C++ File Handling Tutorial
Table of contents
- Introduction to File Streams
- Writing and Reading to/from Files
- Appending to Files
- Binary File Operations
- Error Handling
- Practical Example: Phone Directory
- Summary
This tutorial covers the basics of file handling in C++. We’ll explore how to read from and write to files using various techniques.
Introduction to File Streams
C++ provides several classes for file handling through the <fstream> header:
ofstream: Output file stream for writing to filesifstream: Input file stream for reading from filesfstream: File stream for both reading and writing
Writing and Reading to/from Files
// Program 01_basic_write.cpp
#include <iostream>
#include <fstream>
using namespace std;
int main() {
// Create an output file stream
ofstream outFile("basic_output.txt");
// Check if file opened successfully
if (!outFile) {
cerr << "Error: Could not open file for writing!" << endl;
return 1;
}
// Write some text to the file
outFile << "Hello, this is my first file!" << endl;
outFile << "Writing to files in C++ is easy." << endl;
outFile << "This is line number 3." << endl;
// File is automatically closed when outFile goes out of scope
cout << "Data has been written to basic_output.txt" << endl;
return 0;
}
This program demonstrates:
- Creating an output file stream with
ofstream - Writing text to a file using the insertion operator (
<<) - Automatic file closing when the stream goes out of scope
// Program 02_basic_read.cpp
#include <iostream>
#include <fstream>
#include <string>
using namespace std;
int main() {
// Create an input file stream
ifstream inFile("basic_output.txt");
// Check if file opened successfully
if (!inFile) {
cerr << "Error: Could not open file for reading!" << endl;
return 1;
}
// Read the file line by line
string line;
int lineNumber = 1;
while (getline(inFile, line)) {
cout << "Line " << lineNumber << ": " << line << endl;
lineNumber++;
}
// File is automatically closed when inFile goes out of scope
cout << "Finished reading the file." << endl;
return 0;
}
This program demonstrates:
- Opening a file for reading with
ifstream - Reading a file line by line using
getline() - Error checking to ensure the file opened successfully
Writing Numbers to File
// Program 03_numbers_write.cpp
#include <iostream>
#include <fstream>
using namespace std;
int main() {
// Create an output file stream
ofstream outFile("numbers.txt");
if (!outFile) {
cerr << "Error: Could not open file for writing!" << endl;
return 1;
}
// Write squares of numbers from 1 to 10
cout << "Writing squares of numbers 1-10 to numbers.txt" << endl;
for (int i = 1; i <= 10; i++) {
outFile << i << " " << (i * i) << endl;
}
cout << "Data has been written to numbers.txt" << endl;
return 0;
}
This program shows how to:
- Write formatted numerical data to a file
- Use loops to generate and write multiple values
Reading Numbers from Files
// Program 04_numbers_read.cpp
#include <iostream>
#include <fstream>
#include <string>
#include <iomanip>
using namespace std;
int main() {
// Create an input file stream
ifstream inFile("basic_output.txt");
if (!inFile) {
cerr << "Error: Could not open file for reading!" << endl;
return 1;
}
// Read the file word by word
string word;
int wordCount = 0;
cout << "Reading file word by word:" << endl;
while (inFile >> word) {
wordCount++;
cout << "Word " << wordCount << ": " << word << endl;
}
cout << "Total words: " << wordCount << endl;
return 0;
}
This program shows how to:
- Read numerical data from a file
- Use the extraction operator (
>>) to read formatted data - Format the output for better readability
Reading Word by Word
// Program 05_word_by_word.cpp
#include <iostream>
#include <fstream>
#include <string>
using namespace std;
int main() {
// Create an input file stream
ifstream inFile("basic_output.txt");
if (!inFile) {
cerr << "Error: Could not open file for reading!" << endl;
return 1;
}
// Read the file word by word
string word;
int wordCount = 0;
cout << "Reading file word by word:" << endl;
while (inFile >> word) {
wordCount++;
cout << "Word " << wordCount << ": " << word << endl;
}
cout << "Total words: " << wordCount << endl;
return 0;
}
This program demonstrates:
- Reading a file word by word instead of line by line
- Counting words in a file
Appending to Files
// Program 06_append_to_file.cpp
#include <iostream>
#include <fstream>
using namespace std;
int main() {
// Open file in append mode
ofstream outFile("basic_output.txt", ios::app);
if (!outFile) {
cerr << "Error: Could not open file for appending!" << endl;
return 1;
}
// Append new content to the file
outFile << "\nThis line was appended later." << endl;
outFile << "Appending is done using ios::app flag." << endl;
cout << "Data has been appended to basic_output.txt" << endl;
return 0;
}
This program shows how to:
- Open a file in append mode using the
ios::appflag - Add new content to the end of an existing file
Binary File Operations
Binary files store data in the same format as it’s represented in memory, without any formatting or conversion.
