fmt::Debug a number field with differing base (rust)

March 27, 2022

I want to the Debug format of a struct number field to show a decimal value and a hexadecimal value.

So given rust struct (rust playground)

#[derive(Default)]
pub struct S1 {
    pub num1: u64,
}

fn main() {
    let mut s1 = S1::default();
    s1.num1 = 0xFF;
    println!("{:?}", s1)
}

I would like fmt::Debug implementation (i.e. format string "{:?}") to show two different based values for field num1, i.e. to print

S1 { num1: 255 (0xFF) }

(I’m not sure how to solve the easier problem: format the numeric field as hexadecimal).

Here is what I have for fmt::Debug implementation

impl std::fmt::Debug for S1 {
    fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
        f.debug_struct("S1")
            .field("num1", &self.num1)
            .finish()
    }
}

I do not know which part of std::fmt module would help me.

>Solution :

You can use format_args! to construct the formatting arguments in a low-cost way:

impl std::fmt::Debug for S1 {
    fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
        f.debug_struct("S1")
            .field("num1", &format_args!("{0:?} (0x{0:X})", &self.num1))
            .finish()
    }
}