Cannot use moved BufReader after for loop with bufreader.lines()

问题

I'm trying to read some lines from a file, skipping the first few and printing the rest, but I keep getting errors about used value after move:

use std::fs::File;
use std::io::{self, BufRead, BufReader, Read};
use std::path::Path;

fn skip_and_print_file(skip: &usize, path: &Path) {
    let mut skip: usize = *skip;

    if let Ok(file) = File::open(path) {
        let mut buffer = BufReader::new(file);
        for (index, line) in buffer.lines().enumerate() {
            if index >= skip {
                break;
            }
        }
        print_to_stdout(&mut buffer);
    }
}

fn print_to_stdout(mut input: &mut Read) {
    let mut stdout = io::stdout();
    io::copy(&mut input, &mut stdout);
}

fn main() {}

This is the error I'm getting:

error[E0382]: use of moved value: `buffer`
  --> src/main.rs:15:30
   |
10 |         for (index, line) in buffer.lines().enumerate() {
   |                              ------ value moved here
...
15 |         print_to_stdout(&mut buffer);
   |                              ^^^^^^ value used here after move
   |
   = note: move occurs because `buffer` has type `std::io::BufReader<std::fs::File>`, which does not implement the `Copy` trait

回答1:

In order to avoid the move, use the Read::by_ref() method. That way, you only borrow the BufReader:

for (index, line) in buffer.by_ref().lines().enumerate() { ... }
//                         ^^^^^^^^^
// you can still use `buffer` here

回答2:

As Lukas Kalbertodt says, use Read::by_ref.

This prevents lines from consuming the BufReader and instead it consumes a &mut BufReader. The same logic applies to iterators.

Instead of implementing skip yourself, you can just use Iterator::take. This has to be driven to completion with something like Iterator::count though:

use std::{
    fs::File,
    io::{self, BufRead, BufReader, Read},
    path::Path,
};

fn skip_and_print_file(skip: usize, path: &Path) {
    if let Ok(file) = File::open(path) {
        let mut buffer = BufReader::new(file);

        for _ in buffer.by_ref().lines().take(skip) {}
        // Or: buffer.by_ref().lines().take(skip).for_each(drop);

        print_to_stdout(&mut buffer);
    }
}

fn print_to_stdout(mut input: &mut Read) {
    let mut stdout = io::stdout();
    io::copy(&mut input, &mut stdout).expect("Unable to copy");
}

fn main() {
    skip_and_print_file(2, Path::new("/etc/hosts"));
}

Note that there's no reason to make the skip variable mutable or even to pass in a reference. You can also take in AsRef<Path> and then callers of skip_and_print_file can just pass in a string literal.

来源：https://stackoverflow.com/questions/39935158/cannot-use-moved-bufreader-after-for-loop-with-bufreader-lines

标签

rust

traits