Published by
Apr 14, 2010 (last update: Apr 22, 2010)

stdiostream

Score: 4.2/5 (14 votes)
*****
This code is being modified... It currently has a couple of bugs.
Hey all, I just wanted to play around with this kind of stuff so I thought I'd share.

This is a stdiobuf implementation of my own design.
The purpose is to allow you to wrap a C stdio FILE* with a std::iostream.
I make no claims as to correctness or efficiency. (But I think I did a pretty good job...)

It is sometimes convenient to do this kind of thing and get the full power of C++ while interfacing with C. It won't prevent you from doing particularly stupid things with it though.

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// stdiostream.hpp
//
// Copyright (c) 2010 Michael Thomas Greer
// Distributed under the Boost Software License, Version 1.0
// (See <a href="http://www.boost.org/LICENSE_1_0.txt">http://www.boost.org/LICENSE_1_0.txt</a> )
//

#pragma once
#ifndef DUTHOMHAS_STDIOSTREAM_HPP
#define DUTHOMHAS_STDIOSTREAM_HPP

#include <cstdio>
#include <iostream>
#include <streambuf>
#include <string>
#include <vector>

namespace duthomhas
  {

  /* /////////////////////////////////////////////////////////////////////////
    basic_stdiobuf
  ///////////////////////////////////////////////////////////////////////// */

  template <
    typename CharType,
    typename CharTraits = std::char_traits <CharType>
    >
  class basic_stdiobuf: public std::basic_streambuf <CharType, CharTraits>
    {
    //------------------------------------------------------------------------
    public:
    //------------------------------------------------------------------------
      typedef CharType                                char_type;
      typedef CharTraits                              traits_type;
      typedef typename traits_type::int_type          int_type;
      typedef typename traits_type::pos_type          pos_type;
      typedef typename traits_type::off_type          off_type;

      typedef basic_stdiobuf <char_type, traits_type> this_type;

      //......................................................................
      basic_stdiobuf( FILE* fp = NULL ):
        fp( fp )
        { }

      //......................................................................
//BUG 1: Hey! I never get called! (How is that?)
      ~basic_stdiobuf()
        {
        this->close();
        }

      //......................................................................
      bool is_open() const throw()
        {
        return fp != NULL;
        }

      //......................................................................
      this_type* open( const char* filename, std::ios_base::openmode mode )
        {
        if (is_open()) return NULL;

        // Figure out the open mode flags  . . . . . . . . . . . . . . . . . .
        std::string fmode;

        bool is_ate = mode & std::ios_base::ate;
        bool is_bin = mode & std::ios_base::binary;
        mode &= ~(std::ios_base::ate | std::ios_base::binary);

        #define _(flag) std::ios_base::flag
        if      (mode == (         _(in)                    )) fmode = "r";
        else if (mode == (                 _(out) & _(trunc))) fmode = "w";
        else if (mode == (_(app)         & _(out)           )) fmode = "a";
        else if (mode == (         _(in) & _(out)           )) fmode = "r+";
        else if (mode == (         _(in) & _(out) & _(trunc))) fmode = "w+";
        else if (mode == (_(app) & _(in) & _(out)           )) fmode = "a+";
        // I would prefer to throw an exception here,
        // but the standard only wants a NULL result.
        else return NULL;
        #undef _
        if (is_bin) fmode.insert( 1, 1, 'b' );

        // Try opening the file  . . . . . . . . . . . . . . . . . . . . . . .
        fp = std::fopen( filename, fmode.c_str() );
        if (!fp) return NULL;

        // Reposition to EOF if wanted . . . . . . . . . . . . . . . . . . . .
        if (is_ate) std::fseek( fp, 0, SEEK_END );

        return this;
        }

      //......................................................................
      this_type* close()
        {
        if (fp)
          {
          std::fclose( fp );
          fp = NULL;
          }
        pushbacks.clear();
        return this;
        }

      //......................................................................
      FILE* stdiofile() const
        {
        return fp;
        }

    //------------------------------------------------------------------------
    protected:
    //------------------------------------------------------------------------

      //......................................................................
      // Get the CURRENT character without advancing the file pointer
      virtual int_type underflow()
        {
        // Return anything previously pushed-back
        if (pushbacks.size())
          return pushbacks.back();

        // Else do the right thing
        fpos_t pos;
        if (std::fgetpos( fp, &pos ) != 0)
          return traits_type::eof();
          
        int c = std::fgetc( fp );
        std::fsetpos( fp, &pos );

        return maybe_eof( c );
        }

      //......................................................................
      // Get the CURRENT character AND advance the file pointer
      virtual int_type uflow()
        {
        // Return anything previously pushed-back
        if (pushbacks.size())
          {
          int_type c = pushbacks.back();
          pushbacks.pop_back();
          return c;
          }

        // Else do the right thing
        return maybe_eof( std::fgetc( fp ) );
        }

      //......................................................................
      virtual int_type pbackfail( int_type c = traits_type::eof() )
        {
        if (!is_open())
          return traits_type::eof();

        // If the argument c is EOF and the file pointer is not at the
        // beginning of the character sequence, it is decremented by one.
        if (traits_type::eq_int_type( c, traits_type::eof() ))
          {
          pushbacks.clear();
          return std::fseek( fp, -1L, SEEK_CUR )
               ? traits_type::eof()
               : 0;
          }

        // Otherwise, make the argument the next value to be returned by
        // underflow() or uflow()
        pushbacks.push_back( c );
        return c;
        }

      virtual int_type overflow( int_type c = traits_type::eof() )
        {
        pushbacks.clear();

