Multi tool use

BufReader: move after for-loop with bufreader.lines()

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


2 Answers
2

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

Read::by_ref()

BufReader

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


As Lukas Kalbertodt says, use Read::by_ref.

Read::by_ref

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

lines

BufReader

&mut BufReader

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

skip

Iterator::take

Iterator::count

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.

skip

AsRef<Path>

skip_and_print_file

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