Recursive lambdas in C++14

main.cpp

// lifted from stackoverflow documentation
// the example can currently be found hosted at:
// http://www.riptutorial.com/cplusplus/example/8508/recursive-lambdas
// original contributors:
// - Barry (https://stackoverflow.com/users/2069064/barry)
// - Yakk - Adam Nevraumont (https://stackoverflow.com/users/1774667/yakk-adam-nevraumont)
// thanks!

#include <iostream>
#include <utility>

template <class F> struct y_combinator {
  F f; // the lambda will be stored here

  // a forwarding operator():
  template <class... Args> decltype(auto) operator()(Args &&... args) const {
    // we pass ourselves to f, then the arguments.
    // the lambda should take the first argument as `auto&& recurse` or similar.
    return f(*this, std::forward<Args>(args)...);
  }
};
// helper function that deduces the type of the lambda:
template <class F> y_combinator<std::decay_t<F>> make_y_combinator(F &&f) {
  return {std::forward<F>(f)};
}

int main() {
  auto gcd = make_y_combinator([](auto &&gcd, int a, int b) -> int {
    return b == 0 ? a : gcd(b, a % b);
  });
  std::cout << gcd(21, 9) << "\n";
  return 0;
}