The LINT Public Interface: Members and Friends - Cryptography in C and C++ - page 347

15.3.2 Manipulators
Building on the previous mechanisms, we would like in this section to obtain
more convenient possibilities for controlling the output format for LINT objects.
To this end we use manipulators , which are placed directly into the output stream
and thus display the same effects as occur in calling the above status functions.
Manipulators are addresses of functions for which there exist special insert
operators that on their part accept a pointer to a function as argument. As an
example we consider the following function:
ostream& LintHex (ostream& s)
{
LINT::setf (s, LINT::linthex);
return s;
}
This function calls the status function setf(s, LINT::linthex) in the context
of the specified output stream ostream& s and thereby effects the output of LINT
objects as hexadecimal numbers. The name LintHex of the function without
parentheses is viewed as a pointer to the function (cf. [Lipp], page 202) and can
be set in an output stream as a manipulator with the help of the insert operator
ostream& ostream::operator<< (ostream& (*pf)(ostream&))
{
return (*pf)(*this);
}
defined in the class ostream :
LINT a ("0x123456789abcdef0");
cout << LintHex << a;
ostream s;
s << LintDec << a;
The LINT manipulator functions operate according to the same pattern as
the standard manipulators in the C++ library, for example dec , hex , oct , flush ,
and endl : The insert operator << simply calls the manipulator function LintHex()
or LintDec() at the appropriate place. The manipulators ensure that the status
flags belonging to the output streams cout , respectively s , are set. The overloaded
operator<< for the output of LINT objects takes over the representation of the LINT
object a in the requested form.