Assume there are 3 source files:

main.cpp:

#include <iostream>
#include <string>
#include <vector>

#include "funcs.hpp"

void possible_characters(std::string &digits) {
  std::vector<std::string> res;
  
  print_vec(res);
}

int main() {
  std::string ss = "2";
  possible_characters(ss);

  return 0;
}

funcs.hpp

#include <vector>

template <typename T> void print_vec(std::vector<T> &v);

funcs.cpp:

#include "funcs.hpp"

template <typename T> void print_vec(std::vector<T> &v) {
  for (auto &x : v) {
    std::cout << x << ", ";
  }
  std::cout << std::endl;
}

When compiling these srcs using the following cmd:

g++ main.cpp funcs.cpp -o tele_digit

the error occurs:

undefined reference to `void print_vec()...`