Debug Derived Trait

This is from lower on this page:

https://doc.rust-lang.org/book/ch05-02-example-structs.html

TODO: Make sure you've already talked about the debug print format {:?} and !dbg and how !dbg takes ownership and then returns it and prints to stderr

You can't print a struct directly.

struct Widget { alfa: bool }

fn main() {

  let thing = Widget { alfa: true };

  println!("thing is {}", thing);

}

TODO: Show that error

The {:?} debug syntax won't work either.

struct Widget { alfa: bool }

fn main() {

  let thing = Widget { alfa: true };

  println!("thing is {:?}", thing);

}

TODO: Show the "error[E0277]: Widget doesn't implement Debug" error.

If you add the outter attribute for debugging with: #[derive(Debug)] it'll allow for {:?} or {:#?} to output a basic representation of the struct. (The {:#?}is the same thing as{:?} with a little nicer formatting and is that's shown here)

#[derive(Debug)]
struct Widget { alfa: bool }

fn main() {

  let thing = Widget { alfa: true };

  println!("thing is {:#?}", thing);

}

Using !dbg

Probably you should just show this for the debuggin stuff instead of {:?} in println!() for the default way to do debugging. TODO: look into the differences to see about that, but almost certianly move the {:?} to a later section.

NOTE: this isn't showing up on the playground possibly because only stdout and not stderr is returned? Need to look into that.

#[derive(Debug)]
struct Widget { alfa: bool }

fn main() {

  let thing = Widget { alfa: true };

  dbg!(thing);

}

Use dbg! Around Expressions

You can use around expressions. This means you can do things like this (TODO: figure out if dbg! can output on the rust playground.)

fn main() {

  let alfa = 3;
  let bravo = 5;

  let charlie = dbg!(alfa * bravo);

  println!("charlie is {}", charlie);

}