C++: Advanced File Operations

Q: What if a file can't be opened?

Check with if (!file) , combined with exception handling.

Last updated: 2026-08-26

In lesson 40 we learned about exception handling.

Now, we'll dive into file operations — a core skill for persistent data in programs.

Whether it's configuration files, logs, or databases, file operations are essential.

1. File Stream Overview

(1) 1.1 Three File Stream Classes

C++ uses three classes to handle files:

Class	Purpose
`std::ifstream`	Input file stream (reading files)
`std::ofstream`	Output file stream (writing files)
`std::fstream`	Input/output file stream (reading and writing)

(2) 1.2 Opening a File

Example: Opening a file (Difficulty ⭐)

▶ Example 2: File Operation Demo (Difficulty ⭐)

TEXT 📖 Display only

#include <iostream>
#include <fstream>

int main() {
 std::ifstream file("data.txt");
 
 if (!file) {
 std::cerr << "Unable to open file" << std::endl;
 return 1;
 }
 
 std::cout << "File opened successfully" << std::endl;
 file.close();
 
 return 0;
}

Output:

TEXT 📖 Display only

File opened successfully

2. Text File Reading and Writing

(1) 2.1 Reading Line by Line

Example: Counting file lines (Difficulty ⭐)

CPP

#include <iostream>
#include <fstream>
#include <string>

int main() {
 std::ifstream file("data.txt");
 std::string line;
 int count = 0;
 
 while (std::getline(file, line)) {
 count++;
 }
 
 std::cout << "Number of lines: " << count << std::endl;
 file.close();
 
 return 0;
}

(2) 2.2 Formatted Reading and Writing

Example: Reading and writing a struct (Difficulty ⭐⭐)

CPP

#include <iostream>
#include <fstream>

struct Student {
 char name[50];
 int age;
 double score;
};

int main() {
 Student s = {"Zhang San", 20, 85.5};
 
 // Write to file
 std::ofstream out("student.txt");
 out << s.name << std::endl;
 out << s.age << std::endl;
 out << s.score << std::endl;
 out.close();
 
 // Read from file
 Student s2;
 std::ifstream in("student.txt");
 in >> s2.name >> s2.age >> s2.score;
 in.close();
 
 std::cout << "Name: " << s2.name << std::endl;
 std::cout << "Age: " << s2.age << std::endl;
 std::cout << "Score: " << s2.score << std::endl;
 
 return 0;
}

⚠️ Warning:

This approach doesn't work well for names with spaces (>> stops at whitespace)

3. Binary Files

(1) 3.1 Why Use Binary Files?

Comparison	Text File	Binary File
Readability	Human-readable	Not readable
Size	Larger	Small
Speed	Slower	Fast
Cross-platform	Good	Poor (endianness issues)

(2) 3.2 Reading and Writing Binary Files

Example: Binary read/write of a struct (Difficulty ⭐⭐⭐)

CPP

#include <iostream>
#include <fstream>

struct Student {
 char name[50];
 int age;
 double score;
};

int main() {
 Student s = {"Zhang San", 20, 85.5};
 
 // Write binary file
 std::ofstream out("student.bin", std::ios::binary);
 out.write(reinterpret_cast<char*>(&s), sizeof(s));
 out.close();
 
 // Read binary file
 Student s2;
 std::ifstream in("student.bin", std::ios::binary);
 in.read(reinterpret_cast<char*>(&s2), sizeof(s2));
 in.close();
 
 std::cout << "Name: " << s2.name << std::endl;
 std::cout << "Age: " << s2.age << std::endl;
 std::cout << "Score: " << s2.score << std::endl;
 
 return 0;
}

💡 Tip:

Use write and read for binary I/O
You must specify the number of bytes (sizeof)

4. Random Access

(1) 4.1 seekg and seekp

Random access allows you to jump to any position in a file for reading or writing.

Function	Purpose
`seekg(pos)`	Set read position
`seekp(pos)`	Set write position
`tellg()`	Get current read position
`tellp()`	Get current write position

(2) 4.2 Example: Modifying content in the middle of a file (Difficulty ⭐⭐⭐)

CPP

#include <iostream>
#include <fstream>

int main() {
 std::fstream file("data.txt", std::ios::in | std::ios::out);
 
 // Jump to the 10th byte
 file.seekp(10);
 
 // Write data (overwrite)
 file << "Hello";
 
 file.close();
 return 0;
}

Output:

TEXT 📖 Display only

Hello, File!
C++ File Operations

5. File Stream State

(1) 5.1 State Flags

File streams have 4 state flags:

Flag	Description
`good()`	Everything is fine
`eof()`	Reached end of file
`fail()`	Logical error (e.g. type mismatch)
`bad()`	Serious error (e.g. disk failure)

