#include <iostream>
#include <vector>

template<class T>
struct SumClass {
  T init() { throw 1; }
  T add(const T& a, const T& b) { throw 1; }
};

template<class T>
T Sum(const std::vector<T>& values) {
  SumClass<T> summer = SumClass<T>();
  T res = summer.init();
  for (const T& val : values) {
    res = summer.add(res, val);
  }
  return res;
}

template<>
struct SumClass<int> {
  int init() { return 0; }
  int add(const int& a, const int& b) { return a + b; }
};

template<>
struct SumClass<std::string> {
  std::string init() { return ""; }
  std::string add(const std::string& a, const std::string& b) { return a + b; }
};

int main() {
  std::vector<std::string> strings = {"a", "b", "c"};
  std::cout << Sum(strings) << std::endl; // prints abc
  std::vector<int> ints = {1, 2, 3};
  std::cout << Sum(ints) << std::endl; // prints 6
  return 0;
}