WChars, Encodings, Standards and Portability

Question

The following may not qualify as a SO question; if it is out of bounds, please feel free to tell me to go away. The question here is basically, "Do I understand the C standard correctly and is this the right way to go about things?"

I would like to ask for clarification, confirmation and corrections on my understanding of character handling in C (and thus C++ and C++0x). First off, an important observation:

Portability and serialization are orthogonal concepts.

Portable things are things like C, unsigned int, wchar_t. Serializable things are things like uint32_t or UTF-8. "Portable" means that you can recompile the same source and get a working result on every supported platform, but the binary representation may be totally different (or not even exist, e.g. TCP-over-carrier pigeon). Serializable things on the other hand always have the same representation, e.g. the PNG file I can read on my Windows desktop, on my phone or on my toothbrush. Portable things are internal, serializable things deal with I/O. Portable things are typesafe, serializable things need type punning. </preamble>

When it comes to character handling in C, there are two groups of things related respectively to portability and serialization:

• wchar_t, setlocale(), mbsrtowcs()/wcsrtombs(): The C standard says nothing about "encodings"; in fact, it is entirely agnostic to any text or encoding properties. It only says "your entry point is main(int, char**); you get a type wchar_t which can hold all your system's characters; you get functions to read input char-sequences and make them into workable wstrings and vice versa.

• iconv() and UTF-8,16,32: A function/library to transcode between well-defined, definite, fixed encodings. All encodings handled by iconv are universally understood and agreed upon, with one exception.

The bridge between the portable, encoding-agnostic world of C with its wchar_t portable character type and the deterministic outside world is iconv conversion between WCHAR-T and UTF.

So, should I always store my strings internally in an encoding-agnostic wstring, interface with the CRT via wcsrtombs(), and use iconv() for serialization? Conceptually:

                        my program
    <-- wcstombs ---  /==============\   --- iconv(UTF8, WCHAR_T) -->
CRT                   |   wchar_t[]  |                                <Disk>
    --- mbstowcs -->  \==============/   <-- iconv(WCHAR_T, UTF8) ---
                            |
                            +-- iconv(WCHAR_T, UCS-4) --+
                                                        |
       ... <--- (adv. Unicode malarkey) ----- libicu ---+

Practically, that means that I'd write two boiler-plate wrappers for my program entry point, e.g. for C++:

// Portable wmain()-wrapper
#include <clocale>
#include <cwchar>
#include <string>
#include <vector>

std::vector<std::wstring> parse(int argc, char * argv[]); // use mbsrtowcs etc

int wmain(const std::vector<std::wstring> args); // user starts here

#if defined(_WIN32) || defined(WIN32)
#include <windows.h>
extern "C" int main()
{
  setlocale(LC_CTYPE, "");
  int argc;
  wchar_t * const * const argv = CommandLineToArgvW(GetCommandLineW(), &argc);
  return wmain(std::vector<std::wstring>(argv, argv + argc));
}
#else
extern "C" int main(int argc, char * argv[])
{
  setlocale(LC_CTYPE, "");
  return wmain(parse(argc, argv));
}
#endif
// Serialization utilities

#include <iconv.h>

typedef std::basic_string<uint16_t> U16String;
typedef std::basic_string<uint32_t> U32String;

U16String toUTF16(std::wstring s);
U32String toUTF32(std::wstring s);

/* ... */

Is this the right way to write an idiomatic, portable, universal, encoding-agnostic program core using only pure standard C/C++, together with a well-defined I/O interface to UTF using iconv? (Note that issues like Unicode normalization or diacritic replacement are outside the scope; only after you decide that you actually want Unicode (as opposed to any other coding system you might fancy) is it time to deal with those specifics, e.g. using a dedicated library like libicu.)

Updates

Following many very nice comments I'd like to add a few observations:

• If your application explicitly wants to deal with Unicode text, you should make the iconv-conversion part of the core and use uint32_t/char32_t-strings internally with UCS-4.

• Windows: While using wide strings is generally fine, it appears that interaction with the console (any console, for that matter) is limited, as there does not appear to be support for any sensible multi-byte console encoding and mbstowcs is essentially useless (other than for trivial widening). Receiving wide-string arguments from, say, an Explorer-drop together with GetCommandLineW+CommandLineToArgvW works (perhaps there should be a separate wrapper for Windows).

• File systems: File systems don't seem to have any notion of encoding and simply take any null-terminated string as a file name. Most systems take byte strings, but Windows/NTFS takes 16-bit strings. You have to take care when discovering which files exist and when handling that data (e.g. char16_t sequences that do not constitute valid UTF16 (e.g. naked surrogates) are valid NTFS filenames). The Standard C fopen is not able to open all NTFS files, since there is no possible conversion that will map to all possible 16-bit strings. Use of the Windows-specific _wfopen may be required. As a corollary, there is in general no well defined notion of "how many characters" comprise a given file name, as there is no notion of "character" in the first place. Caveat emptor.

