std::string path_join(...);

path_join.cc

#include <iostream>
#include <sstream>

inline void path_join_append(std::ostringstream* o) { }

template<class... Types>
void path_join_append(std::ostringstream* o, const char* v, Types... args) {
    constexpr char kPathSeparator[] = "/";
    *o << kPathSeparator << v;
    path_join_append(o, args...);
}

template<class... Types>
std::string path_join(const char* value, Types... args) {
    std::ostringstream o;
    o << value;
    path_join_append(&o, args...);
    return o.str();
}

int main() {
    std::cout << path_join("A") << '\n';
    std::cout << path_join("A", "B") << '\n';
    std::cout << path_join("A", "B", "C") << '\n';
    std::cout << path_join("A", "B", "C", "D") << '\n';
    std::cout << path_join("A", "B", "C", "D", "E") << '\n';
}