How to format a negative hex number in alternate form

help
1
  • LowerHex apparently prints negative numbers in twos complement form
  • Adding a sign is still possible even though it always prints a positive number?

I expected the following to print -0x00ab, but instead it prints +0xff55

let num: i16 = -0xab;
println!("{num:+#07x}")

Is the only way to manually check the sign before formatting?

2

It seems that you need to use the NewType pattern to implement either the LowerHex or the UpperHex formatter traits (or both).

3

I'll quote the relevant part here

use std::fmt::{self, Formatter, UpperHex};
use num_traits::Signed;

struct ReallySigned<T: PartialOrd + Signed + UpperHex>(T);

impl<T: PartialOrd + Signed + UpperHex> UpperHex for ReallySigned<T> {
    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
        let prefix = if f.alternate() { "0x" } else { "" };
        let bare_hex = format!("{:X}", self.0.abs());
        f.pad_integral(self.0 >= T::zero(), prefix, &bare_hex)
    }
}

fn main() {
    println!("{:#X}", -0x12345678);
    println!("{:#X}", ReallySigned(-0x12345678));
}

Playground

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