#include <iostream>

void increment_by_value(int x) {
  x += 1;
}

void increment_by_reference1(int* x) {
  *x += 1;
}

void increment_by_reference2(int& x) {
  x += 1;
}

int main() {

  int x = 100;
  std::cout << "original: " << x << std::endl;

  increment_by_value(x);
  std::cout << "after by-value: " << x << std::endl;

  increment_by_reference1(&x);
  std::cout << "after by-reference (pointer): " << x << std::endl;

  increment_by_reference2(x);
  std::cout << "after by-reference (reference): " << x << std::endl;
}