Doing UTF-8 in Windows

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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 its 65000 characters 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 rules given below:

• 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 the narrow 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 = 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 command line arguments:

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

The second one provides directly a vector of strings:

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.

Conclusion

I hope this article and the included code shows that using UTF-8 encoding in Windows programs doesn't have to be too painful.

The next chapters in this series are:

• "Tolower or not to Lower" shows how to solve  case converion issues in UTF-8
• "INI Files" shows an implementation of Windows API for working with UTF-8 in Windows INI files

History

• 02 August, 2020 - Links to other articles in the series, code updated
• 22 November, 2019 - Initial version