Display printing puts a value into a readable message. In `println!`, `{}` asks Rust to print a value using its `Display` behavior: `println!("Answer: {}", 42)`. Not every type can do this. A trait describes what a type can do, so a type needs the `Display` trait to work with `{}`. section 1.8
When `{}` does not work
Try `println!("This will not print: {}", ());`. The compiler reports that `()` does not implement `Display` and suggests `{:?}` or `{:#?}` instead. Treat that message as a useful hint: use Debug printing when you need to inspect a value that cannot use Display. section 1.8
`{:?}` uses Debug printing, which usually shows more detail and is less polished. `{:#?}` spreads Debug output over more lines when that makes it easier to read. Use `{}` when you want the value’s Display output; use Debug when you want to inspect its structure. section 1.8
use std::fmt;
struct Cat {
name: String,
age: u8,
}
impl fmt::Display for Cat {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
write!(f, "{} is a cat who is {} years old", self.name, self.age)
}
}
fn main() {
let mr_mantle = Cat {
name: "Reggie Mantle".to_string(),
age: 4,
};
println!("{mr_mantle}");
}
This complete example implements `Display` for a `Cat`. Its `fmt` method uses `write!` to choose the sentence that `println!("{mr_mantle}")` prints. section 7.1.1
Deriving `Debug` can print a struct’s fields, but implementing `Display` lets you choose wording suited to the reader. Once a type implements `Display`, it also gains `.to_string()`, which produces a `String` from that formatting. section 7.1.1
A cleaner application screen
An application struct can implement `Display` so its article title, description, and extract appear as a clean screen rather than Debug-style struct output. section 23.3.3
