Opening stream via function

问题

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.

回答1:

What about:

ifstream * open_ifstream(const string &filename);

回答2:

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

回答3:

Here's a moderately unintrusive idea:

#include <iconv.h>
#include <algorithm>

void windowify(std::string & filename)
{
#ifdef WIN32
  assert(filename.length() < 1000);

  wchar_t wbuf[1000];
  char    cbuf[1000];
  char * ip = &cbuf[0];
  char * op = reinterpret_cast<char*>(&wbuf[0]);

  size_t ib = filename.length(), ob = 1000;

  std::fill(cbuf + filename.length(), cbuf + 1000, 0);
  std::copy(filename.begin(), filename.end(), cbuf);

  iconv_t cd = iconv_open("WCHAR_T", "UTF-8");
  iconv(cd, &ip, &ib, &op, &ob);
  iconv_close(cd);

  wchar_t sfnbuf[1000];
  std::fill(cbuf, cbuf + 1000, 0);

  ib = GetShortPathNameW(wbuf, sfnbuf, 1000);
  ob = 1000;
  ip = reinterpret_cast<char*>(&wbuf[0]);
  op = &cbuf[0];

  cd = iconv_open("UTF-8", "WCHAR_T");
  iconv(cd, &ip, &ib, &op, &ob);
  iconv_close(cd);

  filename = std::string(cbuf);
#endif
}

Usage:

std::string filename = getFilename();
windowify(filename);
std::ifstream infile(filename.c_str());

回答4:

I would suggest a small improvement: use _wopen (or _wsopen_s) instead of _wfopen. You will get a file descriptor (int) that you can pass to the stdio_filebuf in place of the the FILE*. In this way you should avoid leaking any resource (as pointed out by marcin)

来源：https://stackoverflow.com/questions/6524821/opening-stream-via-function

标签

c++

c++11

copy-constructor

istream

deleted-functions