Two overloads called join_strings() are listed in the following code. One takes a container of strings and a pointer to a sequence of characters representing a separator, while the other takes two random access iterators, representing the first and one past the last element of a range, and a separator. They both return a new string created by concatenating all the input strings, using an output string stream and the std::copy function. This general-purpose function copies all the elements in the specified range to an output range, represented by an output iterator. We are using here an std::ostream_iterator that uses operator<< to write the assigned value to the specified output stream each time the iterator is assigned a value:

template <typename Iter>
std::string join_strings(Iter begin, Iter end, 
                         char const * const separator)
{
   std::ostringstream os;
   std::copy(begin, end-1, 
             std::ostream_iterator<std::string>(os, separator));
   os << *(end-1);
   return os.str();
}

template <typename C>
std::string join_strings(C const & c, char const * const separator)
{
   if (c.size() == 0) return std::string{};
   return join_strings(std::begin(c), std::end(c), separator);
}

int main()
{
   using namespace std::string_literals;
   std::vector<std::string> v1{ "this","is","an","example" };
   std::vector<std::string> v2{ "example" };
   std::vector<std::string> v3{ };
 
   assert(join_strings(v1, " ") == "this is an example"s);
   assert(join_strings(v2, " ") == "example"s);
   assert(join_strings(v3, " ") == ""s);
}