Files

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garanews
Published Sep 1, 2021Updated Nov 14, 2024
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Computers use file systems to store and retrieve data. Each file is an individual container of related information.

The fstream library, short for “file stream”, enables working with files in C++. To use the fstream library in the C++ program, include both the standard <iostream> and the <fstream> header files in the C++ program:

#include <iostream>
#include <fstream>

There are three classes included in the fstream library, which are used to create, write or read files:

  • ofstream (“output file stream”): Create files and write to files.
  • ifstream (“input file stream”): Read from files.
  • fstream: A combination of ofstream and ifstream (create, read, and write to files).

Create and Write to a File

To create a file, use either the ofstream or fstream class, and specify the name of the file.

To write to the file, use the insertion operator (<<). For example:

#include <iostream>
#include <fstream>


int main() {
  // Create and open a text file
  std::ofstream MyFile("journal.txt");


  // Write to the file
  MyFile << "Today is the greatest\n";
  MyFile << "Day I've ever known";


  // Close the file
  MyFile.close();
}

In the same folder, there should be a new text file called journal.txt. There should be two lines of text inside:

Today is the greatest
Day I've ever known

Read a File

To read from a file, use either the ifstream or fstream class, and the name of the file.

Note: A while loop is used together with the getline() function (which belongs to the ifstream class) to read the file line by line, and to print the content of the file:

#include <iostream>
#include <fstream>


int main() {
  // Create a text string, which is used to output the text file
  std::string myText;


  // Read from the text file
  std::ifstream MyReadFile("journal.txt");


  // Output the file line by line
  while (getline (MyReadFile, myText)) {
    std::cout << myText << "\n";
  }


  // Close the file
  MyReadFile.close();
}

The output would be:

Today is the greatest
Day I've ever known

Appending to a File

In some cases, you may want to append text to an existing file instead of overwriting its content. To do this, you can open the file in append mode using the std::ios::app flag with ofstream or fstream. Here’s an example:

#include <iostream>
#include <fstream>
int main() {
  // Open the file in append mode
  std::ofstream MyFile("journal.txt", std::ios::app);
  // Append text to the file
  MyFile << "\nSmashing Pumpkins lyrics\n";
  // Close the file
  MyFile.close();
}

The file journal.txt will now contain:

Today is the greatest
Day I've ever known
Smashing Pumpkins lyrics

Checking if a file exists

Before opening a file, checking if the file exists is a good practice. This can prevent issues like reading a file that doesn’t exist. You can use the .is_open() function for this:

#include <iostream>
#include <fstream>
int main() {
  std::ifstream MyFile("journal.txt");
  // Check if the file opened successfully
  if (MyFile.is_open()) {
    std::cout << "File opened successfully.\n";
  } else {
    std::cout << "File does not exist.\n";
  }
  // Close the file
  MyFile.close();
}

If the file journal.txt exists, the output will be:

File opened successfully.

If the file journal.txt does not exist, the output will be:

File does not exist.

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