Read/Write file with unicode file name with plain C++/Boost

I want to read / write a file with a unicode file name using boost filesystem, boost locale on Windows (mingw) (should be platform independent at the end).

This is my code:

#include <boost/locale.hpp>
#define BOOST_NO_CXX11_SCOPED_ENUMS
#include <boost/filesystem.hpp>
#include <boost/filesystem/fstream.hpp>
namespace fs = boost::filesystem;

#include <string>
#include <iostream>

int main() {

  std::locale::global(boost::locale::generator().generate(""));
  fs::path::imbue(std::locale());

  fs::path file("äöü.txt");
  if (!fs::exists(file)) {
    std::cout << "File does not exist" << std::endl;
  }

  fs::ofstream(file, std::ios_base::app) << "Test" << std::endl;
}

The fs::exists really checks for a file with the name äöü.txt. But the written file has the name äöü.txt.

Reading gives the same problem. Using fs::wofstream doesn't help either, since this just handles wide input.

How can I fix this using C++11 and boost?

Edit: Bug report posted: https://svn.boost.org/trac/boost/ticket/9968

To clarify for the bounty: It is quite simple with Qt, but I would like a cross platform solution using just C++11 and Boost, no Qt and no ICU.

This can be complicated, for two reasons:

1. There's a non-ASCII string in your C++ source file. How this literal gets converted to the binary representation of a const char * would depend on compiler settings and/or OS codepage settings.

2. Windows only works with Unicode filenames through the UTF-16 encoding, while Unix uses UTF-8 for Unicode filenames.

Constructing the path object

To get this working on Windows, you can try to change your literal to wide characters (UTF-16):

const wchar_t *name = L"\u00E4\u00F6\u00FC.txt";
fs::path file(name);

To get a full cross-platform solution, you'll have to start with either a UTF-8 or a UTF-16 string, then make sure it gets properly converted to the path::string_type class.

Opening the file stream

Unfortunately, the C++ (and thus Boost) ofstream API does not allow specifying wchar_t strings as the filename. This is the case for both the constructor and the open method.

You could try to make sure that the path object does not get immediately converted to const char * (by using the C++11 string API) but this probably won't help:

std::ofstream(file.native()) << "Test" << std::endl;

For Windows to work, you might be able have to call the Unicode-aware Windows API, CreateFileW, convert the HANDLE to a FILE *, then use the FILE * for the ofstream constructor. This is all described in another StackOverflow answer, but I'm not sure if that ofstream constructor will exist on MinGW.

Unfortunately basic_ofstream doesn't seem to allow subclassing for custom basic_filebuf types, so the FILE * conversion might be the only (completely non-portable) option.

An alternative: Memory-mapped files

Instead of using file streams, you can also write to files using memory-mapped I/O. Depending on how Boost implements this (it's not part of the C++ standard library), this method could work with Windows Unicode file names.

Here's a boost example (taken from another answer) that uses a path object to open the file:

#include <boost/filesystem.hpp>
#include <boost/iostreams/device/mapped_file.hpp>
#include <iostream>

int main()
{ 
  boost::filesystem::path p(L"b.cpp");
  boost::iostreams::mapped_file file(p); // or mapped_file_source
  std::cout << file.data() << std::endl;
}