Re: Error Handling in C++

 James Kanze <>
Thu, 09 Aug 2007 13:43:45 -0000
On Aug 9, 3:32 am, "Axel Gallus" <> wrote:

I am desperately looking for some resources dealing with error handling in

Don't get me wrong.... I have very detailed knowledge of exceptions,
assertions and invariants,
but what is missing it the following:

    When developing a commercial application, how does the
    usage of exceptions and assertions as well as other
    mechanisms look like in the development lifecycle and in
    the end-product? E.g. do the developer only use
    assertions in debug phase? When do they use assertions
    when exceptions. Do they use exceptions for such daily
    things as to report if some functions parameters weren't
    delivered right? Moreover if performance is crucial, how
    can that shift the usage of exceptions, assertions and
    other mechanisms.

Typically, any reasonably sized application will use a
combination of mechanisms to report errors. Assertions,
exceptions and return codes serve different purposes, and will
all be used, each according to what is needed. In general:

 -- If the error represents a programming error, or a can't
    possibly occur error, it's an assertion failure (and
    obviously, assertions stay in, even in delivered code). If
    the internal state of the program is inconsistent (and if
    you arrive in an "impossible" state, it is), then the only
    reasonable thing to do is to abort.

 -- There are some errors which can't be recovered from. These
    depend on the application, of course, but for a lot of
    applications, out of memory would be an example. And for
    most applications, so would errors during start-up and
    initialization---it's usually (but not always) better to
    stop immediately, for example, if there is an error in a
    configuration file. In such cases, an error message and
    exit is the most appropriate behavior. (Not an abort,
    because there's nothing wrong with the program.)

 -- There are errors which, while not requiring that the entire
    process be stopped, do mean that the operation (transaction)
    currently in progress be aborted. These are best handled by
    exceptions, which propagate the error over many layers of
    function calls in a more or less painless fashion.

 -- There are more or less everyday errors, which you really
    expect (e.g. cannot open file, with a user supplied
    filename, or a syntax error in user supplied input). In
    many cases, they can be handled more or less locally, and
    will occur more or less frequently; they are best handled by
    return codes.

 -- There are also errors which can occur almost everywhere
    during processing: write error to a file, for example. If
    these can be made "sticky", the easiest (and often
    acceptable) solution is just to ignore them, except that the
    level where they were detected, and only test them at the
    end of a complete set of operations. Thus, for example,
    when writing to an ofstream, you'll usually ignore the
    possibility of an error until after the close(), and only
    test then.

Note that the same type of error may have different
implications, and call for a different treatment, depending on
the application, or where it occurs in the application. If you
are opening a file whose name was given by interactive input,
and the open fails, you return an error, and the calling code
asks the user for another name; if you are opening a
configuration file whose name was obtained from a command line
argument, you probably don't want the program to go any further.
In such cases, where you cannot decide at the level where the
error is detected, it's probably best to handle it in the least
critical manner reasonable (i.e. as the lowest point in the
above); it's easier to convert a return code into an exception
than vice versa, and of course, if you terminate the program,
there's no way to convert that into anything less drastic.

    How does the logic of a more sophisticated error-handler
    class system look like? E.g. how looks a system which
    capabilities to identify and reporting the problem to
    other subroutines/user, recovering from error and
    correcting the problem?

Because the requirements are so varied, solutions tend to be ad
hoc. Most applications will define a global inheritance
hierarchy (based on std::exception), and most will define a
return code type (which triggers an assertion failure if
destructed without being read), but that's about it.

    How does a error handler system which should be prepared
    for mutilanguage support as well as concurrency (threads)
    look like?

For the most part, errors that don't terminate the program will
be handled in the thread where they occur. At least in the
types of applications I've written; a lot depends on why you are
using threads to begin with.

James Kanze (GABI Software)
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