#include <iostream>

#include <fstream>

#include <string>


int main() {

  // Open a file for reading

  std::ifstream input_file("input.txt");


  // Check if the file was successfully opened

  if (!input_file) {

    std::cerr << "Error opening input file" << std::endl;

    return 1;

  }


  // Read the contents of the file into a string

  std::string input_str;

  std::getline(input_file, input_str);


  // Close the file

  input_file.close();


  // Print the contents of the string

  std::cout << "The contents of the input file are: " << input_str << std::endl;


  // Open a file for writing

  std::ofstream output_file("output.txt");


  // Check if the file was successfully opened

  if (!output_file) {

    std::cerr << "Error opening output file" << std::endl;

    return 1;

  }


  // Write the contents of the string to the file

  output_file << input_str;


  // Close the file

  output_file.close();


  return 0;

}

output :

This program reads the contents of a file called "input.txt" and stores it in a string. It then writes the contents of the string to a file called "output.txt".

To use this program, you will need to create a file called "input.txt" in the same directory as the program and put some text in it. When you run the program, it will read the contents of the file and write it to "output.txt".