Map function on C++

map.cc

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

template <class T, class F>
decltype(auto) map(const std::vector<T> a, const F fn) {
  std::vector<decltype( fn(a[0]) )> result = {};
  std::transform(a.cbegin(), a.cend(), std::back_inserter(result), fn);
  return result;
}

int main() {
  const std::vector<int> v = { 3, 5, 6};
  const auto result = map(v, [](const int x){return std::to_string(x * 2); });

  std::for_each(result.begin(), result.end(),
                [](const std::string& s) { std::cout << s << std::endl; });

  std::for_each(v.begin(), v.end(),
                [](const int n) { std::cout << n << std::endl; });

}