Theory on error handling?

Question

Most advice concerning error handling boils down to a handful of tips and tricks (see this post for example). These hints are helpful but I think they don\'t answer all question

Answer 1

How to decide if an error should be handled locally or propagated to higher level code?

Error handling should be done at the highest affected level. If it only impacts the lower level code, then it should be handled there. If the error affects higher level code, then the error needs to be handled at the higher level. This is to prevent some higher level code from going on its merry way after an error has caused its actions to be incorrect. It should know what is going on, provided it is impacted.

How to decide between logging an error, or showing it as an error message to the user?

You should always log the error. You should show the error to the user when they are affected by it. If it is something they will never notice and does not have a direct impact (e.g. two sockets failed to open before the third finally opened, resulting in a very short delay for the user should not be reported), then they should not be notified.

Is logging something that should only be done in application code? Or is it ok to do some logging from library code.

Too much logging is rarely a bad thing. You will regret not logging things when you have to hunt down a library bug more than you will be frustrated by extra logs when hunting down other bugs.

In case of exceptions, where should you generally catch them? In low-level or higher level code?

Similar to error handling above, it should be caught where the impact is, and where the error can be corrected/handled effectively. This will vary from case to case.

Should you strive for a unified error handling strategy through all layers of code, or try to develop a system that can adapt itself to a variety of error handling strategies (in order to be able to deal with errors from 3rd party libraries).

This is largely a personal decision. My internal error handling is much different than the error handling I use for anything that touches a third party library. I have a general idea of what to expect from my code, but the third party stuff could have anything happen to it.

Does it make sense to create a list of error codes? Or is that old fashioned these days? Depends how much you expect to have errors thrown. You might love your list of error codes if you spend a lot of time bug hunting, as they can help point you in the right direction. However, any time spent building these is less time spent on coding/bug fixing, so its a mixed bag. This largely comes down to personal preference.