An example of implementing type classes in C++.

tc.cpp

/*
  To compile and run -

  % g++ tc.cpp -std=c++11 -o tc
  % ./tc
  2, 4, 6, 8
*/

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

using namespace std;

template <template <class, class...> class f>
struct functor {
  template<typename a, typename b>
  static f<b>* map(const f<a>* fa, function<b(a)> ab);
};

template<>
struct functor<vector> {
  template<typename a, typename b>
  static vector<b>* map(const vector<a>* va, function<b(a)> ab) {
    vector<b>* vb = new vector<b>(va->size());
    for (unsigned i = 0; i < va->size(); ++i) {
      const a aval = (*va)[i];
      const b bval = ab(aval);
      (*vb)[i] = bval;
    }
    return vb;
  }
};

int main(int argc, char** argv) {
  vector<int> vi { 1, 2, 3, 4 };
  vector<string>* vs = functor<vector>::map<int, string>(&vi, [](int i) { return to_string(i*2); });
  for (auto it = vs->begin(); it < vs->end(); ++it) {
    if (it > vs->begin()) cout << ", ";
    cout << *it;
  }
  cout << '\n';
  return 0;
}