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

Note

It is always a good practice to group I/O related logics in a minimal number of classes/functions because they are not testable with automatic testing.

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, ',');

      Warning

      differentiate it from the cin.getline() function!

    • 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

        Note

        All above manipulators are sticky except setw. Sticky means that once it is set, it will be affective until it is set again later. However, the effect of setw will only affect the next insertion operation.

  • 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

Note

You can also manipulate strings using string methods. Check the string document for more information.

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)) { ... }

Warning

while (inFS >> value) will work but while (inFS >> value == true) will fail! The expression must be directly used in the parenthesis. Same rule applies to getline(inFS, line) call.