Lesson 36: File I/O Streams

File I/O Streams\n\nReading from and writing to files in C++ is performed through the <fstream> library, which provides std::ifstream, std::ofstream, and std::fstream.\n\n---\n\n## Example: Writing a text file\ncpp\n#include <fstream>\n\nint main() {\n std::ofstream out(\"data.txt\");\n if (!out) {\n std::cerr << \"Failed to open file for writing\\n\";\n return 1;\n }\n out << \"Header,Value\\n\";\n for (int i = 0; i < 5; ++i) {\n out << i << \",\" << i * i << '\\n';\n }\n // out is closed automatically when it goes out of scope\n return 0;\n}\n\n\n## Example: Reading the file back in binary mode\ncpp\n#include <fstream>\n#include <vector>\n\nint main() {\n std::ifstream in(\"data.bin\", std::ios::binary);\n if (!in) {\n std::cerr << \"Cannot open file\\n\";\n return 1;\n }\n // Determine file size\n in.seekg(0, std::ios::end);\n std::size_t size = in.tellg();\n in.seekg(0, std::ios::beg);\n std::vector<char> buffer(size);\n in.read(buffer.data(), size);\n std::cout << \"Read \" << size << \" bytes\\n\";\n return 0;\n}\n\n\n> Tip: Use std::filesystem::exists to check file existence before opening.\n\n---\n\n<Alert type="warning">Always verify stream state (if (!stream)) before reading or writing to catch I/O errors early.