Rust by Example

1.2.2 Display

fmt::Debug hardly looks compact and clean, so it is often advantageous to customize the output appearance. This is done by manually implementing fmt::Display, which uses the {} print marker. Implementing it looks like this:

// Import (via `use`) the `fmt` module to make it available.
use std::fmt;

// Define a structure which `fmt::Display` will be implemented for. This is simply
// a tuple struct containing an `i32` bound to the name `Structure`.
struct Structure(i32);

// In order to use the `{}` marker, the trait `fmt::Display` must be implemented
// manually for the type.
impl fmt::Display for Structure {
    // This trait requires `fmt` with this exact signature.
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        // Write strictly the first element into the supplied output
        // stream: `f`. Returns `fmt::Result` which indicates whether the
        // operation succeeded or failed. Note that `write!` uses syntax which
        // is very similar to `println!`.
        write!(f, "{}", self.0)
    }
}

fmt::Display may be cleaner than fmt::Debug but this presents a problem for the std library. How should ambiguous types be displayed? For example, if the std library implemented a single style for all Vec<T>, what style should it be? Either of these two?

  • Vec<path>: /:/etc:/home/username:/bin (split on :)
  • Vec<number>: 1,2,3 (split on ,)

No, because there is no ideal style for all types and the std library doesn't presume to dictate one. fmt::Display is not implemented for Vec<T> or for any other generic containers. fmt::Debug must then be used for these generic cases.

This is not a problem though because for any new container type which is not generic,fmt::Display can be implemented.

use std::fmt; // Import `fmt`

// A structure holding two numbers. `Debug` will be derived so the results can
// be contrasted with `Display`.
#[derive(Debug)]
struct MinMax(i64, i64);

// Implement `Display` for `MinMax`.
impl fmt::Display for MinMax {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        // Use `self.number` to refer to each positional data point.
        write!(f, "({}, {})", self.0, self.1)
    }
}

// Define a structure where the fields are nameable for comparison.
#[derive(Debug)]
struct Point2 {
    x: f64,
    y: f64,
}

// Similarly, implement for Point2
impl fmt::Display for Point2 {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        // Customize so only `x` and `y` are denoted.
        write!(f, "x: {}, y: {}", self.x, self.y)
    }
}

fn main() {
    let minmax = MinMax(0, 14);

    println!("Compare structures:");
    println!("Display: {}", minmax);
    println!("Debug: {:?}", minmax);

    let big_range =   MinMax(-300, 300);
    let small_range = MinMax(-3, 3);

    println!("The big range is {big} and the small is {small}",
             small = small_range,
             big = big_range);

    let point = Point2 { x: 3.3, y: 7.2 };

    println!("Compare points:");
    println!("Display: {}", point);
    println!("Debug: {:?}", point);

    // Error. Both `Debug` and `Display` were implemented but `{:b}`
    // requires `fmt::Binary` to be implemented. This will not work.
    // println!("What does Point2D look like in binary: {:b}?", point);
}

So, fmt::Display has been implemented but fmt::Binary has not, and therefore cannot be used. std::fmt has many such traits and each requires its own implementation. This is detailed further in std::fmt.

Activity

After checking the output of the above example, use the Point2 struct as guide to add a Complex struct to the example. When printed in the same way, the output should be:

Display: 3.3 + 7.2i
Debug: Complex { real: 3.3, imag: 7.2 }

See also

derive, std::fmt, macros, struct, trait, and use