33.1 A Recap of Manipulators

We showed examples of manipulators in Section 28.3. There we learned that:

• Manipulators are objects that can be inserted into or extracted from a stream.

• Such insertions and extractions have specific desirable side effects.

As a recap, here is a typical example of two manipulators:

std::cout << std::setw(10) << 10.55 << std::endl;

The inserted objects std::setw(10) and std::endl are the manipulators. As its only side effect, the manipulator setw(10) sets the stream's field width to 10. Similarly, the manipulator std::endl inserts the end of line character and flushes the output.

As we mentioned previously, extensibility is a major advantage of iostreams. We've seen in the previous Section 32.3 how you can implement inserters and extractors for user-defined types that behave like the built-in input and output operations. Additionally, you can add user-defined manipulators that fit seamlessly into the iostreams framework. In this section, we show how to do this.

First of all, to be extracted or inserted, a manipulator must be an object of a type that we call manipT, for which overloaded versions of the shift operators exist. (Associated with the manipulator type manipT, there is usually a function called f_manipT()that we explain in detail later.) Here's the pattern for the manipulator extractor:

template <class charT, class Traits>
std::basic istream<charT,Traits>&
operator>>(std::basic istream<charT,Traits>& istr,
           const manipT& manip)
{   return fmanipT(istr, ...);   }

With this extractor defined, you can extract a manipulator Manip, which is an object of type manipT, by simply saying:

std::cin >> Manip;

This results in a call to the operator>>() sketched out above. The manipulator inserter is analogous.

A manipulator's side effect is often created by calling an associated function f_manipT() that takes a stream and returns the stream. There are several ways to associate the manipulator type manipT to the function f_manipT(), which we explore in the following sections. The iostream framework does not specify a way to implement manipulators, but there is a simple way to write your own manipulators. We explain this technique along with other useful approaches.

It turns out that there is a major difference between manipulators with parameters like std::width(10) and manipulators without parameters like std::endl. Let's start with the simpler case of manipulators without parameters.