Doing UTF-8 in Windows

Doing UTF-8 in Windows #

Introduction #

This article shows how to handle UTF-8 encoding on a platform that still encourages the UTF-16 encoding. I am also providing a small library for this purpose. The code works, it is clean, easy to understand and small.

This is an implementation of the solution advocated in the UTF-8 Everywhere manifesto. I would strongly encourage you to go read the whole document to get indoctrinated ☺.

Background #

Let me rehash some of the points made in the manifesto mentioned above:

  • UTF-16 (variously called Unicode, widechar or UCS-2) was introduced back in early ‘90-es and, at the time, it was believed that it’s 65000 codes will be enough for all characters,
  • Except in particular cases, UTF-16 is not more efficient or easier to use than UTF-8. In fact in many cases the opposite is true.
  • In UTF-16 characters have also variable width encoding (two or four bytes) and counting characters is as difficult as in UTF-8.

If you want to work with UTF-8 encoding in Windows (and you should), and you don’t want go insane or your program to crash unexpectedly you must follow the following rules:

  • Define _UNICODE when compiling your program (or select “Use Unicode Character Set” in Visual Studio).
  • Use wchar_t or std::wstring only in arguments to API function calls. Use char or std::string everywhere else.
  • Use widen() and narrow() functions to go between UTF-8 and UTF-16.

The functions provided in this package will make your life much easier.

Calling Library Functions #

All functions live in the utf8 namespace and I would advise you not to place a using directive for this namespace. This is because many/most functions have the same name as the traditional C functions. For instance, if you had a function call:

mkdir (folder_name);

and you want to start using UTF-8 characters, you just have to change it to:

utf8::mkdir (folder_name);

Prefixing the function with the namespace makes it obvious what function you are using.

Basic Conversion Functions #

Following the same manifesto, the basic conversion functions are narrow(), to go from UTF-16 to UTF-8 and widen() to go in the opposite direction. Their signatures are:

    std::string narrow (const wchar_t* s);
	std::string narrow (const std::wstring&; s);
    
    std::wstring widen (const char* s);
    std::wstring widen (const std::string&; s);

In addition there are two more functions for conversion from and to UTF-32:

    std::string narrow (const std::u32string&; s);
	std::u32string runes (const std::string&; s);

Internally, the conversion is done using the WideCharToMultiByte and MultiByteToWideChar functions.

There are also functions for counting the number of characters in a UTF-8 string (length()), to check if a string is valid (valid()), and to advance a pointer/iterator in character string (next()).

Wrappers #

Pretty much all the other functions are wrappers around traditional C/C++ functions or structures:

  • directory manipulation functions: mkdir, rmdir, chdir, getcwd
  • file operations: fopen, chmod, access, rename, remove
  • streams: ifstream, ofstream, fstream
  • path manipulation functions: splitpath and makepath
  • environment access functions putenv and `getenv
  • character classification functions is... (isalnum, isdigit, isalpha, etc.)

The parameters for all these functions mimic the standard parameters. For some of them however, like access, rename, etc., the return type is bool with true indicating success and false indicating failure. This is contrary to standard C functions that return 0 for success. Caveat emptor!

Return Values #

For API functions that return a character string, you would need to setup a wchar_t buffer to receive the value, convert it to UTF-8 using a narrowing function and eventually release the buffer. Below is an example of how this would look like. The code retrieves the name of temporary file:

wstring wpath (_MAX_PATH, L'\0');
wstring wfname (_MAX_PATH, L'\0');

GetTempPath (wpath.size (), const_cast<wchar_t*>(wpath.data ()));
GetTempFileName (wpath.c_str(), L"ABC", 1, const_cast<wchar_t*>(wfname.data ()));

string result = utf8::narrow(wfname);

This seemed a bit too cumbersome and error prone so I made a small object destined to hold returned values. It has operators to convert it to a wchar_t buffer and then to a UTF-8 string. For lack of a better name, I called it buffer. Using this object, the same code fragment becomes:

utf8::buffer path (_MAX_PATH);
utf8::buffer fname (_MAX_PATH);

GetTempPath (path.size (), path);
GetTempFileName (path, L"ABC", 1, fname);

string result = fname;

Internally, a buffer object contains UTF-16 characters but the string conversion operator invokes the utf8::narrow function to convert the string to UTF-8.

Program Arguments #

There are two functions for accessing and converting UTF-16 encoded program arguments: the get_argv function returns an argv like array of pointers to commend line arguments:

int argc;
char **argv = utf8::get_argv (&amp;argc);

The second one provides directly a vector of stings:

std::vector<std::string> argv = utf8::argv ();

When using the first function, one has to call utf8::free_argv function to release the memory allocated for argv array.

Unexpected Twists #

The C++20 standard has added an additional type char8_t, designed to keep UTF-8 encoded characters, and a string type std::u8string. By making it a separate type from char and unsigned char, the committee has also created a number of incompatibilities. For instance the following fragment will produce an error:

std::string s {"English text"}; //this is ok
s = {u8"日本語テキスト"}; //"Japaneese text" - error

You would have to change it to something like:

std::u8string s {u8"English text"}; 
s = {u8"日本語テキスト"};

In my opinion, by introducing the char8_t type, the committee went against the very principles of UTF-8 encoding. The purpose of the encoding was to extend usage of the char type to additional Unicode code points. It has been so successful that it is now the de-facto standard used all across the Internet. Even Windows, that has been a bastion of UTF-16 encoding, is now slowly moving toward UTF-8.

In this context, the use of char data type for anything other than holding encodings of strings, seems out of place. In particular, arithmetic computations with char or unsigned char entities are just a small fraction of the use cases. The standard should try to simplify usage in the most common cases leaving the infrequent ones to bear the burden of complexity.

Following this principle, you would want to write:

std::string s {"English text"};
s += " and ";
s += "日本語テキスト";

with the implied assumption that all char strings are UTF-8 encoded character strings.

Recently (June, 2022) the committee seems to have changed position and introduced a compatibility and portability fix - DR2513R3 allowing initialization of arrays of char or unsigned char with UTF-8 string literals. Until the defect report makes its way into the next standard edition, the solution for Visual C++ users who compile under C++20 standard rules is to use the Zc:char8_t- compiler option.

History #

  • 22-Nov-2019 - Initial version.
  • 30-May-2023 - Added discussion about C++20 char8_t