Traits and Generics

Derive Debug

Printing a Struct

#[derive(Debug)] generates a debug formatter so a struct can be printed with {:?}.

Program

Play the program to print a point using its derived Debug output.

x
derive_debug.rs
Replay: real traced execution (multi-file project)
#[derive(Debug)]
struct Point {
    x: i32,
    y: i32,
}

fn main() {
    let x = 1;
    let y = 2;
    let p = Point { x, y };
    println!("{p:?}");
}

#[derive(Debug)]
struct Point {
    x: i32,
    y: i32,
}

fn main() {
    let x = -3;
    let y = 2;
    let p = Point { x, y };
    println!("{p:?}");
}

#[derive(Debug)]
struct Point {
    x: i32,
    y: i32,
}

fn main() {
    let x = 5;
    let y = 2;
    let p = Point { x, y };
    println!("{p:?}");
}

  1. x ← 1, y ← 2, p ← Point { x: 1, y: 2 }

    7fn main() {8    let → 1x = 1; //@x=1, 5, -39    let → 2y = 2;10    let → Point { x: 1, y: 2 }p = Point { 1x, 2y };11    println!("{p:?}");12}
    outputPoint { x: 1, y: 2 }

  1. x ← -3, y ← 2, p ← Point { x: -3, y: 2 }

    7fn main() {8    let → -3x = -3;9    let → 2y = 2;10    let → Point { x: -3, y: 2 }p = Point { -3x, 2y };11    println!("{p:?}");12}
    outputPoint { x: -3, y: 2 }

  1. x ← 5, y ← 2, p ← Point { x: 5, y: 2 }

    7fn main() {8    let → 5x = 5;9    let → 2y = 2;10    let → Point { x: 5, y: 2 }p = Point { 5x, 2y };11    println!("{p:?}");12}
    outputPoint { x: 5, y: 2 }

Follow the Debug Print

  1. x starts as 1.
  2. y starts as 2.
  3. Point { x, y } builds Point { x: 1, y: 2 }.
  4. #[derive(Debug)] lets println!("{p:?}") print the struct fields.
  5. The program prints Point { x: 1, y: 2 }. | x | y | debug output | | --- | --- | --- | | 1 | 2 | Point { x: 1, y: 2 } | | 5 | 2 | Point { x: 5, y: 2 } | | -3 | 2 | Point { x: -3, y: 2 } |
derive `#[derive(Debug)]` auto-generates trait code.
Debug The `Debug` trait enables developer-facing formatting.
{:?} `{:?}` prints the derived debug representation.

Exercise: derive_debug.rs

Reproduce Point { x: 1, y: 2 }, then use the pinned x variants 5 and -3 to predict Point { x: 5, y: 2 } and Point { x: -3, y: 2 }.