Code for my blog post on Higher Order Functions in C++.

higherorder.cc

#include <cassert>
#include <functional>

//
// Classical Function Definition
//
int add(int x, int y) {
  return x + y;
}

int mul(int x, int y) {
  return x * y;
}

//
// Function Objects
//
class Adder {
  public: 
    int operator()(int x, int y) const {
      return x + y;
    }
};

class Multiplier {
  public: 
    int operator()(int x, int y) const {
      return x * y;
    }
};

//
// Function Pointers
//
int resultOfCalculation_ptr(int (*op)(int, int), int x,
                               int y) {
  return (*op)(x, y);
}

//
// std::function
//
int resultOfCalculation_fun(const std::function<int(int, int)>& op, int x,
                            int y) {
  return op(x, y);
}

//
// Templates
//
template <typename FN>
int resultOfCalculation_tpl(const FN& op, int x, int y) {
  return op(x, y);
}


int main() {
  // Instantiate function objects
  Adder add_o;
  Multiplier mul_o;

  // Define lambdas
  auto add_l = [](int x, int y) { return x + y; };
  auto mul_l = [](int x, int y) { return x * y; };

  // Wrap in std::function
  std::function<int(int, int)> add_f(add), add_of(add_o), add_lf(add_l),
      mul_f(mul), mul_of(mul_o), mul_lf(mul_l);

  // Check results
  int x = 1,
      y = 2,
      s = 3,
      p = 2;
  assert(add(x, y) == s);
  assert(add_o(x, y) == s);
  assert(add_l(x, y) == s);
  assert(mul(x, y) == p);
  assert(mul_o(x, y) == p);
  assert(mul_l(x, y) == p);
  assert(add_f(x, y) == s);
  assert(add_of(x, y) == s);
  assert(add_lf(x, y) == s);
  assert(mul_f(x, y) == p);
  assert(mul_of(x, y) == p);
  assert(mul_lf(x, y) == p);

  assert(resultOfCalculation_ptr(add, x, y) == s);
  assert(resultOfCalculation_ptr(mul, x, y) == p);
  //assert(resultOfCalculation_ptr(add_o, x, y) == s);
  //assert(resultOfCalculation_ptr(mul_o, x, y) == p);
  assert(resultOfCalculation_ptr(add_l, x, y) == s);
  assert(resultOfCalculation_ptr(mul_l, x, y) == p);

  assert(resultOfCalculation_fun(add, x, y) == s);
  assert(resultOfCalculation_fun(mul, x, y) == p);
  assert(resultOfCalculation_fun(add_o, x, y) == s);
  assert(resultOfCalculation_fun(mul_o, x, y) == p);
  assert(resultOfCalculation_fun(add_l, x, y) == s);
  assert(resultOfCalculation_fun(mul_l, x, y) == p);

  assert(resultOfCalculation_tpl(add, x, y) == s);
  assert(resultOfCalculation_tpl(mul, x, y) == p);
  assert(resultOfCalculation_tpl(add_o, x, y) == s);
  assert(resultOfCalculation_tpl(mul_o, x, y) == p);
  assert(resultOfCalculation_tpl(add_l, x, y) == s);
  assert(resultOfCalculation_tpl(mul_l, x, y) == p);
}