What is the difference between println format styles?

Question

I am very sorry to ask such a simple question ... A day ago, I began to study Rust and tried the println! method println! .

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

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

I know that {} String instead, but I don't understand a different formatting style. How do these styles differ from each other? I think {:?} Is an array or a vector. Is it correct?

Please explain this formatting style with a sample code :(

Answer 1

For thoroughness, the formatting syntax for std::fmt consists of two parts:

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

Where:

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

By default, if there is no position, name or format, select the argument corresponding to the index {} and use the Display flag. However, there are different features! By the link above:

• nothing ⇒ Display
• ? ⇒ Debugging
• o ⇒ Octal
• x ⇒ LowerHex
• x ⇒ UpperHex
• p ⇒ Index
• b ⇒ Binary
• e ⇒ LowerExp
• e ⇒ UpperExp

and if necessary, new features may be added in the future.

Answer 2

println!() is a macro that uses the std::fmt syntax and {} specifies parameters. If the brackets are left empty ( {} ), the corresponding argument must implement the Display flag, and if they contain :? , this means that you should use the Debug implementation instead.

The bottom line is that the type of parameters does not apply here, but the properties that they implement . For example, Vec tors implement Debug , but they do not implement Display , and therefore you cannot use {} against them, and {:?} Works just fine.