C++ : How to write and read with only one file open for both operations

cpp_file_simulatneous_read_write.cpp

#include <iostream>
#include <iomanip>
#include <ctime>
#include <fstream>
#include <string>
 
std::ostream & format_2_l0(std::ostream & iStream)
{
  return iStream << std::setw(2) << std::setfill('0');
}
 
int main(int argc, char* argv[])
{ 
  std::string my_file("file.txt");
  std::fstream file_p(my_file.c_str(), std::ios::in | std::ios::app); // <- Pour ajouter à la fin du contenu
  // std::fstream file_out(my_file, std::ios::in | std::ios::out); // <- Pour écraser le contenu
  if(file_p)
  {
    // Ecrire
    std::time_t now = std::time(0);
    std::tm * tm_now = std::localtime(&now);
    format_2_l0(file_p) << tm_now->tm_hour << "h";
    format_2_l0(file_p) << tm_now->tm_min << " ";
    format_2_l0(file_p) << tm_now->tm_sec << "s";
    file_p << std::endl;
 
    // Lire
    std::string ligne;
    file_p.seekg(0,std::ios::beg);
    while ( std::getline( file_p, ligne ) )
      std::cout << ligne << std::endl;
 
 
    file_p.close();
  }
  return 0;
}

This snippet stacks the current time into a file then displays its content by opening only one file pointer.