求子类可替代基类。这只是一个最佳实践。

(b) I would apply LSP here. If someone calls what() on your exception, then it's observable behavior should match that of std::exception. I don't think that it is really a standards conformance issue as much as a correctness issue. The Standard doesn't require that subclasses are substitutable for base classes. It is really just a "best practice".

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