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 Debug, which
handles cases where the argument type is left unspecified: {} for instance.
#![feature(core)]
use std::fmt::{self, Formatter, Display};
// For .abs()
use std::num::Float;
struct City {
name: &'static str,
// Latitude
lat: f32,
// Longitude
lon: f32,
}
impl Display for City {
// `f` is a buffer, 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)
}
}
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 },
].iter() {
println!("{}", *city);
}
}
Here's the full list of formatting traits and their respective argument types: