Re: iomanip - alignment

 James Kanze <>
Fri, 19 Oct 2007 09:10:37 -0000
On Oct 18, 11:04 pm, brekehan <> wrote:

I can't seem to get the alignment to switch back and forth combined
with a set width. I tryed a search on this group and alot of googling,
but still can't deduce the problem.

It works fine for me, both with g++ and with Sun CC. However...

#include <iostream>
#include <iomanip>

using namespace std;

int main()
   // using cout instead of ostream & operator << method, for example pur=


   // hardcoding values instead of class data for example purposes

   cout << setiosflags(ios::fixed);

I'm not sure that this is defined behavior. I'd normally use
    std::cout << std::fixed ;
    std::cout.setf( std::ios::fixed, std::ios::floatfield ) ;

(In a real program, of course, you'll almost always be using
custom manipulators, defined in accordance with the semantics of
the data being output.)

   cout << left << setw(25) << "attribute name:";
   cout << right << setw( 6) << 0.111;

Note that the setw here doesn't do anything, since you're
generating more than six characters anyway ("0.111000").

   cout << endl;

   cout << left << setw(25) << "attribute name";
   cout << right << setw( 6) << 0.123556;

Same comment as above for the setw. This generates "0.123556".

   cout << endl;

   // etc etc

   cout << resetiosflags(ios::fixed);

Again, this isn't guaranteed to do what you think. "std::cout
<< std::resetiosflags( ios::floatfield )" would do the trick,
"std::cout << std::defaultfloat" is probably more readable, but
normally, if the goal is to restore the previous context, you'll
read the previous context before changing it, typically by means
of some sort of RAII class. (Of course, if you're using custom
manipulators, you'll arrange for them to restore the original
state at the end of the full expression, so this won't be

   return 0;


The output seems to always be left aligned.

Try putting markers around it, and I think you'll see what the
problem is. Something like:
    std::cout << '|' << std::left << std::setw( 25) << "attr:" <<
'|' ;
    std::cout << '|' << std::right << std::setw( 6 ) << 1.23 << '|' ;
This will show exactly how each field is being formatted.

I've also tryed using <<
setiosflags and << resetiosflags with the same results.

Can anyone clear this up for me? I hate to admit that I've forgotten
something so basic.

What is the lifetime of the differant iomanip members used here?
How are right and left alignments represented internally? i.e do you
need to reset left before using right and vica versa? or are they two
states of the same bit.

Alignment is a field in ios, consisting of at least 2 bits. To
assign to it, you have to do something like:

    std::setf( std::ios::left, std::ios::alignfield ) ;

The second argument tells setf to reset the bits first. This is
exactly what std::left does, however, so there should be no
problem using the manipulator.

James Kanze (GABI Software)
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