Debug - Rust By Example
Description: A description

# Debug

All types which want to use `std::fmt` formatting `traits` require an implementation to be printable. Automatic implementations are only provided for types such as in the `std` library. All others must be manually implemented somehow.

The `fmt::Debug` `trait` makes this very straightforward. All types can `derive` (automatically create) the `fmt::Debug` implementation. This is not true for `fmt::Display` which must be manually implemented.

    # #![allow(unused_variables)]
    #fn main() {
    // This structure cannot be printed either with `fmt::Display` or
    // with `fmt::Debug`.
    struct UnPrintable(i32);

    // The `derive` attribute automatically creates the implementation
    // required to make this `struct` printable with `fmt::Debug`.
    #[derive(Debug)]
    struct DebugPrintable(i32);
    #}

All `std` library types automatically are printable with `{:?}` too:

    // Derive the `fmt::Debug` implementation for `Structure`. `Structure`
    // is a structure which contains a single `i32`.
    #[derive(Debug)]
    struct Structure(i32);

    // Put a `Structure` inside of the structure `Deep`. Make it printable
    // also.
    #[derive(Debug)]
    struct Deep(Structure);

    fn main() {
        // Printing with `{:?}` is similar to with `{}`.
        println!("{:?} months in a year.", 12);
        println!("{1:?} {0:?} is the {actor:?} name.",
                 "Slater",
                 "Christian",
                 actor="actor's");

        // `Structure` is printable!
        println!("Now {:?} will print!", Structure(3));

        // The problem with `derive` is there is no control over how
        // the results look. What if I want this to just show a `7`?
        println!("Now {:?} will print!", Deep(Structure(7)));
    }

So `fmt::Debug` definitely makes this printable but sacrifices some elegance. Rust also provides "pretty printing" with `{:#?}`.

    #[derive(Debug)]
    struct Person<'a> {
        name: &'a str,
        age: u8
    }

    fn main() {
        let name = "Peter";
        let age = 27;
        let peter = Person { name, age };

        // Pretty print
        println!("{:#?}", peter);
    }

One can manually implement `fmt::Display` to control the display.

### See also

attributes, `derive`, `std::fmt`, and `struct`