Exceptional wisdom

In Visual C++ 7.1 and earlier, “catch(…)” would catch all exceptions, both C++ and SEH. The behavior has changed with Visual C++ 8.0. This has caused some confusion.

The details get a bit tricky, but the generally accepted wisdom among the C++ gurus that have advised me is:

  1. try/catch is for C++ exceptions.

    • Corollary: Don’t catch C++ exceptions with __except.

  2. __try/__except is for Windows Structured Exception Handling (SEH) exceptions.

    • Corollary: Don’t catch SEH exceptions with catch.

    • Corollary: Don’t use _set_se_translator. Use __except, then throw your C++ exception from the __except block.

    • Corollary: Don’t use /EHa. The only reason to use /EHa is to catch SEH exceptions with catch or via _set_se_translator, which you shouldn’t be doing in the first place.

  3. Don’t use both C++ exception handling and SEH exception handling in the same function.

    • Creating an instance of an object that has a destructor implicitly generates a try/catch block, so don’t create destructable objects in a function that uses __try/__except or might directly raise an SEH exception.

  4. Don’t catch anything you don’t know how to handle.

  5. You don’t know how to handle Access Violations.

    • Corollary: Don’t catch Access Violations. Ever.

  6. It was always kind of weird (many would say horrible and bad) that the C++ catch would sometimes catch SEH exceptions. This was never dependable. Don’t rely on this behavior.

  7. It is a compiler implementation detail that __except can catch C++ exceptions. Don’t rely on this behavior.

  8. Visual C++ 8.0 changed the semantics of “catch(…)”.

    • catch(…) no longer catches all SEH exceptions. This is a good thing (see rules 4 and 5).

    • If you were following the above rules, you didn’t see any changes in your program’s behavior.

Comments (2)

  1. Rick Wheeler says:

    This rule set is way to simple minded. There are real occasions in mission critical apps where SEH must be used (including access violations) to maximize operations even amidst failures. And try/catch offers decent programmatic structure for the business logic of such apps. The _set_se_translator (with some other work) provides one way of trying to bridge the gap. A more consolidated and sanctioned approach would be welcomed, but be assured… both are needed in a cooperative framework.

  2. krishnan.selvaraj says:

    HI ,

    Is there a way to catch SEH? I get a  SEH in  a function where I use CStirng and so it does it let me use  __try and __catch.