Writing Binary Data
// Program 07_binary_write.cpp
#include <iostream>
#include <fstream>
using namespace std;
struct Student {
char name[50];
int age;
double gpa;
};
int main() {
// Create sample student data
Student students[5] = {
{"Anne Shirley", 11, 3.98}, // Highly imaginative and intelligent
{"Diana Barry", 11, 3.65}, // Anne's kindred spirit and best friend
{"Gilbert Blythe", 12, 3.89}, // Anne's academic rival
{"Ruby Gillis", 11, 3.20}, // One of Anne's schoolmates
{"Josie Pye", 11, 3.10} // Another schoolmate who often competes with Anne
};
// Open file in binary mode for writing
ofstream outFile("students.bin", ios::binary);
if (!outFile) {
cerr << "Error: Could not open file for binary writing!" << endl;
return 1;
}
// Write the array of structures to the file
outFile.write(reinterpret_cast<char*>(students), sizeof(students));
cout << "Binary data has been written to students.bin" << endl;
return 0;
}
This program demonstrates:
- Creating a structure to hold data
- Opening a file in binary mode
- Using
write()to write binary data to a file
Reading Binary Data
// Program 08_binary_read.cpp
#include <iostream>
#include <fstream>
#include <iomanip>
using namespace std;
struct Student {
char name[50];
int age;
double gpa;
};
int main() {
// Create an array to hold the read data
Student students[3];
// Open file in binary mode for reading
ifstream inFile("students.bin", ios::binary);
if (!inFile) {
cerr << "Error: Could not open file for binary reading!" << endl;
return 1;
}
// Read the array of structures from the file
inFile.read(reinterpret_cast<char*>(students), sizeof(students));
// Display the read data
cout << "Student Information:" << endl;
cout << "-------------------" << endl;
for (const auto& student : students) {
cout << "Name: " << student.name << "\n"
<< "Age: " << student.age << "\n"
<< "GPA: " << std::fixed << std::setprecision(2) << student.gpa << "\n"
<< "-------------------\n";
}
return 0;
}
This program shows how to:
- Open a binary file for reading
- Use
read()to read binary data from a file - Display the structured data
Error Handling
// Program 10_error_handling.cpp
#include <iostream>
#include <fstream>
#include <cstring>
using namespace std;
int main() {
// Try to open a file that doesn't exist
ifstream inFile("nonexistent_file.txt");
// Check if file opened successfully
if (!inFile) {
cerr << "Error: Could not open nonexistent_file.txt" << endl;
cerr << "Error state: " << strerror(errno) << endl << endl;
}
// Try to create a file in a directory that doesn't exist
ofstream outFile("nonexistent_directory/new_file.txt");
if (!outFile) {
cerr << "Error: Could not create file in nonexistent directory" << endl;
cerr << "Error state: " << strerror(errno) << endl << endl;
}
// Try to open a file with proper error handling
string filename = "test_file.txt";
ifstream testFile(filename);
try {
if (!testFile) {
throw runtime_error("Could not open file: " + filename);
}
// File operations would go here
} catch (const exception& e) {
cerr << "Exception: " << e.what() << endl << endl;
}
cout << "Program continues despite file errors" << endl;
return 0;
}
This program shows:
- How to check if a file opened successfully
- Error reporting using
cerr - Using exceptions for more robust error handling
Practical Example: Phone Directory
Reading a book
#include <iostream>
#include <fstream>
#include <string>
using namespace std;
int main() {
// Open the file for reading
ifstream anneFile("Anne_of_Green_Gables.txt");
// Check if the file opened successfully
if (!anneFile) {
cerr << "Error: Could not open Anne_of_Green_Gables.txt" << endl;
