Should I inherit from std::exception?

Question

I\'ve seen at least one reliable source (a C++ class I took) recommend that application-specific exception classes in C++ should inherit from std::exception. I\

Answer 1

I once participated in the clean up of a large codebase where the previous authors had thrown ints, HRESULTS, std::string, char*, random classes... different stuff everywhere; just name a type and it was probably thrown somewhere. And no common base class at all. Believe me, things were much tidier once we got to the point that all the thrown types had a common base we could catch and know nothing was going to get past. So please do yourself (and those who'll have to maintain your code in future) a favor and do it that way from the start.

Answer 2

Whether to derive from any standard exception type or not is the first question. Doing so enables a single exception handler for all standard library exceptions and your own, but it also encourages such catch-them-all handlers. The problem is that one should only catch exceptions one knows how to handle. In main(), for example, catching all std::exceptions is likely a good thing if the what() string will be logged as a last resort before exiting. Elsewhere, however, it's unlikely to be a good idea.

Once you've decided whether to derive from a standard exception type or not, then the question is which should be the base. If your application doesn't need i18n, you might think that formatting a message at the call site is every bit as good as saving information and generating the message at the call site. The problem is that the formatted message may not be needed. Better to use a lazy message generation scheme -- perhaps with preallocated memory. Then, if the message is needed, it will be generated on access (and, possibly, cached in the exception object). Thus, if the message is generated when thrown, then a std::exception derivate, like std::runtime_error is needed as the base class. If the message is generated lazily, then std::exception is the appropriate base.