Rust uses Result. Checked exceptions are Java's analogue of Result.

I disagree. Exceptions are a fundamentally different control flow. Exceptions are exceptions. Return values are not.

I would not call std::optional in C++ any form of checked exception, and the difference isn't that std::optional doesn't carry value-missing metadata.

So what do you do in C++ if the input to your function is not what you expect?

I think a language's standard library sets a good pattern for how to write idiomatic code in that language.

C++ throws on memory allocation error, but that's about it. Memory allocation errors are special in all languages. Because of lazy allocation and overcommit, unless you specifically set your environment to work otherwise, your program will probably just crash when it gets its first page fault that can't be honored.

Open a file? fstream sets .is_open() (or its operator bool, so just "if (!f)")

Write fails? Sets .fail()

POSIX stuff usually return an error code, and set errno.

Modern C++ has std::optional.

But there's of course another answer to this, and that is "C++ has zero cost abstractions", meaning for example if you don't check for nullptr, then neither will the language. There's no NullPointerException because C++ just says that this is Undefined Behavior.

Oh, here's one: If you use dynamic_cast to try to downcast into the wrong type, that'll throw an exception. But first of all: don't downcast, and second of all: This is not an "unexpected input to function". This is a complete programming error and it's probably best to terminate. I.e. this is something Go style would panic about, not return an error.

Do you have more specific examples about unexpected input to a function that you would want to return an error for?

Ugh. Actually std::stoi() violates this pattern. If std::optional existed in C++11 it would probably have been used here.

> Memory allocation errors are special in all languages.

Actually no, not in Java! You can catch java.lang.OutOfMemoryError just like any other exception. If the memory pressure is high though, it's possible that another OOM would be thrown from the code that handles it.

You can catch std::bad_alloc in C++, too. That's not my point. Especially because destructors free immediately (not wait for GC) I would expect C++ to handle this as a language much better than Java.

But when memory pressure is high you can get killed at any time. E.g. on Linux the OOM killer might decide that it's best for the system that your process dies, even if you've not done memory allocations or needed to page fault for hours.

IIRC OpenBSD doesn't overcommit memory, but in my experience its system stability is much worse when memory is low.

Return an error or fatally exit the program, depending.

If you return an error, it needs to be checked for in every place where this function is called. Yes, I know C libraries and OS APIs do this often, but that's C, it's the only thing it can do. This just invites human error. Besides, it's often desirable to handle multiple different error conditions (arising from different steps as you process the input data) in one place, which is complicated with this approach. Java-like exception handling makes it easy to handle all errors in a single place and be sure none go unnoticed. This is especially useful when you're accepting external input (a file, a network packet/stream, an HTTP request, etc).

If you exit the program on error, this does work in some cases like command-line utilities, but your users would not be happy if your GUI app crashes when you open a malformed file.

Entire books, I'm sure, have been written about the pro and cons of exceptions for error handling.

Yes, I called it "idiomatic C++" before, but reasonable people disagree about the best option.

Smarter people than me have written good things in the E section of the C++ Core Guidelines. The people involved have overlap with the C++ standards committee:

http://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#...

At least if you use exceptions for errors C++ doesn't need "finally", because it has working RAII, unlike Java.

In modern Java, and by modern I mean 8 and newer, you no longer need "finally", there's "try with resources" instead that would close everything upon leaving the try block:

    try(FileInputStream in=new FileInputStream(file)){
        // do something with the file data
    }catch(IOException x){
        x.printStackTrace();
    }

It's been a very long time since I last wrote "something.close()".

I'm not arguing for or against this approach, merely responding to a question with a factual answer.

But since you brought it up ;) I bounce back and forth between which approach I like. Sometimes exceptions seem bulky and unwieldy and honestly a bit lazy. Returning an error feels verbose and annoying and bulky as well. But it also feels like returning an error forces you to think about what should happen, where exceptions let you kick the can down the road.

Neither are great, but I find that code that uses exceptions ends up being poorer in design and functioning, but also errors-on-return tends to be harder to read.

This is fine, this is exception handling which you are describing. What exceptions should not be for is handling normal control flow. That is what branches are for. That is what the article is about and what people replying are complaining about.