How to deal with last comma, when making comma separated string?

Detecting the one before last is always tricky, detecting the first is very easy.

bool first = true;
stringstream ss;
BOOST_FOREACH(int i, VecInts)
{
  if (!first) { ss << ","; }
  first = false;
  ss << i;
}

Using Karma from Boost Spirit - has a reputation for being fast.

#include <iostream>
#include <vector>
#include <boost/spirit/include/karma.hpp>

int main()
{
  std::vector<int> v;
  v.push_back(1);
  v.push_back(2);
  v.push_back(3);

  using namespace boost::spirit::karma;
  std::cout << format(int_ % ',', v) << std::endl;
}

How about this:

#include <iostream>
#include <vector>
#include <algorithm>
#include <iterator>
#include <string>
#include <sstream>

int main()
{
   std::vector<int> v;

   v.push_back(1);
   v.push_back(2);
   v.push_back(3);
   v.push_back(4);
   v.push_back(5);

   std::ostringstream ss;
   if(!v.empty()) {
      std::copy(v.begin(), std::prev(v.end()), std::ostream_iterator<int>(ss, ", "));
      ss << v.back();
   }
   std::cout << ss.str() << "\n";
}

No need to add extra variables and doesn't even depend on boost! Actually, in addition to the "no additional variable in the loop" requirement, one could say that there is not even a loop :)