(2) 5.2 Clearing State

Example: Handling end-of-file (Difficulty ⭐)

CPP

#include <iostream>
#include <fstream>

int main() {
 std::ifstream file("data.txt");
 std::string line;
 
 while (true) {
 std::getline(file, line);
 
 if (file.eof()) {
 break; // Reached end of file
 }
 
 std::cout << line << std::endl;
 }
 
 file.clear(); // Clear state to continue using
 file.close();
 
 return 0;
}

6. Practice: Simple Database

▶ Example 1: Student Record Management (Difficulty ⭐⭐⭐)

TEXT 📖 Display only

#include <iostream>
#include <fstream>
#include <vector>
#include <string>

struct Student {
 int id;
 char name[50];
 int age;
};

void saveStudents(const std::vectorStudent& students, const std::string& filename) {
 std::ofstream file(filename, std::ios::binary);
 for (const auto& s : students) {
 file.write(reinterpret_cast<const char*>(&s), sizeof(s));
 }
}

void loadStudents(std::vectorStudent& students, const std::string& filename) {
 std::ifstream file(filename, std::ios::binary);
 Student s;
 while (file.read(reinterpret_cast<char*>(&s), sizeof(s))) {
 students.push_back(s);
 }
}

int main() {
 std::vector<Student> students = {
 {1, "Zhang San", 20},
 {2, "Li Si", 21}
 };
 
 // Save
 saveStudents(students, "students.bin");
 
 // Load
 std::vectorStudent loaded;
 loadStudents(loaded, "students.bin");
 
 std::cout << "Loaded " << loaded.size() << " records" << std::endl;
 
 return 0;
}

Output:

TEXT 📖 Display only

Loaded 2 records

▶ Example 3: Simple File Read/Write (Difficulty ⭐)

CPP

#include <iostream>
#include <fstream>
#include <string>

int main() {
    // Write to file
    std::ofstream outFile("test.txt");
    outFile << "Hello, File!" << std::endl;
    outFile << "C++ File Operations" << std::endl;
    outFile.close();
    
    // Read from file
    std::ifstream inFile("test.txt");
    std::string line;
    while (std::getline(inFile, line)) {
        std::cout << line << std::endl;
    }
    inFile.close();
    
    return 0;
}

▶ Try it Yourself

Output:

TEXT 📖 Display only

(program output)

Expected output:

TEXT 📖 Display only

Hello, File!
C++ File Operations

❓ FAQ

Q Which is better — text files or binary files?

A It depends on your needs: - Need human readability → text files - Need performance/space efficiency → binary files - Need cross-platform compatibility → text files

Q How do I handle Chinese characters?

A C++ has poor Unicode support. Suggestions: - Save text files in UTF-8 encoding - Use third-party libraries (like iconv) - Or stick to English/pinyin only

Q What if a file can't be opened?

A Check with if (!file), combined with exception handling.

📖 Summary

Topic	Key Points
File stream classes	`ifstream`/`ofstream`/`fstream`
Text files	`<<` and `>>` operators
Binary files	`write` and `read`
Random access	`seekg`/`seekp`
State checking	`eof()`/`fail()`/`bad()`

📝 Exercises

Basic (Difficulty ⭐): Use ofstream to write a text file (three lines of content), then use ifstream to read and display it on screen.
Intermediate (Difficulty ⭐⭐): Use fstream to open a file in ios::binary mode, write an array of structs, then read and verify.
Challenge (Difficulty ⭐⭐⭐): Use seekg/tellg to implement random file access. Create an "index file" that records the position of each record, supporting direct jumps to a specified record by index.

File streams: ifstream for reading / ofstream for writing / fstream for both
Open modes: in/out/app/binary/trunc
File state checking: is_open/good/fail/eof
Text files vs binary files: text is readable but slower
Random access: seekg/tellg to position read/write locations

Next lesson: Regular Expressions (#42)

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C++: Advanced File Operations

1. File Stream Overview

(1) 1.1 Three File Stream Classes

(2) 1.2 Opening a File

▶ Example 2: File Operation Demo (Difficulty ⭐)

2. Text File Reading and Writing

(1) 2.1 Reading Line by Line

(2) 2.2 Formatted Reading and Writing

3. Binary Files

(1) 3.1 Why Use Binary Files?

(2) 3.2 Reading and Writing Binary Files

4. Random Access

(1) 4.1 seekg and seekp

(2) 4.2 Example: Modifying content in the middle of a file (Difficulty ⭐⭐⭐)

5. File Stream State

(1) 5.1 State Flags

(2) 5.2 Clearing State

6. Practice: Simple Database

▶ Example 1: Student Record Management (Difficulty ⭐⭐⭐)

▶ Example 3: Simple File Read/Write (Difficulty ⭐)

❓ FAQ

📖 Summary

📝 Exercises

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