8.4. Inputs/Outputs (I/O) in C++

8.4.1. Streams

The class std::ifstream implements input operations on file based streams.

• In order to use the class std::ifstream you need to include the file fstream with the directive :

  #include <fstream>
  

• the constructor :

  std::ifstream::ifstream(const char* name)

  constructs the named file stream:

  {
    std::ifstream f ("figures.dat");
  
    if (!f)
    {
      std::cerr << "Cannot open the file figures.dat" << std::endl;
      return (1);
    }
    ...
  }
  

• the eof() method

  bool std::ifstream::eof() const

  returns true if the end of the given file stream has been reached.

  {
    std::ifstream f ("figures.dat");
  
    while (!f.eof ())
    {
      ... // Loop on file reading
    }
  
    f.close ();
    ...
  }
  

• the close() method

  void std::ifstream::close()

  closes the file.

• the operator >> enables reading the next word.

  {
    std::ifstream f ("figures.dat");
  
    // Reading word by word
    while (!f.eof ())
    {
      std::string word;
  
      f >> word;
    }
  
    f.close ();
    ...
  }
  

• the getline() method offers two versions:

  1. std::istream& std::getline(char* str, int count)

  reads characters from in input stream until delimiting character (end-of-line per default) is found and saves them to the given string str. If delimiter is found, it is discarded.

  {
    std::ifstream f ("figures.dat");
  
    // Line by line reading
    while (!f.eof ())
    {
      char ligne[256];
      std::string s;
  
      f.getline (ligne, sizeof (ligne));
      s = ligne;
    }
  
    f.close ();
    ...
  }
  

  2. std::istream& std::getline(std::istream& stream, std::string& line)

  reads characters from in input stream until delimiting character (end-of-line per default) is found and saves them to the given string str. If delimiter is found, it is discarded.

  {
    std::ifstream f ("figures.dat");
  
    // Line by line reading
    while (!f.eof ())
    {
      std::string s;
  
      std::getline (f, s);
    }
  
    f.close ();
    ...
  }
  

For more information, see : streams