        // Do nothing
        if (traits_type::eq_int_type( c, traits_type::eof() ))
          return 0;

        // Else write a character
        return maybe_eof( std::fputc( c, fp ) );
        }

      //......................................................................
      virtual this_type* setbuf( char* s, std::streamsize n )
        {
        return std::setvbuf( fp, s, (s and n) ? _IOLBF : _IONBF, (size_t)n )
             ? NULL
             : this;
        }

      //......................................................................
      virtual pos_type seekoff(
        off_type                offset,
        std::ios_base::seekdir  direction,
        std::ios_base::openmode which = std::ios_base::in | std::ios_base::out
        ) {
        pushbacks.clear();
        return std::fseek( fp, offset,
          (direction == std::ios_base::beg) ? SEEK_SET :
          (direction == std::ios_base::cur) ? SEEK_CUR :
                                              SEEK_END
          ) ? (-1) : std::ftell( fp );
        }

      //......................................................................
      virtual pos_type seekpos(
        pos_type                position,
        std::ios_base::openmode which = std::ios_base::in | std::ios_base::out
        ) {
        pushbacks.clear();
        return std::fseek( fp, position, SEEK_SET )
             ? (-1)
             : std::ftell( fp );
        }

      //......................................................................
      virtual int sync()
        {
        pushbacks.clear();
        return std::fflush( fp )
             ? traits_type::eof()
             : 0;
        }

    //------------------------------------------------------------------------
    private:
    //------------------------------------------------------------------------
      FILE*                  fp;
      std::vector <int_type> pushbacks;  // we'll treat this like a stack

      //......................................................................
      // Utility function to make sure EOF gets translated to the proper value
      inline int_type maybe_eof( int value ) const
        {
        return (value == EOF) ? traits_type::eof() : value;
        }
    };


  /* /////////////////////////////////////////////////////////////////////////
    basic_stdiostream
  ///////////////////////////////////////////////////////////////////////// */

  template <
    typename CharType,
    typename CharTraits = std::char_traits <CharType>
    >
  struct basic_stdiostream: public std::basic_iostream <CharType, CharTraits>
    {
    typedef CharType                                      char_type;
    typedef CharTraits                                    traits_type;

    typedef basic_stdiobuf      <char_type, traits_type>  sbuf_type;
    typedef basic_stdiostream   <char_type, traits_type>  this_type;
    typedef std::basic_iostream <char_type, traits_type>  base_type;

    //......................................................................
    basic_stdiostream( FILE* fp = NULL ):
      base_type( new sbuf_type( fp ) )
      { }

    //......................................................................
    basic_stdiostream( const char* filename, std::ios_base::openmode mode ):
//BUG 2: Oops! This is a potential memory leak!
      base_type( (new sbuf_type)->open( filename, mode ) )
      { }

    //......................................................................
    void open(
      const char*             filename,
      std::ios_base::openmode mode = std::ios_base::in | std::ios_base::out
      ) {
      sbuf_type* buf = static_cast <sbuf_type*> ( this->rdbuf() );
      if (!(buf->open( filename, mode )))
        this->setstate( std::ios_base::badbit );
      }

    //......................................................................
    void close()
      {
      sbuf_type* buf = static_cast <sbuf_type*> ( this->rdbuf() );
      buf->close();
      }
    };


  /* /////////////////////////////////////////////////////////////////////////
    Useful typedefs
  ///////////////////////////////////////////////////////////////////////// */

  typedef basic_stdiobuf    <char> stdiobuf;
  typedef basic_stdiostream <char> stdiostream;

  } // namespace duthomhas

#endif

// end stdiostream.hpp

You can also use it much like you use a normal std::fstream, except it works through a FILE*, of course...

It is most convenient, however, if you have an open FILE* that you wish to perform some C++ manipulations on...

All commentary and feedback welcome.

--Michael
Thanks! :-)

Uses Use it whenever you must use a FILE* but still want C++ super powers.
Typical instances are when interfacing with a C library and when playing with tmpfile() and the like. For example:

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// Play with a tmpfile()
//
// BTW, the tmpfile() function is not a very good choice on Windows. The following
// code may not work for you. (If it does it will probably stick the temporary in the root
// directory of the current drive, so make sure you have access permissions for it first.)
//
#include <cstdlib>
#include <fstream>
#include <iostream>
#include <limits>
#include <string>

#include "stdiostream.hpp"

using namespace std;
using namespace duthomhas;

int main()
  {

  // Open a temporary file to play with
  stdiostream tf( tmpfile() );
  if (!tf)
    {
    cerr << "Fooey!\n";
    return 1;
    }

  // Write some stuff to it...
  tf << "3.";
  tf << "141592\n";
  tf << "Hello ";
  tf << "world!\n";

  // rewind
  tf.seekg( 0, ios::beg );

  // Get the stuff we wrote..
  double pi;
  tf >> pi;
  tf.ignore( numeric_limits <streamsize> ::max(), '\n' );

  string greeting;
  getline( tf, greeting );

  // Show whether suceessful:
  cout << greeting << endl;
  cout << "pi = " << pi << endl;

  // Play with the user
  cout << "\nNow is the time to check that the temporary file actually exists.\n"
          "Press ENTER once done.\n";
  cin.ignore( numeric_limits <streamsize> ::max(), '\n' );

  // All done.
  cout << "\nOK, now it should be gone...\n";
  return 0;
  }

Other Issues An unresolved issue is whether the destructor should endeavor to automatically close the file or not, or whether this should be a flaggable option.