simple map reduce with C++ 11. The sample maps an integer to a string, and reduces with reverse concatenation.

map_reduce

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

template <typename R, typename T, typename M>
auto map_reduce(const std::vector<T>& input, std::function<M(const T&)> mapper, std::function<R(const M& m, const R& r)> reducer, const R& init) -> decltype(reducer(std::declval<const M&>(), std::declval<const R&>()))
{
  R r = init;
  for (auto const & i: input) r = reducer(mapper(i), r);
  return r;
};

int main()
{
  std::vector<int> input({1, 2, 3, 4, 5, 6});
  std::string x = map_reduce<std::string, int, std::string>(input,
    [](const int& i) { return std::to_string(i); },
    [](const std::string& b, const std::string& r) { return r.empty()? b: b + "," + r; },
    ""
  );

  std::cout << x << std::endl;

  return 0;
}