Display - Rust By Example
Description: A description

# 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:

    # #![allow(unused_variables)]
    #fn main() {
    // 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 Point2D {
        x: f64,
        y: f64,
    }

    // Similarly, implement for Point2D
    impl fmt::Display for Point2D {
        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 = Point2D { 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 `Point2D` 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