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Re: Error codes vs. exceptions

Cross-posted to 2 groups.

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On 29.05.2012 06:15, mike3 wrote:
> I've heard about this, and wonder when is it right to use codes, and
> when to use exceptions for reporting errors? I've heard various stuff,
> such as that exceptions should only be used to indicate "exceptional"
> conditions. Yet what does that mean?

Finally it is up to the programmer to take the decision.
But there are some performance aspects as well as some considerations 
about exception safe code.

Depending on your language, throwing an exception could be an expensive 
task. C++ is yet mostly harmless, but managed languages like Java or 
*.NET create an expensive stack trace at the time of the exception 
construction or at throw respectively. So it is in general a bad advise 
to throw and catch exceptions over and over in loops.

Furthermore exceptions are an unexpected way of changing the execution 
flow. While this is intended in most cases there are some pitfalls. Most 
code is not fully exception safe. I.e. is shows undefined behavior if 
the exception occur at the evaluation of certain (sub-)expressions. 
Writing fully exception safe code can be almost as complicated than 
writing thread-safe code.
So in fact you should know which of your functions throw exceptions and 
which don't. At the end, the advantage of the exceptions, easy code, 
might no longer be that large.


> I've heard that, e.g. a user
> inputting invalid input should not be considered "exceptional", but
> something like running out of memory should be.

Well, I dislike general statements like this.

If a wrong user input causes the normal execution flow to be interrupted 
at a certain point, a exception might be quite OK. If the execution 
continues with the next input box that might be also become red, 
exceptions might not hit the nail on the head. You remember, no catch in 
a loop (over the controls).


> Also, how exactly does one go about determining what is and is not
> "exceptional"?

You can't determine this. You have to /define/ this.
What causes your program to abandon normal operation (for a while)?


> non-exceptional, but what about something else, like say in a game,
> where you have a grid representing a game level, and a request for a
> tile of the level is made with a coordinate that is off the map (like
> a 64x64 map and something requests a tile at (100, 100).). Would it be
> OK for the function working on the tile to throw?

Does this abort normal program flow?
What is the proper action to be taken next?

> Or should it give an
> "out of range" error code? And as for that mixing: consider, e.g. C++
> and probably many other languages: a function has a single definite
> return type. Suppose our grid had a function that extracts an
> attribute from a cell. What to do when there's an out-of-bounds
> request? Throw exception?

Unless your semantics have some default value which could be used to 
continue processing, you won't come around to abort the execution flow 
here. But it could be a good advice to check this before you call any 
function that will not work properly. I.e. you could turn the array 
bounds condition into a precondition which could be checked by an assertion.


> But what to do? Make every
> function return an error code, using pointers to output variables to
> store output, and only use exceptions for a rare few kinds of "system-
> related" error?

Don't do that. Use exception where they make the code easier to read and 
easier to understand. And if you run into performance problems, then 
look for a work around. But in C++ this is unlikely as long as you do 
not abuse them extensively.
A prominent exception to this rule are APIs that can cross language 
boundaries. In this case you mostly have no chance. You need error codes.

Another rule of thumb: if throw and catch are close together with 
respect to the call stack it is more likely that an error code fit your 
needs. If they are well separated, then exceptions might be preferable.


[...]
> Would that be OK or excessive use of exceptions? And if we
> are to mix error codes and exceptions, does this mean we should have
> the lists of codes and exceptions correspond + a translator to
> translate between the two?

Converting an error code into an exception might be a good idea. But the 
other way around is most likely not.

And another rule of thumb: the number of exceptions thrown during the 
processing of a complete request should be finite. I.e. it must not 
scale with the amount of data processed.


Marcel

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Error codes vs. exceptions mike3 <mike4ty4@yahoo.com> - 2012-05-28 21:15 -0700
  Re: Error codes vs. exceptions BGB <cr88192@hotmail.com> - 2012-05-29 00:10 -0500
    Re: Error codes vs. exceptions mike3 <mike4ty4@yahoo.com> - 2012-05-28 22:41 -0700
      Re: Error codes vs. exceptions BGB <cr88192@hotmail.com> - 2012-05-29 01:34 -0500
    Re: Error codes vs. exceptions Adam Skutt <askutt@gmail.com> - 2012-05-29 07:16 -0700
      Re: Error codes vs. exceptions BGB <cr88192@hotmail.com> - 2012-05-29 09:45 -0500
  Re: Error codes vs. exceptions Marcel Müller <news.5.maazl@spamgourmet.com> - 2012-05-29 09:08 +0200
    Re: Error codes vs. exceptions mike3 <mike4ty4@yahoo.com> - 2012-05-31 13:52 -0700
      Re: Error codes vs. exceptions Ian Collins <ian-news@hotmail.com> - 2012-06-01 09:08 +1200
        Re: Error codes vs. exceptions mike3 <mike4ty4@yahoo.com> - 2012-05-31 22:32 -0700
  Re: Error codes vs. exceptions "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> - 2012-05-29 09:37 +0200
  Re: Error codes vs. exceptions "io_x" <a@b.c.invalid> - 2012-05-29 10:00 +0200
  Re: Error codes vs. exceptions Jorgen Grahn <grahn+nntp@snipabacken.se> - 2012-05-29 12:24 +0000
  Re: Error codes vs. exceptions Nobody <nobody@nowhere.com> - 2012-05-30 15:23 +0100
    Re: Error codes vs. exceptions BGB <cr88192@hotmail.com> - 2012-05-30 09:40 -0500
      Re: Error codes vs. exceptions Adam Skutt <askutt@gmail.com> - 2012-05-30 08:04 -0700
        Re: Error codes vs. exceptions BGB <cr88192@hotmail.com> - 2012-05-30 12:01 -0500
  Re: Error codes vs. exceptions Jeff Flinn <TriumphSprint2000@hotmail.com> - 2012-05-30 14:18 -0400
    Re: Error codes vs. exceptions Patricia Shanahan <pats@acm.org> - 2012-05-30 13:00 -0700
    Re: Error codes vs. exceptions Adam Skutt <askutt@gmail.com> - 2012-05-30 21:25 -0700
  Re: Error codes vs. exceptions Pavel <pauldontspamtolk@removeyourself.dontspam.yahoo> - 2012-05-30 22:44 -0400
    Re: Error codes vs. exceptions Nobody <nobody@nowhere.com> - 2012-05-31 05:04 +0100
    Re: Error codes vs. exceptions Pavel <pauldontspamtolk@removeyourself.dontspam.yahoo> - 2012-06-01 21:44 -0400
  Re: Error codes vs. exceptions "Alf P. Steinbach" <alf.p.steinbach+usenet@gmail.com> - 2012-06-02 08:39 +0200
    Re: Error codes vs. exceptions BGB <cr88192@hotmail.com> - 2012-06-02 16:31 -0500
    Re: Error codes vs. exceptions mike3 <mike4ty4@yahoo.com> - 2012-06-02 15:49 -0700
      Re: Error codes vs. exceptions Ian Collins <ian-news@hotmail.com> - 2012-06-03 11:17 +1200
      Re: Error codes vs. exceptions Rui Maciel <rui.maciel@gmail.com> - 2012-06-03 16:38 +0100
    Re: Error codes vs. exceptions Pavel <pauldontspamtolk@removeyourself.dontspam.yahoo> - 2012-06-03 20:26 -0400
  Re: Error codes vs. exceptions gremnebulin <peterdjones@yahoo.com> - 2012-06-20 12:05 -0700

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