File Input/Output

Up to this point we have only dealt with the i/o streams cin and cout. A program can create its own i/o stream, called a file stream, that can be associated with a file.

To use a file stream:

1. include <iostream> and <fstream>

2. Create the stream:

For input:    ifstream  in_stream_name;
For output:   ofstream  out_stream_name;

3. Open the stream:

stream_name.open( fileName );

stream_name.open( fileName, ios::binary);

There is a possibility that the open operation could fail. As before, the name of the stream can be used as a boolean expression to test for a successful open.

The easiest way to do this is by using the assert() function, which is included in the <cassert> library.

ifstream infile;

infile.open("input.txt");
assert(infile);

Once a stream is open, it may be used the same way as cin and cout.

int i;
ifstream infile;
ofstream outfile;

infile.open("input.txt");
assert(infile);

outfile.open("output.txt");
assert(outfile);

infile >> i;

outfile << "The value of i is" << setw(4) << i << endl;

With the exception of reading from a file with binary data. In C, fread() was used. In C++, we will use the read() function. The format for this is:

read ((char *) &buf, size);

&buf -- is the address of where the data should be stored
size -- the number of bytes to be read (think about using sizeof(buf))

There are a few different ways to test for end of file:

1. use the stream name as a boolean expression
2. use the eof() function

1.  infile >> i;
    
    while (infile)                          while ( infile >> i )
      {                                        outfile << "i = " << i << endl;
      outfile << "i = " << i << endl;
      infile >> i;
      }


2. infile >> i;
    
    while ( !infile.eof() )
      {
      outfile << "i = " << i << endl;
      infile >> i;
      }

Once processing of the stream is complete, it should be closed.

infile.close();
outfile.close();

I/O redirection

After a program has been compiled and linked, it can be run via i/o redirection so that data can be read from and written to files.

An input file will be read from as if a user was typing values in via a keyboard.

lx%  ./executable_file_name < input_file_name

lx%  ./executable_file_name > output_file_name

lx%  ./executable_file_name < input_file_name > output_file_name