General-purpose tokenizer with multi-delimiters in C++

#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;
using std::stoi;
using std::stod;

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

// Declare a generic conversion function...
template <typename T>
T decode(const string& x);

template <>
inline string decode(const string& x) {
  return x;
}

template <>
inline int decode(const string& x) {
  return stoi(x);
}

template <>
inline double decode(const string& x) {
  return stod(x);
}

// given a string with delimiters inside, parse it into
//  individual tokens stored in a vector<T>
template <typename T>
void tokenize(const string& str, vector<T>& 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) {
    // Add the extracted token after converting it to the proper type.
    tokens.emplace_back(decode<T>(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
  }
}

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;
}