BufReader

BufReader is a buffered reader adaptor in Rust's standard library (std::io) that wraps another type implementing Read to provide buffered input. By storing data in an internal buffer, it reduces the number of calls to the underlying reader, improving performance when reading from files, network streams, or other slow sources. BufReader implements both Read and BufRead, making it usable wherever these traits are required.

Construction and capacity: BufReader::new(inner) creates a buffered reader with a default internal buffer size (typically around 8 KiB). BufReader::with_capacity(cap, inner) allows specifying a custom buffer size. The method into_inner consumes the BufReader and returns the underlying reader.

How it works: The wrapper fills its internal buffer from the inner reader and serves subsequent reads from that buffer until it is exhausted. Methods like fill_buf and consume manage the internal buffer directly. BufReader also exposes standard BufRead operations such as read_line, read_until, and read_to_end, which operate on the buffered data.

Methods and traits: Read is implemented by delegating to the buffer, while BufRead provides utilities for buffered input, including lines() to iterate over lines. read_line appends to a provided string, including the newline if present; lines() yields strings without trailing newline. The exact error semantics follow the underlying reader and the standard library's conventions.

Usage and caveats: BufReader is especially beneficial for many small reads from a source, reducing system calls and latency. It is not inherently thread-safe; Sync/Send depend on the inner reader. It is generic over any T: Read, enabling use with files, sockets, or in-memory sources. Example: let f = std::fs::File::open("foo.txt").unwrap(); let mut r = std::io::BufReader::new(f); let mut line = String::new(); r.read_line(&mut line).unwrap();