Fixed-point (Y) combinator in C++

fix.cpp

#include <functional>
#include <iostream>
using namespace std;

template <typename F>
struct Y
{
  Y(F f) : m_f(f) {}

  template <typename T>
  auto operator()(T&& t) const
  {
    return m_f(*this, std::forward<T>(t));
  }

  F m_f;
};

template <typename F>
Y<F> fix(F&& f)
{
  return Y<F>(forward<F>(f));
}

int main()
{
  auto f = fix(
      [] (auto f, int n)
      {
        if (n == 0) return 1;
        return n * f(n-1);
      });

  cout << f(4) << endl;

  return 0;
}