14.7.2 Parameterized Manipulators

One of the parameterized manipulators that is included in iomanip.h is setfill. setfill sets the character that is used to fill out field widths. It is implemented as shown in the following example:

//file setfill.cc
#include<iostream.h>
#include<iomanip.h>

//the private manipulator
static ios& sfill(ios& i, int f) {
         i.fill(f);
         return i;
}
//the public applicator
smanip_int setfill(int f) {
       return smanip_int(sfill, f);
}

A parameterized manipulator is implemented in two parts:

• The manipulator. It takes an extra parameter. In the previous code example, it takes an extra int parameter. You cannot place this manipulator function in a sequence of input or output operations, since there is no shift operator defined for it. Instead, you must use an auxiliary function, the applicator.

• The applicator. It calls the manipulator. The applicator is a global function, and you make a prototype for it available in a header file. Usually the manipulator is a static function in the file containing the source code for the applicator. The manipulator is called only by the applicator, and if you make it static, you keep its name out of the global address space.

Several classes are defined in the header file iomanip.h. Each class holds the address of a manipulator function and the value of one parameter. The iomanip classes are described in the man page manip(3CC4). The previous example uses the smanip_int class, which works with an ios. Because it works with an ios, it also works with an istream and an ostream. The previous example also uses a second parameter of type int.

The applicator creates and returns a class object. In the previous code example the class object is an smanip_int, and it contains the manipulator and the int argument to the applicator. The iomanip.h header file defines the shift operators for this class. When the applicator function setfill appears in a sequence of input or output operations, the applicator function is called, and it returns a class. The shift operator acts on the class to call the manipulator function with its parameter value, which is stored in the class.

In the following example, the manipulator print_hex:

• Puts the output stream into the hex mode.

• Inserts a long value into the stream.

• Restores the conversion mode of the stream.

The class omanip_long is used because this code example is for output only, and it operates on a long rather than an int:

#include <iostream.h>
#include <iomanip.h>
static ostream& xfield(ostream& os, long v) {
        long save = os.setf(ios::hex, ios::basefield);
        os << v;
        os.setf(save, ios::basefield);
        return os;
    }
omanip_long print_hex(long v) {
       return omanip_long(xfield, v);
   }