C++ exception class design

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(Closed). This question is opinion-based. It is not currently accepting answers. question from:https://stackoverflow.com/questions/1335561/c-exception-class-design

Answer 1

Use a shallow hierarchy of exception classes. Making the hierarchy too deep adds more complexity than value.

Derive your exception classes from std::exception (or one of the other standard exceptions like std::runtime_error). This allows generic exception handlers at the top level to deal with any exceptions you don't. For example, there might be an exception handler that logs errors.

If this is for a particular library or module, you might want a base specific to your module (still derived from one of the standard exception classes). Callers might decide to catch anything from your module this way.

I wouldn't make too many exception classes. You can pack a lot of detail about the exception into the class, so you don't necessarily need to make a unique exception class for each kind of error. On the other hand, you do want unique classes for errors you expect to handle. If you're making a parser, you might have a single syntax_error exception with members that describe the details of the problem rather than a bunch of specialty ones for different types of syntax errors.

The strings in the exceptions are there for debugging. You shouldn't use them in the user interface. You want to keep UI and logic as separate as possible, to enable things like translation to other languages.

Your exception classes can have extra fields with details about the problem. For example, a syntax_error exception could have the source file name, line number, etc. As much as possible, stick to basic types for these fields to reduce the chance of constructing or copying the exception to trigger another exception. For example, if you have to store a file name in the exception, you might want a plain character array of fixed length, rather than a std::string. Typical implementations of std::exception dynamically allocate the reason string using malloc. If the malloc fails, they will sacrifice the reason string rather than throw a nested exception or crashing.

Exceptions in C++ should be for "exceptional" conditions. So the parsing examples might not be good ones. A syntax error encountered while parsing a file might not be special enough to warrant being handled by exceptions. I'd say something is exceptional if the program probably cannot continue unless the condition is explicitly handled. Thus, most memory allocation failures are exceptional, but bad input from a user probably isn't.