#include <iterator>

template<typename ValueType>
class FormattedBackInserter: public std::iterator<std::output_iterator_tag, void, void, void, void>
{
   public:
      explicit FormattedBackInserter(std::string& target): 
         first_(true),
         target_(target)
      { }

      FormattedBackInserter<size_t>& operator=(const ValueType& value)
      {
         if (!first_)
            target_.push_back(' ');
         else
            first_ = false;

         insert(value);

         return *this;
      }

      /// Simply returns *this.
      FormattedBackInserter& operator*()
      { return *this; }

      /// Simply returns *this.  (This %iterator does not "move".)
      FormattedBackInserter& operator++()
      { return *this; }

      /// Simply returns *this.  (This %iterator does not "move".)
      FormattedBackInserter operator++(int)
      { return *this; }

   private:
      void insert(const ValueType& value)
      {
         if (value > 9)
            insert(value/10);

         target_.push_back('0' + value % 10);
      }

      bool first_;
      std::string& target_;
};