How can I print a list of elements separated by commas?

I know how to do this in other languages, but not in C++, which I am forced to use here.

I have a set of strings (keywords) that I'm printing to out as a list, and the strings need a comma between them, but not a trailing comma. In Java, for instance, I would use a StringBuilder and just delete the comma off the end after I've built my string. How can I do it in C++?

auto iter = keywords.begin();
for (iter; iter != keywords.end( ); iter++ )
{
    out << *iter << ", ";
}
out << endl;

I initially tried inserting the following block to do it (moving the comma printing here):

if (iter++ != keywords.end())
    out << ", ";
iter--;

Use an infix_iterator:

// infix_iterator.h 
// 
// Lifted from Jerry Coffin's 's prefix_ostream_iterator 
#if !defined(INFIX_ITERATOR_H_) 
#define  INFIX_ITERATOR_H_ 
#include <ostream> 
#include <iterator> 
template <class T, 
          class charT=char, 
          class traits=std::char_traits<charT> > 
class infix_ostream_iterator : 
    public std::iterator<std::output_iterator_tag,void,void,void,void> 
{ 
    std::basic_ostream<charT,traits> *os; 
    charT const* delimiter; 
    bool first_elem; 
public: 
    typedef charT char_type; 
    typedef traits traits_type; 
    typedef std::basic_ostream<charT,traits> ostream_type; 
    infix_ostream_iterator(ostream_type& s) 
        : os(&s),delimiter(0), first_elem(true) 
    {} 
    infix_ostream_iterator(ostream_type& s, charT const *d) 
        : os(&s),delimiter(d), first_elem(true) 
    {} 
    infix_ostream_iterator<T,charT,traits>& operator=(T const &item) 
    { 
        // Here's the only real change from ostream_iterator: 
        // Normally, the '*os << item;' would come before the 'if'. 
        if (!first_elem && delimiter != 0) 
            *os << delimiter; 
        *os << item; 
        first_elem = false; 
        return *this; 
    } 
    infix_ostream_iterator<T,charT,traits> &operator*() { 
        return *this; 
    } 
    infix_ostream_iterator<T,charT,traits> &operator++() { 
        return *this; 
    } 
    infix_ostream_iterator<T,charT,traits> &operator++(int) { 
        return *this; 
    } 
};     
#endif 

Usage would be something like:

#include "infix_iterator.h"

// ...
std::copy(keywords.begin(), keywords.end(), infix_iterator(out, ","));

In an experimental C++17 ready compiler coming soon to you, you can use std::experimental::ostream_joiner:

#include <algorithm>
#include <experimental/iterator>
#include <iostream>
#include <iterator>

int main()
{
    int i[] = {1, 2, 3, 4, 5};
    std::copy(std::begin(i),
              std::end(i),
              std::experimental::make_ostream_joiner(std::cout, ", "));
}

Live examples using GCC 6.0 SVN and Clang 3.9 SVN

Because everyone has decided to do this with while loops, I'll give an example with for loops.

for (iter = keywords.begin(); iter != keywords.end(); iter++) {
  if (iter != keywords.begin()) cout << ", ";
  cout << *iter;
}

Assuming a vaguely normal output stream, so that writing an empty string to it does indeed do nothing:

const char *padding = "";
for (auto iter = keywords.begin(); iter != keywords.end(); ++iter) {
    out << padding << *iter;
    padding = ", "
}

One common approach is to print the first item prior to the loop, and loop only over the remaining items, PRE-printing a comma before each remaining item.

Alternately you should be able to create your own stream that maintains a current state of the line (before endl) and puts commas in the appropriate place.

EDIT: You can also use a middle-tested loop as suggested by T.E.D. It would be something like:

if(!keywords.empty())
{
    auto iter = keywords.begin();
    while(true)
    {
        out << *iter;
        ++iter;
        if(iter == keywords.end())
        {
            break;
        }
        else
        {
            out << ", ";
        }
    }
}

I mentioned the "print first item before loop" method first because it keeps the loop body really simple, but any of the approaches work fine.