Extending the C++ Standard Library by inheritance?

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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) 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:

  • 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.

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

(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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