return 1;
}
cout << "=== Anne of Green Gables ===" << endl;
cout << "Reading the file line by line:" << endl;
cout << "----------------------------" << endl;
// Read and display the file line by line
string line;
int lineCount = 0;
while (getline(anneFile, line)) {
lineCount++;
cout << line << endl;
// Pause after every 20 lines to make reading easier
if (lineCount % 20 == 0) {
cout << "\nPress Enter to continue...";
cin.get();
cout << "----------------------------" << endl;
}
}
cout << "\nEnd of file reached." << endl;
cout << "Total lines read: " << lineCount << endl;
// File is automatically closed when anneFile goes out of scope
return 0;
}
This program:
- Opens the Anne_of_Green_Gables.txt file
- Reads it line by line
- Displays each line to the console
- Pauses after every 20 lines to make it easier to read
- Shows the total number of lines at the end
Writing to a Phone Directory
// Program 11_phone_directory_write.cpp
#include <iostream>
#include <fstream>
#include <string>
using namespace std;
int main() {
// Open file for writing
ofstream phoneFile("phone_directory.txt");
if (!phoneFile) {
cerr << "Error: Could not open file for writing!" << endl;
return 1;
}
long phoneNumber;
string name;
cout << "Phone Directory Entry System" << endl;
cout << "===========================" << endl;
cout << "Enter phone number and name (enter 0 as phone number to quit)" << endl;
while (true) {
cout << "\nPhone Number: ";
cin >> phoneNumber;
if (phoneNumber == 0)
break;
// Clear the input buffer
cin.ignore();
cout << "Name: ";
getline(cin, name);
// Write to file
phoneFile << phoneNumber << "," << name << endl;
cout << "Entry added successfully!";
}
cout << "\nAll entries have been saved to phone_directory.txt" << endl;
return 0;
}
This program demonstrates:
- Creating a simple phone directory application
- Reading user input and writing it to a file
- Using a comma as a delimiter between fields
Reading from a Phone Directory
// Program 12_phone_directory_read.cpp
#include <iostream>
#include <fstream>
#include <string>
#include <sstream>
#include <iomanip>
using namespace std;
int main() {
// Open file for reading
ifstream phoneFile("phone_directory.txt");
if (!phoneFile) {
cerr << "Error: Could not open phone directory file!" << endl;
return 1;
}
cout << "Phone Directory" << endl;
cout << "==============" << endl;
cout << setw(15) << left << "Phone Number" << " | " << "Name" << endl;
cout << "--------------- | ---------------" << endl;
string line;
long phoneNumber;
string name;
while (getline(phoneFile, line)) {
// Parse the line using stringstream
stringstream ss(line);
// Extract phone number and name separated by comma
if (getline(ss, line, ',')) {
phoneNumber = stol(line);
if (getline(ss, name)) {
cout << setw(15) << left << phoneNumber << " | " << name << endl;
}
}
}
cout << "\nEnd of directory" << endl;
return 0;
}
This program shows:
- Reading and parsing a CSV-like file format
- Using
stringstreamto parse each line - Displaying formatted output
Summary
This tutorial covered the essential aspects of file handling in C++:
- Basic Operations: Opening, reading, writing, and closing files
- Text Files: Reading and writing text data
- Binary Files: Reading and writing binary data
- File Positioning: Moving within a file
- Error Handling: Dealing with file operation errors
- Practical Application
File handling is a fundamental skill in C++ programming, allowing you to store and retrieve data persistently. These examples provide a foundation for more advanced file operations in your applications.