Formatting

We've seen that formatting is specified via a format string:

  • format!("{}", foo) -> "3735928559"
  • format!("0x{:X}", foo) -> "0xDEADBEEF"
  • format!("0o{:o}", foo) -> "0o33653337357"

The same variable (foo) can be formatted differently depending on which argument type is used: X vs o vs unspecified.

This formatting functionality is implemented via traits, and there is one trait for each argument type. The most common formatting trait is Display, which handles cases where the argument type is left unspecified: {} for instance.

use std::fmt::{self, Formatter, Display};

struct City {
    name: &'static str,
    // Latitude
    lat: f32,
    // Longitude
    lon: f32,
}

impl Display for City {
    // `f` is a buffer, and this method must write the formatted string into it.
    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
        let lat_c = if self.lat >= 0.0 { 'N' } else { 'S' };
        let lon_c = if self.lon >= 0.0 { 'E' } else { 'W' };

        // `write!` is like `format!`, but it will write the formatted string
        // into a buffer (the first argument).
        write!(f, "{}: {:.3}°{} {:.3}°{}",
               self.name, self.lat.abs(), lat_c, self.lon.abs(), lon_c)
    }
}

#[derive(Debug)]
struct Color {
    red: u8,
    green: u8,
    blue: u8,
}

fn main() {
    for city in [
        City { name: "Dublin", lat: 53.347778, lon: -6.259722 },
        City { name: "Oslo", lat: 59.95, lon: 10.75 },
        City { name: "Vancouver", lat: 49.25, lon: -123.1 },
    ] {
        println!("{}", city);
    }
    for color in [
        Color { red: 128, green: 255, blue: 90 },
        Color { red: 0, green: 3, blue: 254 },
        Color { red: 0, green: 0, blue: 0 },
    ] {
        // Switch this to use {} once you've added an implementation
        // for fmt::Display.
        println!("{:?}", color);
    }
}

You can view a full list of formatting traits and their argument types in the std::fmt documentation.

Activity

Add an implementation of the fmt::Display trait for the Color struct above so that the output displays as:

RGB (128, 255, 90) 0x80FF5A
RGB (0, 3, 254) 0x0003FE
RGB (0, 0, 0) 0x000000

Two hints if you get stuck:

See also:

std::fmt