C++ exception class design [closed]

Answer 1

2: No you should not mix user interface (=localized messages) with program logic. Communication to the user should be done at an outer level when the application realises that it cannot handle the issue. Most of the information in an exception is too much of an implementation detail to show a user anyway.
3: Use boost.exception for this
5: No dont do this. See 2. The decision to log should always be at the error handling site.

Dont use only one type of exception. Use enough types so the application can use a separate catch handler for each type of error recovery needed

Answer 2

Use virtual inheritance. This insight is due to Andrew Koenig. Using virtual inheritance from your exception's base class(es) prevents ambiguity problems at the catch-site in case someone throws an exception derived from multiple bases which have a base class in common.

Other equally useful advice on the boost site

Answer 3

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.

Answer 4

Not directly related to the design of an exception class hierarchy, but important (and related to using those exceptions) is that you should generally throw by value and catch by reference.

This avoids problems related to managing the memory of the thrown exception (if you threw pointers) and with the potential for object slicing (if you catch exceptions by value).

Answer 5

Since std::nested_exception and std::throw_with_nested have become available with C++11, I would like to point to answers on StackOverflow here and here

Those answers describe how you can get a backtrace on your exceptions inside your code without need for a debugger or cumbersome logging, by simply writing a proper exception handler which will rethrow nested exceptions.

The exception design there, in my opinion, also suggests to not create exception class hierarchies, but to only create a single exception class per library (as already pointed out in an answer to this question).

Answer 6

A good design is not to create a set of exception classes - just create one per library, based on std::exception.

Adding information is fairly easy:

try {
  ...
}
catch( const MyEx & ex ) {
   throw MyEx( ex.what() + " more local info here" );
}

And exception handlers have the information they need because they are exception handlers - only the functions in the try blocks can cause exceptions, so the handlers only need to consider those errors. And not you should not really be using exceptions for general error handling.

Basically, exceptions should be as simple as possible - a bit like logfiles, which which they should have no direct connection.

This has been asked before, I think, but I can't find it right now.