fstream

fstream is a component of the C++ standard library that provides file input and output streams. It is defined in the <fstream> header and is based on the generic iostreams framework. The library supplies three types: std::ifstream for input from files, std::ofstream for output to files, and std::fstream for both input and output from files. These types are aliases of the templated std::basic_fstream<char> specialized for char.

Opening files: A file stream is opened by constructing it with a filename and an opening mode, or by default-constructing and calling open. Common modes include std::ios::in for reading, std::ios::out for writing, std::ios::app to append, and std::ios::trunc to truncate existing content. For binary data, add std::ios::binary. Modes can be combined; for example, std::fstream f("data.dat", std::ios::in | std::ios::out | std::ios::binary).

Reading and writing: Data are transferred using the extraction operator (>>) and insertion operator (<<), as well as member functions read and write. For random access, use seekg/seekp and tellg/tellp. It is common to check the stream's state with if (fs) or fs.good(), and to verify whether opening succeeded with is_open() after open() or construction.

Error handling and text vs binary: Streams maintain state flags such as goodbit, eofbit, failbit, and badbit. Exceptions can be enabled via exceptions() to throw on errors. In text mode, some platforms perform newline translation; binary mode avoids translation. When using ifstream/ofstream/fstream, you may need to call close() to release resources, especially before re-opening a file.

Use in practice: fstream is widely used for reading and writing configuration files, logs, and data interchange. It integrates with the standard C++ iostreams and supports locale, formatting, and synchronization features as part of the standard library.

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