how to read and write in a file c++

#include <iostream>
#include <fstream>
using namespace std;

ifstream file_variable; //ifstream is for input from plain text files
file_variable.open("input.txt"); //open input.txt

file_variable.close(); //close the file stream
/*
Manually closing a stream is only necessary
if you want to re-use the same stream variable for a different
file, or want to switch from input to output on the same file.
*/
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//You can also use cin if you have tables like so:
while (cin >> name >> value)// you can also use the file stream instead of this
{
 cout << name << value << endl;
}
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//ifstream file_variable; //ifstream is for input from plain text files
ofstream out_file;
out_file.open("output.txt");

out_file << "Write this scentence in the file" << endl;// basic file operations
#include <iostream>
#include <fstream>
using namespace std;

int main () {
  ofstream myfile;
  myfile.open ("example.txt");
  myfile << "Writing this to a file.\n";
  myfile.close();
  return 0;
}// reading a text file
#include <iostream>
#include <fstream>
#include <string>
using namespace std;

int main () {
  string line;
  ifstream myfile ("example.txt");
  if (myfile.is_open())
  {
    while ( getline (myfile,line) )
    {
      cout << line << '\n';
    }
    myfile.close();
  }

  else cout << "Unable to open file"; 

  return 0;
}