More on I/O

• The stream pointer is associated with a stream.
  • It indicates the place that the operation will occur.
• Remember, we said that the code cin >> varName will
  • Skip leading whitespace
  • Attempt to read a data value of the type of varName.
  • Will stop when it can't read any further or fails.
  • If it is successful, it will store the new value in varName.
• We can picture what happens like this.
  • If varName is a string.
  • Assume that ^ is the stream pointer.

  • \n
    ^
    ____\t__Hello___\n 

  • It will first skip whitespace, include space (_), tab (\t) and newlines (\n).

  • \n
    ____\t__Hello___\n
            ^
    

  • It will then read in all non-whitespace characters and store them in varName.

  • \n
    ____\t__Hello___\n
                 ^ 
    

  • It will stop on the first whitespace character it encounters. varName will now contain "Hello". And the next character in the input buffer is a space.
    • Model what happens when reading in an int and the stream contains +3.14
    • There are other input modes. We are also concerned about getline
      • reference
      • getline(input stream, string identifier [, char delimiter])
      • This will:
        • Starting at the current position of the stream pointer
        • Read in all characters until the delimiter is encountered.
        • Store these characters in the string.
        • Discard the delimiter.
      • This allows us to read in whitespace.
      • By default the delimiter is the new line character (\n).

      • string name
        cout << "Enter your full name =>";
        getline(cin, name);
                

      • Marry, Queen of Scots\n
        ^
        next line\n
                

      • After the read, the stream will look like this:

      • Marry, Queen of Scots\n
        next line\n
        ^
                

      • And name will contain Mary, Queen of Scots.
      • The \n will be discarded.
    • Caution:

      • int n;
        string s;
        
        cout << "Enter a number =>";
        cin >> n;
        cout << "Enter a phrase =>";
        getline(cin, s);
                

      • This code will produce unexpected results:

      • 5\n
        ^
        Marry, Queen of Scots\n
        next line\n
                

      • After reading in the integer the stream will look like this

      • 5\n
        Marry, Queen of Scots\n
        ^
        next line\n
                

      • And name will contain blank. Why?
    • Solution:
      • Two possible solutions.
        1. Do the getline twice.
        2. inputstream.ignore(int count, char delim)
           • This will skip either
             • count characters, including white space.
             • Or until the character delim is encountered.
             whichever comes first.

      • int n;
        string s;
        
        cout << "Enter a number =>";
        cin >> n;
        cin.ignore(100, '\n');
        cout << "Enter a phrase =>";
        getline(cin, s);

      • We just guess at the number of spaces to skip.
    • There are other cin functions.
      • cin.get() returns the next character, regardless of whitespace. It also advance the stream pointer.
      • cin.unget() backs the stream pointer up one character.