Trait core2::io::BufRead

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pub trait BufRead: Read {
    // Required methods
    fn fill_buf(&mut self) -> Result<&[u8]>;
    fn consume(&mut self, amt: usize);
}

Expand description

A BufRead is a type of Reader which has an internal buffer, allowing it to perform extra ways of reading.

For example, reading line-by-line is inefficient without using a buffer, so if you want to read by line, you’ll need BufRead, which includes a read_line method as well as a lines iterator.

§Examples

A locked standard input implements BufRead:

use std::io;
use std::io::prelude::*;

let stdin = io::stdin();
for line in stdin.lock().lines() {
    println!("{}", line.unwrap());
}

If you have something that implements Read, you can use the [BufReader type][BufReader] to turn it into a BufRead.

For example, File implements Read, but not BufRead. [BufReader] to the rescue!

use std::io::{self, BufReader};
use std::io::prelude::*;
use std::fs::File;

fn main() -> io::Result<()> {
    let f = File::open("foo.txt")?;
    let f = BufReader::new(f);

    for line in f.lines() {
        println!("{}", line.unwrap());
    }

    Ok(())
}

Required Methods§

source

fn fill_buf(&mut self) -> Result<&[u8]>

Returns the contents of the internal buffer, filling it with more data from the inner reader if it is empty.

This function is a lower-level call. It needs to be paired with the consume method to function properly. When calling this method, none of the contents will be “read” in the sense that later calling read may return the same contents. As such, consume must be called with the number of bytes that are consumed from this buffer to ensure that the bytes are never returned twice.

An empty buffer returned indicates that the stream has reached EOF.

§Errors

This function will return an I/O error if the underlying reader was read, but returned an error.

§Examples

A locked standard input implements BufRead:

use std::io;
use std::io::prelude::*;

let stdin = io::stdin();
let mut stdin = stdin.lock();

let buffer = stdin.fill_buf().unwrap();

// work with buffer
println!("{:?}", buffer);

// ensure the bytes we worked with aren't returned again later
let length = buffer.len();
stdin.consume(length);

source

fn consume(&mut self, amt: usize)

Tells this buffer that amt bytes have been consumed from the buffer, so they should no longer be returned in calls to read.

This function is a lower-level call. It needs to be paired with the fill_buf method to function properly. This function does not perform any I/O, it simply informs this object that some amount of its buffer, returned from fill_buf, has been consumed and should no longer be returned. As such, this function may do odd things if fill_buf isn’t called before calling it.

The amt must be <= the number of bytes in the buffer returned by fill_buf.