C++ exception class design [closed]

Question

What is a good design for a set of exception classes?

I see all sorts of stuff around about what exception classes should and shouldn't do, but not a simple design which is easy to use and extend that does those things.

1. The exception classes shouldn't throw exceptions, since this could lead straight to the termination of the process without any chance to log the error, etc.
2. It needs to be possible to get a user friendly string, preferable localised to their language, so that there's something to tell them before the application terminates itself if it can't recover from an error.
3. It needs to be possible to add information as the stack unwinds, for example, if an XML parser fails to parse an input stream, to be able to add that the source was from a file, or over the network, etc.
4. Exception handlers need easy access to the information they need to handle the exception.
5. Write formatted exception information to a log file (in English, so no translations here).

Getting 1 and 4 to work together is the biggest issue I'm having, since any formatting and file output methods could potentially fail.

EDIT: So having looked at exception classes in several classes, and also in the question Neil linked to, it seems to be common practice to just completely ignore item 1 (and thus the boost recommendations), which seems to be a rather bad idea to me.

Anyway, I thought I'd also post the exception class I'm thinking of using.

class Exception : public std::exception {     public:         // Enum for each exception type, which can also be used         // to determine the exception class, useful for logging         // or other localisation methods for generating a         // message of some sort.         enum ExceptionType         {             // Shouldn't ever be thrown             UNKNOWN_EXCEPTION = 0,              // The same as above, but it has a string that             // may provide some information             UNKNOWN_EXCEPTION_STR,              // For example, file not found             FILE_OPEN_ERROR,              // Lexical cast type error             TYPE_PARSE_ERROR,              // NOTE: in many cases functions only check and             //       throw this in debug             INVALID_ARG,              // An error occured while trying to parse             // data from a file             FILE_PARSE_ERROR,         }          virtual ExceptionType getExceptionType()const throw()         {             return UNKNOWN_EXCEPTION;         }          virtual const char* what()throw(){return "UNKNOWN_EXCEPTION";} };   class FileOpenError : public Exception {     public:         enum Reason         {             FILE_NOT_FOUND,             LOCKED,             DOES_NOT_EXIST,             ACCESS_DENIED         };         FileOpenError(Reason reason, const char *file, const char *dir)throw();         Reason getReason()const throw();         const char* getFile()const throw();         const char* getDir ()const throw();      private:         Reason reason;         static const unsigned FILE_LEN = 256;         static const unsigned DIR_LEN  = 256;         char file[FILE_LEN], dir[DIR_LEN]; }; 

Point 1 is addressed since all strings are handled by copying to an internal, fixed size buffer (truncating if needed, but always null terminated).

Although that doesn't address point 3, however I think that point is most likely of limited use in the real world anyway, and could most likely be addressed by throwing a new exception if needed.

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.