I have a class that can perform many types of transformations to a given object.
All data is supplied by the users (via hand crafted command files) but for the most part there is no way to tell if the file commands are valid until we start transforming it (if we checked everything first we’d end up doing the same amount of work as the transformations themselves)
These transformation methods are called on the object and it returns a newly transformed object however if there is a problem we throw an exception since returning an object (even a blank object) could be confused with success whereas an exception sends a clear signal there was a problem and an object can not be returned. It also means we don’t need to call a “get last error” type function to find out exactly what went wrong (error code, message, statement, etc.)
However from reading numerous answers on SO it seems this is incorrect since invalid user input is not an exceptional circumstance and due to the complexity of this thing its not uncommon for the user to submit incorrect command files.
Performance is not really an issue here because if we hit an error we have to stop and go back to the user.
If I use return codes I’d have to take the output object as a parameter and then to check the return codes I’d need long nested if blocks (ie. the way you check HRESULT from COM)
How would I go about avoiding exceptions in this case while still keeping the code reasonably tidy?
The design of your problem really lends itself for exceptions. The fact that program execution will halt or be invalid once the user supplies a bad input file, is a sign of “exceptional circumstance”. Sure, a lot of program runs will end in an exception being thrown (and caught), but this is in one execution of the program.
The things you are reading about exceptions being slow when used for every other circumstance, is only valid if the program can recover, and has to recover often (e.g. a compiler that fails to find a header in a search directory, and has to look at the next directory in the list of search directories, which really happens a lot).
Use exceptions. When in doubt, measure if this is killing performance. Cleaner code >> following what people say on SO. But then again, that would be a reason to ignore what I just said.