String tokenizer using a string of multiple delimiters

#include <iostream>
#include <fstream>
#include <algorithm>
#include <vector>
#include <string>

constexpr auto str_end = std::string::npos;

using std::vector;
using std::string;
using std::ostream;
using std::ifstream;
using std::getline;
using std::cout;
using std::endl;

// stream vector elements out
template <typename T>
ostream& operator<<(ostream& os, const vector<T>& vec) {
  for (const auto& elem : vec) os << elem << ' ';

  return os;
}

// given a string with delimiters inside, parse it into
//  individual tokens stored in a vector<string>
void tokenize(const string& str, vector<string>& tokens,
              const string& delimiters = " ") {
  auto last_pos = str.find_first_not_of(delimiters, 0);      // first token
  auto curr_pos = str.find_first_of(delimiters, last_pos);   // next delim

  while (curr_pos != str_end || last_pos != str_end) {
    tokens.emplace_back(str.substr(last_pos, curr_pos - last_pos));
    last_pos = str.find_first_not_of(delimiters, curr_pos);  // next token
    curr_pos = str.find_first_of(delimiters, last_pos);      // next delim
  }
}

/*
// general wrapper ?
// -exclude string T's somehow?
template <typename T>
void tokenize(const string& str, vector<T>& tokens,
              const string& delimiters = " ") {
  vector<string> str_tokens;
  tokenize(str, str_tokens, delims);

  for (const auto& e : str_tokens)
    tokens.emplace_back(  ??? (e));  // need T's type, then the right conversion
}
*/

// int wrapper
void tokenize(const string& str, vector<int>& tokens,
              const string& delimiters = " ") {
  vector<string> str_tokens;
  tokenize(str, str_tokens, delims);

  for (const auto& e : str_tokens)
    tokens.emplace_back(stoi(e));  // ints
}

// double wrapper
void tokenize(const string& str, vector<double>& tokens,
              const string& delimiters = " ") {
  vector<string> str_tokens;
  tokenize(str, str_tokens, delims);

  for (const auto& e : str_tokens)
    tokens.emplace_back(stod(e));  // doubles
}

int main() {
  ifstream fs{"data"};
  string tmp{""};
  const string delims{"[,]"};
  //vector<string> tokens;
  //vector<int> tokens;
  vector<double> tokens;

  while (getline(fs, tmp)) tokenize(tmp, tokens, delims);

  cout << tokens << endl;
}