Comment by fluoridation
1 day ago
You're using "contract" in a different sense than I did. When I said "contract" I was referring to the required state of the program when the function is called and the guaranteed state of the program when the function returns. By definition an exception cannot be part of the contract in this sense, because a call that throws does not return. This narrower sense of contract is critical, because the entire point of exceptions is to enable alternate control path when it'd otherwise be impossible, such as in the examples I gave above with overloaded operators and code with evolving requirements.
Okay, but if I may offer a suggestion: in the future, I would probably phrase what you said differently. I think it caused you trouble here not just with me but with other readers. Instead of "when the function cannot fulfill its contract", I would talk about "when returning would not fulfill a function's contract."
There are actually two reasons for this, not one:
- It violates standard terminology. The notion of a contract/postcondition/etc. is not specific to C++ or the particular implementation's mechanisms for exiting a function. It simply means a condition that must hold true after the execution of some piece of code. [1] The intent of this definition is to allow program composition: it enables one to reason about the greater program in terms of the sub-parts. Defining it to be anything else just throws people off, and rather misses the point and utility of the term.
- "Cannot" is actually too strong. A function might, in fact, be able to fulfill its contract, but still choose not to. An easy example is something like a constraint solver (SAT, chess, simulator, constexpr evaluator, or whatever). It's guaranteed to be able to find the solution eventually if it keeps going, but that's probably not always a good idea.
Now, going back to what you wrote here:
> Exceptions aren't meant to report errors, just in general. That's a misuse of them. Exceptions are meant to be thrown when a contract cannot be fulfilled.
I'm still not entirely sure I see what you mean by "report errors". How exactly have you seen people use exceptions to "report errors" that is not for the purpose of indicating that "a contract cannot be fulfilled"? The description makes it sound like using exceptions for the purpose of logging, but that would seem like a strawman... I have never seen anyone write throw instead of log. What are you referring to?
[1] https://en.wikipedia.org/wiki/Postcondition
>How exactly have you seen people use exceptions to "report errors" that is not for the purpose of indicating that "a contract cannot be fulfilled"?
Reporting normal errors to the caller, as opposed to exceptional errors. The distinction between normal errors and exceptional errors is kind of nebulous, but it boils down to normal errors being those that the immediate caller would be interested in, and everything else is exceptional.
For example, you have a file system class that abstracts away different underlying hardware interfaces to present to the client code a virtual file system. Not only would it be impractical for the open() function to include as part of its interface every possible error condition of all the possible backends, it would be of no help to the caller. If you're manipulating an AbstractFileSystem class just to open a path and read data, what could you possibly do with a connection reset error, or with a file system structure corruption error? Do you see what I mean? They're errors happening at different levels of abstraction. Exceptions are meant to be used to communicate error conditions when your call stack looks like
(low abstraction) ---> (high abstraction) ---> (high abstraction) ---> (low abstraction)
often with a module boundary between high abstraction stack frames. You use them to pass errors directly between frames at the same level of abstraction, that are not in direct communication.
So to answer your question, exceptions are misused when they communicate relevant errors within the same level of abstraction, such as an open() function throwing a FileNotFoundException.