file descriptor out buffer : ostream « File Stream « C++ Tutorial

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/* The following code example is taken from the book
 * "The C++ Standard Library - A Tutorial and Reference"
 * by Nicolai M. Josuttis, Addison-Wesley, 1999
 *
 * (C) Copyright Nicolai M. Josuttis 1999.
 * Permission to copy, use, modify, sell and distribute this software
 * is granted provided this copyright notice appears in all copies.
 * This software is provided "as is" without express or implied
 * warranty, and with no claim as to its suitability for any purpose.
 */
#include <iostream>
#include <streambuf>
#include <cstdio>

// for write():
#ifdef _MSC_VER
# include <io.h>
#else
# include <unistd.h>
#endif

class fdoutbuf : public std::streambuf {
  protected:
    int fd;    // file descriptor
  public:
    // constructor
    fdoutbuf (int _fd) : fd(_fd) {
    }
  protected:
    // write one character
    virtual int_type overflow (int_type c) {
        if (c != EOF) {
            char z = c;
            if (write (fd, &z, 1) != 1) {
                return EOF;
            }
        }
        return c;
    }
    // write multiple characters
    virtual
    std::streamsize xsputn (const char* s,
                            std::streamsize num) {
        return write(fd,s,num);
    }
};

class fdostream : public std::ostream {
  protected:
    fdoutbuf buf;
  public:
    fdostream (int fd) : std::ostream(0), buf(fd) {
        rdbuf(&buf);
    }
};


int main()
{
    fdostream out(1);    // stream with buffer writing to file descriptor 1

    out << "31 hexadecimal: " << std::hex << 31 << std::endl;
}

31 hexadecimal: 1f