Conventions for exceptions or error codes

In high-level stuff, exceptions; in low-level stuff, error codes.

The default behaviour of an exception is to unwind the stack and stop the program, if I'm writing a script an and I go for a key that's not in a dictionary it's probably an error, and I want the program to halt and let me know all about that.

If, however, I'm writing a piece of code which I must know the behaviour of in every possible situation, then I want error codes. Otherwise I have to know every exception that can be thrown by every line in my function to know what it will do (Read The Exception That Grounded an Airline to get an idea of how tricky this is). It's tedious and hard to write code that reacts appropriately to every situation (including the unhappy ones), but that's because writing error-free code is tedious and hard, not because you're passing error codes.

Both Raymond Chen and Joel have made some eloquent arguments against using exceptions for everything.

I normally prefer exceptions, because they have more contextual information and can convey (when properly used) the error to the programmer in a clearer fashion.

On the other hand, error codes are more lightweight than exceptions but are harder to maintain. Error checking can inadvertently be omitted. Error codes are harder to maintain because you have to keep a catalog with all error codes and then switch on the result to see what error was thrown. Error ranges can be of help here, because if the only thing we are interested in is if we are in the presence of an error or not, it is simpler to check (e.g., an HRESULT error code greater or equal to 0 is success and less than zero is failure). They can inadvertently be omitted because there is no programmatic forcing that the developer will check for error codes. On the other hand, you cannot ignore exceptions.

To summarize I prefer exceptions over error codes in almost all situations.