INFO: fstream's File Pointers Are Not Independent

• Microsoft C/C++ compiler for MS-DOS, versions 7.0, 7.0a
• Microsoft Visual C++ for Windows, versions 1.0, 1.5, 1.51, 1.52
• Microsoft Visual C++ 32-bit Edition, versions 1.0, 2.0, 2.1, 4.0, 5.0

SUMMARY

The Microsoft "iostream Class Library Reference" in the Books Online included with Visual C++ 32-bit Edition, version 4.0, contains the following as part of the description for class filebuf:public iostream:

   The reserve area, put area, and get area are introduced in the
   streambuf class description. The put area and the get area are
   always the same for filebuf objects. Also, the get pointer and
   put pointers are tied; when one moves, so does the other.

   Although the filebuf object's get and put pointers are
   theoretically independent, the get area and the put area cannot
   both be active at the same time.

MORE INFORMATION

Sample Code

   /* Compile options needed: None. Build as a console .EXE for Windows NT
   */

   #include <fstream.h>
   #include <strstrea.h>
   #include <assert.h>
   #undef NDEBUG    // Make sure assert works.

   void main()
   {
     fstream stream("test",ios::in | ios::out | ios::binary);
     int temp;
     char input;

     cout << "\n\nOpened binary file test" << endl;
     cout << "Get pointer is " << hex << stream.tellg() << endl;
     cout << "Put pointer is " << hex << stream.tellp() << endl;
     cout << "Now writing 256 bytes..." << endl;
     for(temp = 0;temp < 256;temp++)
     {
       stream.put((char)temp);
     }
     cout << "Get pointer is " << hex << stream.tellg() << endl;
     cout << "Put pointer is " << hex << stream.tellp() << endl;

     cout << "\nNow setting the put pointer to hex 50" << endl;
     stream.seekp(0x50);
     cout << "Get pointer is " << hex << stream.tellg() << endl;
     cout << "Put pointer is " << hex << stream.tellp() << endl;

     cout << "\nNow setting the get pointer to hex 40" << endl;
     stream.seekg(0x40);
     cout << "Get pointer is " << hex << stream.tellg() << endl;
     cout << "Put pointer is " << hex << stream.tellp() << endl;

     cout << "\nNow writing one character" << endl;
     stream.put('a');
     cout << "Get pointer is " << hex << stream.tellg() << endl;
     cout << "Put pointer is " << hex << stream.tellp() << endl;

     cout << "\nNow reading one character" << endl;
     stream.get(input);
     cout << "Get pointer is " << hex << stream.tellg() << endl;
     cout << "Put pointer is " << hex << stream.tellp() << endl;
     stream.close();
   }

Keywords          : CPPLngIss kbcode kbfasttip
Version           : MS-DOS:7.0,7.00a;WINDOWS:1.0,1.5,1.51,1.52;WINDOWS NT:1.0,2.0,2.1,4.0,5.0
Platform          : MS-DOS WINDOWS
Issue type        : kbinfo