rust - What is the difference between println's format styles?

Question

I'm so sorry to ask such a simple question... A day ago, I started learning Rust and tried the println! method.

fn main() {
  println!("Hello {}!", "world");
}
-> Hello world!

And then, I found other format styles: {}, {:}, {:?}, {?}, ...

I know that {} is instead String, but I don't understand the other format style. How do those styles differ from each other? I think {:?} is array or vector. Is it correct?

Please explain these format style with sample code :(

Answer 1

For thoroughness, the std::fmt formatting syntax is composed of two parts:

{<position-or-name>:<format>}

where:

• <position-or-name> can be the argument position: println!("Hello {0}!", "world");`, note that it is checked at compile-time
• <position-or-name> can also be a name: println!("Hello {arg}!", arg = "world");
• <format> is one of the following formats, where each format requires the argument to implement a specific trait, checked at compile-time

The default, in the absence of position, name or format, is to pick the argument matching the index of {} and to use the Display trait. There are however various traits! From the link above:

• nothing ? Display
• ? ? Debug
• o ? Octal
• x ? LowerHex
• X ? UpperHex
• p ? Pointer
• b ? Binary
• e ? LowerExp
• E ? UpperExp

and if necessary new traits could be added in the future.