For example

#include <iostream>
using namespace std;

int main() {
  freopen("input.txt", "r", stdin);
  freopen("output.txt", "w", stdout);

  int n;
  cin >> n;
  for (int i = 0; i < n; i++) {
    cout << i;
  }

  return 0;
}

Decrease:

#include <fstream>
int main() {
  std::ifstream y("input.txt");
  std::ofstream z("output.txt");

  int n, i = 0;
  y >> n;
  while(i < n)
    z << i++;

  exit(0);
}

What's about "fstream"?

std::fstream y("input.txt"), z("output.txt")

It's amazing but output is not correct.) "output.txt" isn't remaking. Output is writing from begin of file.

How to decrease code?

Just for fun)