C++ join function that works on strings and chars - c++

Question

I've written a join() function that works on iterators, with this signature:
template <typename Iter>
string join(Iter begin, Iter end, string sep = ", ");
I'd like to work for both iterators to strings and iterators to chars. But it uses string::append(*iter), which doesn't work for chars. How can I specialize it?
Edit: Here is my implementation. It works great for collections of strings, but not collections of chars.
template <typename Iter>
string join(Iter begin, Iter end, string sep = ", ") {
string result;
for (Iter i = begin; i != end; ++i) {
if (result.size() > 0) {
result.append(sep);
}
result.append(*i); // <-- fails if called with a collection of chars
}
return result;
}
#HolyBlackCat's suggestion of += makes it work!

Answer 1

As #HolyBlackCat mentioned, you should be able to use the += operator here. In general you could create concepts though to create overloads for the relevant functionality:
template<class Iterator>
concept StringIterator = requires(Iterator i)
{
{ *i } -> std::convertible_to<std::string const&>;
};
template<class Iterator>
concept CharIterator = requires(Iterator i)
{
{ *i } -> std::convertible_to<char>;
};
template<StringIterator Iter>
void Append(std::string& target, Iter pos)
{
target.append(*pos);
}
template<CharIterator Iter>
void Append(std::string& target, Iter pos)
{
target.push_back(*pos);
}
template<class Iter>
std::string join(Iter begin, Iter end, std::string const& sep = ", ")
{
std::string result;
if (begin != end)
{
Append(result, begin);
for (++begin; begin != end; ++begin)
{
result += sep;
Append(result, begin);
}
}
return result;
}
int main() {
std::vector<std::string> strings{ "foo", "bar", "baz"};
std::string s = "Hello World";
std::cout << join(strings.begin(), strings.end()) << '\n';
std::cout << join(s.begin(), s.end()) << '\n';
}