Using std::istream with an already existing file

[John E]

Is it possible to attach an istream object to an already opened file handle or FILE* (so I could read the file contents into my object?)

I discovered recently that ifstream has a c'tor for this but it doesn't seem to be universal (it's available in some versions of MSVC but not for other compilers). The reason I need an already opened file is that I need to open it from a cross-platform file path (in UTF-8 format). So I can't rely on ifstream directly - nor open / fopen etc, as they don't accept UTF-8 paths on Windows. So for opening the files I use a 3rd party library which does understand UTF-8 (hope that makes sense).

P.S. I've just been reading about something called std::filebuf which apparently CAN connect to std::istream. I'm not sure if that would help at all? (I'd never heard of std::filebuf until a few minutes ago ). Is it even available in MSVC? I couldn't find much about it on MSDN...

[John E]

After some further experiments I've realised that (in MSVC at least) std::filebuf does exist and it offers a c'tor of type filebuf::filebuf(FILE*). So if that's available outside of MSVC my problem's solved. Anyone know if that c'tor is part of the official standard (i.e. it's not just some MSVC-specific extension) ?

[2kaud]

The c++ standard for filebuf is described here http://www.cplusplus.com/reference/fstream/filebuf/

There is no mention of a c'tor taking a type of FILE*

[John E]

The c++ standard for filebuf is described here http://www.cplusplus.com/reference/fstream/filebuf/

There is no mention of a c'tor taking a type of FILE*

Thanks 2kaud. Sorry for the delay in replying.

I did some research into this and while you're right (i.e. a c'tor taking FILE* isn't officially part of the standard) it's often supported as a compiler extension (apparently) because it can be useful when handling pipes & sockets.

If there's anyone here using gcc, would they mind letting me know if the following code will compile with it..?

#include <stdio.h>
#include <iostream>
#include <fstream>

void some_function ()
{
      FILE* fin = fopen ("whatever", "r");

      std::filebuf fb (fin);
      std::istream ifs (&fb);
}

[Philip Nicoletti]

It did not compile using g++ ver 4.8.1 under MinGW/Windows

Here is a link for converting a file descriptor to a ostream or istream with g++.

It compiled under g++ ver 4.8.1 under MinGW/Windows, but I did not test it.

https://gist.github.com/rajatkhanduja/2012695

[Codeplug]

If you don't want #define's for using GCC and MS extensions, then there's boost::filesystem as well (which become standardized).

gg