input and output using files

files.cpp

#include <iostream>
#include <stdio.h>

using namespace std;

int main() {
  // use in.txt as the input stream
  freopen("in.txt", "r", stdin);

  // use out.txt as the output stream
  freopen("out.txt", "w", stdout);
  // make sure that those files are in the same directory of the cpp file
  
  // you can now cin from the input file and cout to the output file
  int x;
  cin>>x; // wil read an integer from the input file
  cout<<x; // will write x to the output file
}