Lesson 10 - File IO

C++ provides the following classes to perform output and input of characters to/from files:

• std::ofstream: Stream class to write to files
• std::ifstream: Stream class to read from files
• std::fstream: Stream class to both read and write to/from files.

These classes are derived directly or indirectly from the classes std::istream and std::ostream. std::cin is an object of class std::istream and std::cout is an object of class std::ostream. File streams are used the same way as std::cin and std::cout, with the only difference that these streams are associated with physical files.

#include <iostream>
#include <fstream>

int main()
{
  std::ofstream myfile;
  myfile.open("example.txt");
  myfile << "Writing this to a file.\n";
  myfile.close();
  return 0;
}

open (filename, mode);

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

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

Combining object construction and stream opening in a single statement. Both forms to open a file are valid and equivalent.

To check if a file stream was successful opening a file, you can do it by calling to member is_open. This member function returns a bool value of true in the case that indeed the stream object is associated with an open file, or false otherwise:

if (myfile.is_open()) { /* ok, proceed with output */ }

Closing a file

When we are finished with our input and output operations on a file we shall close it so that the operating system is notified and its resources become available again. For that, we call the stream's member function close. This member function flushes the associated buffers and closes the file:

myfile.close();

Once this member function is called, the stream object can be re-used to open another file, and the file is available again to be opened by other processes.

In case that an object is destroyed while still associated with an open file, the destructor automatically calls the member function close.

Text files

Text file streams are those where the std::ios::binary flag is not included in their opening mode. These files are designed to store text and thus all values that are input or output from/to them can suffer some formatting transformations, which do not necessarily correspond to their literal binary value.

Writing operations on text files are performed in the same way we operated with std::cout

#include <iostream>
#include <fstream>

int main()
 {
  std::ofstream myfile{“example.txt”};
  if (myfile.is_open())
  {
    myfile << "This is a line.\n";
    myfile << "This is another line.\n";
    myfile.close();
  }
  else std::cout << "Unable to open file" << std::endl;
  return 0;
}
#include <iostream>
#include <fstream>
#include <string>

int main()
{
  std::string line;
  std::ifstream myfile{“example.txt”};
  if (myfile.is_open())
  {
    while ( std::getline(myfile,line) )
    {
      std::cout << line << std::endl;
    }
    myfile.close();
  }
  else std::cout << "Unable to open file" << std::endl; 
  return 0;
}

This last example reads a text file and prints out its content on the screen. We have created a while loop that reads the file line by line, using std::getline function. The value returned by std::getline is a reference to the stream object itself, which when evaluated as a boolean expression (as in this while-loop) is true if the stream is ready for more operations, and false if either the end of the file has been reached or if some other error occurred.

Checking state flags

The following member functions exist to check for specific states of a stream (all of them return a bool value):

• bad() - Returns true if a reading or writing operation fails. For example, in the case that we try to write to a file that is not open for writing or if the device where we try to write has no space left.
• fail() - Returns true in the same cases as bad(), but also in the case that a format error happens, like when an alphabetical character is extracted when we are trying to read an integer number.
• eof() - Returns true if a file open for reading has reached the end.
• good() - It is the most generic state flag: it returns false in the same cases in which calling any of the previous functions would return true. Note that good() and bad() are not exact opposites (good() checks more state flags at once).

The member function clear() can be used to reset the state flags.

Stream positioning

All I/O stream objects keep internally at least one internal position:

• std::ifstream, like std::istream, keeps an internal get position with the location of the element to be read in the next input operation.
• std::ofstream, like std::ostream, keeps an internal put position with the location where the next element has to be written.
• Finally, std::fstream, keeps both, the get and the put position, like std::iostream.

These internal stream positions point to the locations within the stream where the next reading or writing operation is performed. These positions can be observed and modified using the following member functions:

tellg() and tellp()

These two member functions with no parameters return a value of the member type std::streampos, which is a type representing the current get position (in the case of tellg()) or the put position (in the case of tellp()).

seekg() and seekp()

These functions allow to change the location of the get and put positions. Both functions are overloaded with two different prototypes. The first form is:

seekg( position );
seekp( position );

Using this prototype, the stream pointer is changed to the absolute position (counting from the beginning of the file). The type for this parameter is std::streampos, which is the same type as returned by functions tellg() and tellp().

The other form for these functions are:

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