Opening stream via function

Question

I need help with the non-copyable nature of [io](f)streams.

I need to provide a hackish wrapper around fstreams in order to handle files with unicode characters in their filenames on Windows. For this, I devised a wrapper function:

bool open_ifstream( istream &stream, const string &filename )
{
#ifdef __GLIBCXX__
    FILE* result = _wfopen( convert_to_utf16(filename).c_str(), L"r" );
    if( result == 0 )
        return false;

    __gnu_cxx::stdio_filebuf<char>* buffer = new __gnu_cxx::stdio_filebuf<char>( result, std::ios_base::in, 1 );
    istream stream2(buffer);
    std::swap(stream, stream2);

#elif defined(_MSC_VER)
    stream.open( convert_to_utf16(filename) );
#endif
    return !!stream;
}

With of course the std::swap line being the culprit. I also tried returning the stream from the function, but it leads to the same problem. The copy constructor of a std::istream is deleted. I also tried a std::move but that didn't help. How do I work around this problem?

EDIT: I finally found a good way to Keep It Simple (TM) and yet functional, thanks to @tibur's idea. It's still hackish in the sense that it depends on the Windows Standard C++ library used, but as there's only two real ones in use, it's not really a problem for me.

#include <fstream>
#include <memory>
#if _WIN32
# if __GLIBCXX__
#  include<ext/stdio_filebuf.h>
unique_ptr<istream> open_ifstream( const string &filename )
{
    FILE* c_file = _wfopen( convert_to_utf16(filename).c_str(), L"r" );
    __gnu_cxx::stdio_filebuf<char>* buffer = new __gnu_cxx::stdio_filebuf<char>( c_file, std::ios_base::in, 1 );

    return std::unique_ptr<istream>( new istream(buffer) );
}
# elif _MSC_VER
unique_ptr<ifstream> open_ifstream( const string &filename )
{
    return unique_ptr<ifstream>(new ifstream( convert_to_utf16(filename)) );
}
# else
# error unknown fstream implementation
# endif
#else
unique_ptr<ifstream> open_ifstream( const string &filename )
{
    return unique_ptr<ifstream>(new ifstream(filename) );
}
#endif

And in user code:

auto stream_ptr( open_ifstream(filename) );
auto &stream = *stream_ptr;
if( !stream )
    return emit_error( "Unable to open nectar file: " + filename );

Which depends on C++0x <memory> and the auto keyword. Of course you can't just close the resulting stream variable, but the GNU Libstdc++ std::istream destructor does take care of closing the file, so no extra memory management is required anywhere.

Answer 1

What about:

ifstream * open_ifstream(const string &filename);

Answer 2

Couldn't you just use the rdbuf member function to set stream's buffer directly?