Section 6.1 Input/Output with files

The fstream library discussed in this section is included in compilers previous to the ANSI-C++ standard. The specifications of this standard have introduced some changes on the traditional implementation of this library that have been followed (although simplified) in the processing of this document.

C++ incorporates support for input and output with files through the following classes:

• ofstream: File class for writing operations (derived from ostream)
• ifstream: File class for reading operations (derived from istream)
• fstream: File class for both reading and writing operations (derived from iostream)

Open a file

In order to open a file it is used the member function open():

void open (const char * filename, openmode mode);

ios::in	Open file for reading
ios::out	Open file for writing
ios::ate	Incial position: end of file
ios::app	Every output is appended at the end of file
ios::trunc	If the file already existed it is erased
ios::binary	Binary mode

ofstream file;
file.open ("example.bin", ios::out | ios::app | ios::binary);

class	default mode to parameter
ofstream	ios::out | ios::trunc
ifstream	ios::in
fstream	ios::in | ios::out

Since frequently the first task that is performed on an object of classes ofstream, ifstream and fstream is to open a file, the three include a constructor that calls directly to this member function and that has the same parameters as this one. This way, we could also have declared the previous object and conducted the same opening operation just writing:

ofstream file ("example.bin", ios::out | ios::app | ios::binary);

You can check if a file has been correctly opened by calling to the member function is_open():

bool is_open();

Close a file

void close ();

In case that an object is destructed and is still associated to an open file, the destructor will automatically call to member function close.

Text mode files

Generally, with text files they are used the same members of these classes that for the communication with console. Like in the following example, where we use the overloaded insertion operator <<:

// writing on a text file
#include <fstream.h>

int main () {
  ofstream examplefile ("example.txt");
  if (examplefile.is_open())
  {
    examplefile << "This is a line.\n";
    examplefile << "This is another line.\n";
    examplefile.close();
  }
  return 0;
}

Data input from file can also be performed in the same way that with cin:

// reading a text file
#include <iostream.h>
#include <fstream.h>

int main () {
  char buffer[256];
  ifstream examplefile ("example.txt");
  if (! examplefile.is_open())
  { cout << "Error opening file"; exit (1); }

  while (! examplefile.eof() )
  {
    examplefile.getline (buffer,100);
    cout << buffer << endl;
  }
  return 0;
}

This last example reads a text file and prints out its content on the screen. Notice how we have used a new member function, called eof that ifstream inherits from class ios and that returns true in case that the end of the file has been reached.

Verification of state flags

bad()

Returns true if any failure occurs in a reading or writing operation. For example in case that it is tried to write to a file that is not open for writing (the same with reading) or if the device where it is tried to write has no space left.

fail()

Returns true in the same cases than bad() plus in case that happens a format error, like if it is tried to read an integer number and an alphabetical character is received.

eof()

Returns true if a file opened for reading has arrived to the end.

good()

It is the most generic: returns false if when calling to anyone of the previous ones it would be received true in anyone.

get and put stream pointers

• ifstream, like istream, has a pointer known as get pointer that points to the next element to be read.
  
• ofstream, like ostream, has a pointer put pointer that points to the location where the next element has to be written.
  
• Finally fstream, like iostream, inherits both: get and put

These stream pointers that point to the reading or writing locations within a stream can be read and/or manipulated using the following member functions:

tellg() and tellp()

These two member functions do not have any parameter and return a value of type pos_type (according ANSI-C++ standard) that is an integer data type representing the current position of get stream pointer (in case of tellg) or put stream pointer (in case of tellp).

seekg() and seekp()

This pair of functions serves respectively to change the position of stream pointers get and put. Both functions are overloaded with two different prototypes:

seekg ( pos_type position );
seekp ( pos_type position );

This prototype serves to change the stream pointer to an absolute position from the beginning of the file. The type required is of the same one than the returned by functions tellg and tellp.

seekg ( off_type offset, seekdir direction );
seekp ( off_type offset, seekdir direction );

Using this prototype, it can be specified an offset from a concrete point determined by parameter direction, that can be:

ios::beg	offset specified from the beginning of the stream
ios::cur	offset specified from the current position of the stream pointer
ios::end	offset specified from the end of the stream

The following example uses the member functions just seen to obtain the size of a binary file:

// obtaining file size
#include <iostream.h>
#include <fstream.h>

const char * filename = "example.txt";

int main () {
  long l,m;
  ifstream file (filename, ios::in|ios::binary);
  l = file.tellg();
  file.seekg (0, ios::end);
  m = file.tellg();
  file.close();
  cout << "size of " << filename;
  cout << " is " << (m-l) << " bytes.\n";
  return 0;
}

Binary files

File streams include two member functions specially thought for input and output of data sequentially: write and read. The first one (write) is a member function of ostream, also inherited by ofstream. And read is member function of istream and it is inherited by ifstream. Objects of class fstream have both. Their prototypes are:

write ( char * buffer, streamsize size );
 read ( char * buffer, streamsize size );

// reading binary file
#include <iostream.h>
#include <fstream.h>

const char * filename = "example.txt";

int main () {
  char * buffer;
  long size;
  ifstream file (filename, ios::in|ios::binary|ios::ate);
  size = file.tellg();
  file.seekg (0, ios::beg);
  buffer = new char [size];
  file.read (buffer, size);
  file.close();

  cout << "the complete file is in a buffer";

  delete[] buffer;
  return 0;
}

Buffer and Synchronization

When the buffer is emptied, all data that contains is written to the physic media (if it is an out stream) or simply erased (if it is an in stream). This process is called synchronization and it takes place under any of the following circumstances:

• When the file is closed: before closing a file any buffer that has not been finished to be written or read is synchronized.
• When the buffer is full: Buffers have a certain size. When the buffer is full it is automaticamente synchronized.
• Explicitly with manipulators: When some manipulators are inserted with the insertion operator << a synchronization takes place. These manipulators are: flush and endl.
• Explicitly with function sync(): Calling the member function sync() without parameters causes an immediate syncronization. This function returns an int value equal to -1 if the flow does not have any buffer associated or on failure.

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