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iostream manipulator construction and design

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I've covered the iostream operations in Stroustrup but there is a section
that I'm failing to understand in section 21.4.6.1 (of the third edition
of C++ - The Programming Language) which I hope that someone can help
me decipher.  The section discusses Manipultors that take arguments and
Stroustrup states

<QUOTE>
"Manipulators that take arguments can also be useful.  For example, we
might want to write

   cout << setprecision(4) << angle;

to print the value of the floating-point variable angle with four
digits.

To do this, setprecision must return an object that is initialized by 4
and that calls cout::setprcision when involked.  Such a manipulator is a
function object thatr is involed by << rather than by ().  The exact
tyoe of function object is implementation-defined, but it might be
defined like this:

   struct smanip{
      ios_base& (*f)(ios_base&, int); //function to be called
      int i;

      smanip(ios_base& (*ff) (ios_base&, int), int ii) : f(ff, i(ii){}
   };

   template<class Ch, class Tr>
   ostream<Ch, Tr>& operator<<(ostream<Ch, Tr>& os, smanip& m)
   {
      return m.f(os, m.i);
   }

The smanip constructor stores its arguments in f and i and the
operator<< calls f(i).  We can now define setprecision() like this:

   ios_base& set_precision(ios)base& s, int n)  //helper
   {
      return s.setprecision(n); //call member function
   }

   inline smap setprecision(int n)
   {
	 return smanip(set_precision, n);
   }

We can now write:
   cout << setprecision(4) << angle;
"   
</QUOTE>

What don't understand a couple of things here, and this might be an
accumulation of increasing confusion on my part as I've continued
through this chapter.

First, I just don't understand what the problem is that is being solved.
Why can't a manipulator just take an argument of an int and return a
base_stream object, like the manipulators that don't take an argument?

Secondly, the exqmple completely befuddles me.  I'm guessing that don't
understand the order of operations of 

   cout << setprecision(4) << angle;

   is ostream<< called first and then the function setprecision?

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iostream manipulator construction and design Ruben Safir <ruben@mrbrklyn.com> - 2011-07-06 02:55 +0000
  Re: iostream manipulator construction and design Michael DOUBEZ <michael.doubez@free.fr> - 2011-07-06 01:00 -0700
  Re: iostream manipulator construction and design Ruben Safir <ruben@mrbrklyn.com> - 2011-07-06 15:01 +0000
    Re: iostream manipulator construction and design "Alf P. Steinbach /Usenet" <alf.p.steinbach+usenet@gmail.com> - 2011-07-06 19:13 +0200
      Re: iostream manipulator construction and design Ruben Safir <ruben@mrbrklyn.com> - 2011-07-06 18:00 +0000
      Re: iostream manipulator construction and design Ruben Safir <ruben@mrbrklyn.com> - 2011-07-07 02:51 +0000
        Re: iostream manipulator construction and design "Alf P. Steinbach /Usenet" <alf.p.steinbach+usenet@gmail.com> - 2011-07-07 11:13 +0200

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