Working with the File System in C++: A Practical Guide

Introduction: Why Working with the File System in C++ is Fundamental

Any serious program will sooner or later need to save data to disk or read it from there. Configuration files, logs, databases, user documents — all of this requires the ability to interact with the file system. In C++, there are two main approaches for this: classic input/output streams (the <fstream> library) and the more modern, higher-level std::filesystem (since C++17).

In this article, we will cover practical scenarios: from simple reading and writing of text files to navigating directories and error handling. All examples are written in C++17/20 and are ready to use in your projects.

1. Basics: Reading and Writing Files with fstream

The <fstream> library provides three main classes: ifstream (input stream — reading), ofstream (output stream — writing), and fstream (bidirectional stream). Let's look at them with examples.

1.1. Writing to a Text File

The simplest way to write data is to use ofstream. If the file does not exist, it will be created. If it exists, it will be overwritten (ios::trunc mode by default).

#include <iostream>\n#include <fstream>\n#include <string>\n\nint main() {\n    std::ofstream outFile("example.txt");\n    if (!outFile) {\n        std::cerr << "Error opening file for writing\\n";\n        return 1;\n    }\n\n    outFile << "Hello, C++ file system!\\n";\n    outFile << "Line number 2\\n";\n    outFile << 42 << " — is a number.\\n";\n\n    outFile.close(); // closing manually is optional, but good practice\n    std::cout << "Data written.\\n";\n    return 0;\n}

1.2. Reading from a Text File

For reading, we use ifstream. The most reliable way is to read line by line using std::getline.

#include <iostream>\n#include <fstream>\n#include <string>\n\nint main() {\n    std::ifstream inFile("example.txt");\n    if (!inFile) {\n        std::cerr << "File not found or cannot be opened\\n";\n        return 1;\n    }\n\n    std::string line;\n    while (std::getline(inFile, line)) {\n        std::cout << "Read: " << line << '\\n';\n    }\n\n    inFile.close();\n    return 0;\n}

1.3. Open Modes and Binary Files

You can combine open flags: ios::app (append), ios::binary (binary mode), ios::ate (seek to end). For binary files, use the read() and write() methods.

#include <fstream>\n#include <vector>\n\n// Example: writing and reading a vector of integers in binary format\nvoid saveVector(const std::string& filename, const std::vector<int>& data) {\n    std::ofstream file(filename, std::ios::binary);\n    if (!file) return;\n    file.write(reinterpret_cast<const char*>(data.data()), data.size() * sizeof(int));\n}\n\nstd::vector<int> loadVector(const std::string& filename) {\n    std::ifstream file(filename, std::ios::binary);\n    if (!file) return {};\n    file.seekg(0, std::ios::end);\n    size_t size = file.tellg() / sizeof(int);\n    file.seekg(0, std::ios::beg);\n    std::vector<int> data(size);\n    file.read(reinterpret_cast<char*>(data.data()), size * sizeof(int));\n    return data;\n}

2. Modern Approach: std::filesystem (C++17)

Starting from C++17, the standard library includes the <filesystem> module, which allows you to work with paths, files, and directories at a high level, cross-platform.

2.1. Checking Existence and Getting File Information

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