Passing &mut to nested function

1

Hi Friends and sorry - most probably this was asked but I can't figure out suitable search keywords to filter out similar though different cases.

I create mutable string and pass it as &mut to function which should pass it the same way further. This fails with "cannot borrow as mutable" (most probably because I don't correctly understand that makes "simultaneous" borrowing). So what is the correct way (besides moving String::new into function itself)?

use std::io;

fn next_num(s: &mut String) -> i32 {
    io::stdin().read_line(&mut s).unwrap();
    s.trim().parse::<i32>().unwrap()
}

fn main() {
    let mut s = String::new();
    let n = next_num(&mut s);
    println!("{}", n);
}
2

Hi,

You just have to remote the &mut part in the first line of next_num.

As the .read_line( function takes an &mut String, and s has this specific type, you can just pass it directly:

io::stdin().read_line(s).unwrap();
3

Oh, got it, thanks a lot! So it is not about borrowing but I just was so silly to confuse types!

Works now, though it seems I need to add .clear() inside the method if I want to reuse the same string.

1 Like
4

You'll be better off creating a new String each type, clearing an existing one takes time as it needs to be filled with zeroes, so it's not really efficient performance-wise.

1 Like
5

Thanks for hint - didn't know it is filled with zeroes (rather than just length truncated) - will do accordingly! Creating new instances for any small object is something I get used to in java world, but was wondering if it is all right in rust. You clarified this point! :+1:

6

The clear method does not zero the string. Reusing the string is more efficient.

3 Likes
7

Sorry I just checked and indeed it does not rewrite the string, sorry for the mistake ^^'

8

Well, Friends, thanks for this yet further clarification :slight_smile:

I'll check out of curiosity whether efficiency is much improved by reusing the string - though probably for code cleanliness it's better to use new instances where possible. Most probably IO is not extremely efficient part of program...

9

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