1. A colleague of yours doesn't believe you when you explain why reading the last byte in a file doesn't set the eof condition to true. Write down a code fragment that will demonstrate to your colleague that the eof condition is set only when eof is detected, and not when the file pointer is set to the point to just after the last byte in the file.

   Your code fragment need not be syntactically correct C++ code, however it should recognizable as quasi-C++ code without too much work. In particular, the functions called, the arguments used, and the actions taken should be clearly evident in your code fragment. If you don't understand this paragraph, just express your answer in syntactically correct C++ code.

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   If we assume the file pointer for ins is pointing to the last byte in the file, the following code fragment will do the job:

   // Move the file pointer to just after the last byte
   
   ins.read(data, 1)
   cerr << "ins.eof() = "
   if (ins.eof())
     cerr << "true"
   else
     cerr << "false"
   cerr << ".\n"
   
   // Read the non-existent next byte, detecting the eof condition.
   
   ins.read(data, 1)
   cerr << "ins.eof() = "
   if (ins.eof())
     cerr << "true"
   else
     cerr << "false"
   cerr << ".\n"
   

   The first read moves the file pointer to just past the last byte in the file, but doesn't detect the eof condition; ins.eof() returns false in this case. The second read detects and sets the eof condition, and ins.eof() returns true.

   For completeness, here's the whole demonstration program

   #include <fstream>
   #include <cstdlib>
   #include <string>
   
   int main() {
   
     const string fname = "/tmp/junk";
   
     // Create a one-byte file: open the file for writing.
   
        ofstream outs;
        outs.open(fname.c_str());
        if (!outs.good()) {
          cerr << "Can't open " << fname << ".\n";
          exit(EXIT_FAILURE);
          }
   
     // Write one byte and close the file.
   
        char data[1] = { 0 };
        outs.write(data, 1);
        if (!outs.good()) {
          cerr << "Can't write " << fname << ".\n";
          exit(EXIT_FAILURE);
          }
        outs.close();
    
     // Open the one-byte file for reading.
   
        ifstream ins;
        ins.open(fname.c_str());
        if (!ins.good()) {
          cerr << "Can't open " << fname << ".\n";
          exit(EXIT_FAILURE);
          }
   
     // Read the one byte and report the eof condition.
   
        ins.read(data, 1);
        cerr << "ins.eof() = ";
        if (ins.eof())
          cerr << "true";
        else
          cerr << "false";
        cerr << ".\n";
   
     // Read the non-existent byte and report the eof condition in an alternative
     // way.
   
        ins.read(data, 1);
        cerr << "ins.eof() = " << (ins.eof() ? "true" : "false") << ".\n";
   
     // That's it.
   
        ins.close();
     }
   
   

   and here's an example compilation and execution

   $ g++ -o q3ans -ansi -pedantic -Wall q3ans.cc
   
   $ q3ans
   ins.eof() = false.
   ins.eof() = true.
   
   $ 

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This page last modified on 27 July 2001.