C++: Yet Another Kind of error handling (Part 1)

Self document your code and let users decide how to handle error

Saturday 11 June 2016

I have been a C++ developer for a little while, I have seen different methods to provide meaningful API:

  • the C-like: using error code and returning output in the parameter;
  • the exception-like: using exceptions.

The difficulties are to:

  • guarantee that the error handling is documented;
  • ensure it does not change (requires a lot of regression tests);
  • keep undefined behaviours as "exceptional".

That's it: exceptions are for exceptional situations. And again, it is not easy to provide up to date documentation of the kind of exceptions your function may throw.

So, let's try something else. Let's see what other languages have done to try to solve the issue of non self documented APIs.


Certainly my favourite language. In Haskell there are different methods to return error information to the user. The simplest is Either or Maybe.

-- define an error type
data Error = IOError
           | UTF8Error

-- just provide the Function signature to show you how
-- self documented it is
readFile :: FilePath -> Either Error Text

So the function is named: readFile, it takes a FilePath as parameter and it returns either an Error or a Text.

This design provides you with some information about the kind of error handled by this function. I don't say this function won't throw an exception: but the signature of the function tells you what you can expect.

Now, you know how you can use it:

If you want to interrupt your program now:

main = do
   case readFile "document.txt" of
      Left err -> error $ "error in readFile: " ++ show err
      Right txt -> printLn txt

If you want to fail quietly

main = do
      (return ())
      (readFile "document.txt")


The latest language I have been looking into. Full of promise for the System Developers.

The error management is also very simple and self documented: Result.

enum Error {

fn readfile(f: Path) -> Result<Error, String>;

And now you can, the same way you would do in Haskell:

throw the error

// unwrap() will throw an exception (panic!)
let txt = readFile("document.txt").unwrap();
println!("{}", txt);

or fail quietly

// only execute the lamba in the function succeed
  .and_then(|txt| println!("{}", txt));


Personally, I like these kind of APIs. They provide a typed error code and users have the choice to handle errors the way they want. I believe it should be possible to do the same in C++. In a second post, I will try to implement a Rust like Result in C++11.