C++ Cookbook

10.3. Writing Your Own Stream Manipulators

Problem

You need a stream manipulator that does something the standard ones can’t. Or, you want to have a single manipulator set several flags on the stream instead of calling a set of manipulators each time you want a particular format.

Solution

To write a manipulator that doesn’t take an argument (à la left), write a function that takes an ios_base parameter and sets stream flags on it. If you need a manipulator that takes an argument, see the discussion a little later. Example 10-4 shows how to write a manipulator that doesn’t take an argument.

Example 10-4. A simple stream manipulator

#include <iostream>
#include <iomanip>
#include <string>

using namespace std;

// make floating-point output look normal
inline ios_base& floatnormal(ios_base& io) {
   io.setf(0, ios_base::floatfield);
   return(io);
}
int main() {

   ios_base::fmtflags flags =  // Save old flags
      cout.flags();

   double pi = 22.0/7.0;

   cout << "pi = " << scientific   // Scientific mode
        << pi * 1000 << '\n';

   cout << "pi = " << floatnormal
        << pi << '\n';

   cout.flags(flags);
}

Discussion

There are two kinds of manipulators: those that accept arguments and those that don’t. Manipulators that take no arguments are easy to write. All you have to do is write a function that accepts a stream parameter, does something to it (sets a flag or changes a setting), and returns it. Writing a manipulator that takes one or more arguments is more complicated because you need to create additional classes and functions ...