File Processing in C++

Programs often need to store data persistently so it is not lost when the program closes. In C++, this is accomplished by interacting with the file system using the <fstream> library.

Just as <iostream> provides cin and cout to read from and write to the console, <fstream> provides classes to read from and write to files.

1. File Stream Classes

There are three main classes included in the <fstream> library:

ofstream (Output File Stream): Creates and writes to files.
ifstream (Input File Stream): Reads from files.
fstream: Can both read from and write to files.

2. Writing to a File

To write data to a file, you create an object of the ofstream class. If the file does not exist, C++ will create it for you.

cpp

#include <iostream>
#include <fstream>
using namespace std;
 
int main() {
    // 1. Create and open a text file
    ofstream MyFile("student_data.txt");
 
    // 2. Write to the file using the insertion operator (<<)
    MyFile << "Name: John Doe" << endl;
    MyFile << "ID: 2024CSE001" << endl;
    MyFile << "Grade: A" << endl;
 
    // 3. Always close the file when finished
    MyFile.close();
 
    cout << "File written successfully." << endl;
    return 0;
}

3. Reading from a File

To read from an existing file, use the ifstream class. You often read files line-by-line using a while loop and the getline() function.

cpp

#include <iostream>
#include <fstream>
#include <string>
using namespace std;
 
int main() {
    string textLine;
 
    // 1. Open an existing file
    ifstream ReadFile("student_data.txt");
 
    // Check if the file opened successfully
    if (!ReadFile) {
        cout << "Error: File could not be opened!" << endl;
        return 1;
    }
 
    // 2. Read the file line by line
    while (getline(ReadFile, textLine)) {
        // Output the text from the file to the console
        cout << textLine << endl;
    }
 
    // 3. Close the file
    ReadFile.close();

4. File Open Modes

By default, ofstream opens a file and overwrites its contents. You can change this behavior by specifying an open mode flag.

For example, to append to a file instead of overwriting it, use ios::app:

cpp

// Opens the file in append mode
ofstream AppendFile("student_data.txt", ios::app); 
AppendFile << "Status: Active" << endl;
AppendFile.close();

Summary

Use #include <fstream> for file operations.
Use ofstream to create and write to files.
Use ifstream to read from files.
Always remember to .close() the file when you are finished to release operating system resources.

Assessed progress

File Processing in C++

File Processing in C++

1. File Stream Classes

2. Writing to a File

3. Reading from a File

4. File Open Modes

Summary

Key Concepts

File Streams

ofstream

ifstream

fstream

Quick Check

Object Oriented Programming

Object Oriented Programming