How to write string representation of a u32 into a String buffer without extra allocation? [duplicate]

Question

I can convert a u32 into String and append it to an existing string buffer like this

let mut a = String::new();
let b = 1_u32.to_string();
a.push_str(&b[..]);

But this involves allocation of a new string object in the heap.

How to push the string representation of a u32 without allocating a new String?

Should I implement an int-to-string function from scratch?

Answer 1

Use write! and its family:

use std::fmt::Write;

fn main() {
    let mut foo = "answer ".to_string();

    write!(&mut foo, "is {}.", 42).expect("This shouldn't fail");

    println!("The {}", foo);
}

This prints The answer is 42., and does exactly one allocation (the explicit to_string).

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