Answer 1

Is this the right way to write an idiomatic, portable, universal, encoding-agnostic program core using only pure standard C/C++

No, and there is no way at all to fulfill all these properties, at least if you want your program to run on Windows. On Windows, you have to ignore the C and C++ standards almost everywhere and work exclusively with wchar_t (not necessarily internally, but at all interfaces to the system). For example, if you start with

int main(int argc, char** argv)

you have already lost Unicode support for command line arguments. You have to write

int wmain(int argc, wchar_t** argv)

instead, or use the GetCommandLineW function, none of which is specified in the C standard.

More specifically,

• any Unicode-capable program on Windows must actively ignore the C and C++ standard for things like command line arguments, file and console I/O, or file and directory manipulation. This is certainly not idiomatic. Use the Microsoft extensions or wrappers like Boost.Filesystem or Qt instead.
• Portability is extremely hard to achieve, especially for Unicode support. You really have to be prepared that everything you think you know is possibly wrong. For example, you have to consider that the filenames you use to open files can be different from the filenames that are actually used, and that two seemingly different filenames may represent the same file. After you create two files a and b, you might end up with a single file c, or two files d and e, whose filenames are different from the file names you passed to the OS. Either you need an external wrapper library or lots of #ifdefs.
• Encoding agnosticity usually just doesn't work in practice, especially if you want to be portable. You have to know that wchar_t is a UTF-16 code unit on Windows and that char is often (bot not always) a UTF-8 code unit on Linux. Encoding-awareness is often the more desirable goal: make sure that you always know with which encoding you work, or use a wrapper library that abstracts them away.

I think I have to conclude that it's completely impossible to build a portable Unicode-capable application in C or C++ unless you are willing to use additional libraries and system-specific extensions, and to put lots of effort in it. Unfortunately, most applications already fail at comparatively simple tasks such as "writing Greek characters to the console" or "supporting any filename allowed by the system in a correct manner", and such tasks are only the first tiny steps towards true Unicode support.

Answer 2

I would avoid the wchar_t type because it's platform-dependent (not "serializable" by your definition): UTF-16 on Windows and UTF-32 on most Unix-like systems. Instead, use the char16_t and/or char32_t types from C++0x/C1x. (If you don't have a new compiler, typedef them as uint16_t and uint32_t for now.)

DO define functions to convert between UTF-8, UTF-16, and UTF-32 functions.

DON'T write overloaded narrow/wide versions of every string function like the Windows API did with -A and -W. Pick one preferred encoding to use internally, and stick to it. For things that need a different encoding, convert as necessary.

Answer 3

The problem with wchar_t is that encoding-agnostic text processing is too difficult and should be avoided. If you stick with "pure C" as you say, you can use all of the w* functions like wcscat and friends, but if you want to do anything more sophisticated then you have to dive into the abyss.

Here are some things that much harder with wchar_t than they are if you just pick one of the UTF encodings:

• Parsing Javascript: Identifers can contain certain characters outside the BMP (and lets assume that you care about this kind of correctness).

• HTML: How do you turn 𐀀 into a string of wchar_t?

• Text editor: How do you find grapheme cluster boundaries in a wchar_t string?

If I know the encoding of a string, I can examine the characters directly. If I don't know the encoding, I have to hope that whatever I want to do with a string is implemented by a library function somewhere. So the portability of wchar_t is somewhat irrelevant as I don't consider it an especially useful data type.

Your program requirements may differ and wchar_t may work fine for you.

Answer 4

Given that iconv is not "pure standard C/C++", I don't think you are satisfying your own specifications.

There are new codecvt facets coming with char32_t and char16_t so I don't see how you can be wrong as long as you are consistent and pick one char type + encoding if the facets are here.

The facets are described in 22.5 [locale.stdcvt] (from n3242).

---

I don't understand how this doesn't satisfy at least some of your requirements:

namespace ns {

typedef char32_t char_t;
using std::u32string;

// or use user-defined literal
#define LIT u32

// Communicate with interface0, which wants utf-8

// This type doesn't need to be public at all; I just refactored it.
typedef std::wstring_convert<std::codecvt_utf8<char_T>, char_T> converter0;

inline std::string
to_interface0(string const& s)
{
    return converter0().to_bytes(s);
}

inline string
from_interface0(std::string const& s)
{
    return converter0().from_bytes(s);
}

// Communitate with interface1, which wants utf-16

// Doesn't have to be public either
typedef std::wstring_convert<std::codecvt_utf16<char_T>, char_T> converter1;

inline std::wstring
to_interface0(string const& s)
{
    return converter1().to_bytes(s);
}

inline string
from_interface0(std::wstring const& s)
{
    return converter1().from_bytes(s);
}

} // ns

Then your code can use ns::string, ns::char_t, LIT'A' & LIT"Hello, World!" with reckless abandon, without knowing what's the underlying representation. Then use from_interfaceX(some_string) whenever it's needed. It doesn't affect the global locale or streams either. The helpers can be as clever as needed, e.g. codecvt_utf8 can deal with 'headers', which I assume is Standardese from tricky stuff like the BOM (ditto codecvt_utf16).

In fact I wrote the above to be as short as possible but you'd really want helpers like this:

template<typename... T>
inline ns::string
ns::from_interface0(T&&... t)
{
    return converter0().from_bytes(std::forward<T>(t)...);
}

which give you access to the 3 overloads for each [from|to]_bytes members, accepting things like e.g. const char* or ranges.