Question

In writing a function to convert between strings of different encodings (e.g. from UTF-8 to UTF-16), what would be the best way to handle errors (e.g. invalid input UTF-8 byte sequence)? Throwing an exception or returning an error code (even a bool)?

// Throws a C++ exception on error. 
std::wstring ConvertFromUtf8ToUtf16(const std::string& utf8);

// Returns true on success, false on error.
bool ConvertFromUtf8ToUtf16(std::wstring& utf16, const std::string& utf8);

Using exceptions, it would be possible to do chained function calls (when the function return value is used as input for other functions/methods).

But I’m not sure that using exceptions in this case is good; I was thinking of what Eric Lippert in his quality blog post calls vexing exceptions (and related Int32.Parse()/TryParse() example).

For example, if exceptions are used, the caller should be forced to wrap the function call in try/catch blocks to check the case of invalid UTF-8 input:

try
{
   wstring utf16 = ConvertFromUtf8ToUtf16(utf8);
}
catch(const Utf8ConversionException& e)
{
   // Bad UTF-8 byte sequence
   ...
}

Which seems not ideal to me.

Maybe the best thing to do is to just provide both overloads (implementing the conversion code in the non-throwing overload, and in the throwing overload just call the non-throwing version, and in case of error return code throw an exception)?

Answer 1

One guideline is to consider what will happen if users ignore or don’t know that they should check your returned error code.

• If the code could theoretically continue in the face of an error, returning an error could be considered reasonable. And as you mention, the code looks cleaner.
• If ignoring the error would likely lead to Very Bad Behavior later, it’s probably a better idea to throw the exception.

• A third potential choice which somewhat balances the terseness of error codes and forcing the programmer to be aware of potential errors is to make the function require a reference to the error code. This will also work well in exported libraries and with (mostly older) compilers that don’t handle exceptions efficiently.

  StringConversionResult result; // Could be a "success" bool

  wstring utf16 = ConvertFromUtf8ToUtf16(utf8, result);