13.7.1 Using Plain Manipulators

A plain manipulator is a function that performs the following actions:

• Takes a reference to a stream

• Operates on the stream in some way

• Returns its argument

The shift operators taking a pointer to such a function are predefined for iostreams so the function can be put in a sequence of input or output operators. The shift operator calls the function rather than trying to read or write a value. The following example shows a tab manipulator that inserts a tab in an ostream is:

ostream& tab(ostream& os) {
             return os <<’\t’;
            }
...
cout << x << tab << y;

This example is an elaborate way to achieve the following code:

const char tab = ’\t’;
...
cout << x << tab << y;

The following code is another example, which cannot be accomplished with a simple constant. Suppose you want to turn whitespace skipping on and off for an input stream. You can use separate calls to ios::setf and ios::unsetf to turn the skipws flag on and off, or you could define two manipulators.

#include <iostream.h>
#include <iomanip.h>
istream& skipon(istream &is) {
       is.setf(ios::skipws, ios::skipws);
       return is;
}
istream& skipoff(istream& is) {
       is.unsetf(ios::skipws);
       return is;
}
...
int main ()
{
      int x,y;
      cin >> skipon >> x >> skipoff >> y;
      return 1;
}