Re: istringstream and setw

From:

"Alex Blekhman" <tkfx.REMOVE@yahoo.com>

Newsgroups:

microsoft.public.vc.language

Date:

Mon, 12 May 2008 18:43:36 +0300

Message-ID:

<OZBRfZEtIHA.3420@TK2MSFTNGP04.phx.gbl>

"Matt Osborn" wrote:

Is my usage of std::setw incorrect? The following code results
in a single character ('t') placed into the string text. If I
replace 'is >> std::setw (1) >> colon' with is.get (), the code
works as expected.

I'm testing this with VC++ 2005.

#include <iostream>
#include <sstream>
#include <iomanip>

int main (int, char*)
{
std::istringstream is ("123:456 text");
int prefix = 0;
if (is >> prefix)
{
char colon = 0;
if (is >> std::setw (1) >> colon)
{
  int suffix = 0;
  if (is >> suffix)
  {
   std::string text;
   if (is >> text)
   {
    std::cout << text;
   }
  }
}
}
}

Aparently the width value is retained by `std::ios_base' object
and used afterwards for strings. I'm not sure why subsequent
integer extraction is not influenced by this setting. In any case
you can use the following workarounds:

1. Do not use `std::setw' at all. When you extract just one
character there is no sense in setting the width to 1. Only one
character will be extracted anyway.

2. Reset the width value to default (0) before extracting strings:

is >> std::setw (0) >> text

This way you will obtain full string until next delimiter.

3. Use sscanf/sscanf_s. This approach is not as nice and object
oriented as C++ streams, however is much easier to use. I
personally find C++ streams painful when you need to do formatted
input/output.

HTH
Alex