Input/Output (I/O)

General Info

Input/output related logics are essential components in many computational tasks including interaction with users, file read/write, string parsing/generation

Stream Based I/O

All I/O covered in this section is about stream based I/O which handles data as sequential structures.

There are many other I/O patterns. For instance, with block I/O the program read/write data as blocks.

Standard I/O

• Input/output in the command-line context

  • keyboard input as standard input
  • terminal character print/display as standard output

• iostream - the input/output stream library header

• Standard input

  • std::cin
  • a global variable that you can use anywhere in your program
  • an instance/object of the istream class

• Standard output

  • std::cout
  • a global variable that you can use anywhere in your program
  • an instance/object of the ostream class

• Standard Error

  • std::cerr
  • a global variable that you can use anywhere in your program
  • an instance/object of the ostream class
  • used to display error messages

• Redirection

  In UNIX shells, you can redirect standard I/O using operators like <, >, |.

  • < to redirect standard input from a file
  • > to redirect standard output to a file
  • | to pipeline the standard output of the last command as the standard input of the next program

  ./main < input.txt
  ./main > output.txt
  ./main < input.txt > output.txt
  ./cat input.txt | ./main
  

Input/Output Operations

• Input operations

  • Extraction operator >>

    • work with any input stream including cin
    • can be chained
    • can be used as a boolean value (true if the operation succeed, false otherwise)
    • read and convert to desired type
    • will leave '\n' or endl character in buffer

  • getline() function

    • read in a line as string: getline(cin, inputString);
    • work with any input stream including cin
    • read in string only
    • may read in empty string
    • special handling when used after extraction
    • may use other delimiter i.e. getline(cin, inputString, ',');

  • ignore() method

    • any input stream has this method
    • ignore characters until the first delimiter
    • default delimiter is \n
    • std::cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n'); to be used after >> to remove the rest of the line

• Output operations

  • Insertion operator <<

    • work with any output stream including cout

    • can be chained

    • buffered, display upon flushing

      • inserting '\n' or endl character will trigger flush
      • syntax like cin.flush() to force flush

    • formatting

      • #include <iomanip> header
      • fixed
      • showpoint
      • scientific
      • setprecision()
      • setw()
      • setfill()
      • left
      • right

• Error handling

  Stream operations are likely to fail. Error handling is essential.

  • Methods
    • eof() if the last successful read hit the end of the file
    • fail() if the last read fail
  • Reading errors
    • check eof() if read errors are not expected to happen
    • check fail() if read errors are likely to happen
  • Writing errors

String I/O

Used for text parsing or formatted string generation

• sstream header
• istringstream class
  • an input stream based on the content of a string
  • >> extraction
  • getline()
  • constructor to take a string as parameter
  • .str() to take a string as the content
  • .clear() to clear all contents
  • end of file check
    • .eof() call after every read (extraction, getline)
    • Extraction operation as bool value while (inSS >> value) ...
• ostringstream class
  • an output stream accept formatted output and generate a string
  • << insertion
  • all formatting tools from iomanip will work
  • .str() method to generate a string containing the content in the stream

File I/O

• fstream header
• ifstream class
  • read from a file
• ofstream class
  • write to a file
• bind to a file
  • pass the path string to the constructor
  • call .open() method with the path string
• open()
  • methods to check open status
    • is_open() is preferred
    • fail()
  • very likely to fail so must check status after opening
    • read a non-existing file
    • write a existing file
    • other file system problems
• close() should be called when the file stream is no longer needed
• end of file check
  • .eof() or fail() function call after read operations (extraction, getline)
  • simplified syntax to check the status of reading (same as check fail())
    • while (inFS >> value) { ... }
    • while (getline(inFS, line)) { ... }