How to implement the Debug trait for struct that has a trait object member? [duplicate]

问题

My goal is printing the contents of struct that has trait object member but I can't find how to tell Rust compiler that the member also implements other traits like Display or Debug.

For example, in the following program, I want to print the structure of S2 (and S1 for comparison) but I get stuck in the implementation of fmt.

trait Tr {}

impl Tr for usize {}

impl Tr for String {}

#[derive(Debug)]
struct S1<A: Tr + std::fmt::Debug> {
    member: Box<A>,
}

struct S2 {
    member: Box<Tr>,
}

impl std::fmt::Debug for S2 {
    fn fmt(&self, fmt: &mut std::fmt::Formatter<'_>) -> Result<(), std::fmt::Error> {
        // ??
        Ok(())
    }
}

fn main() {
    let s1 = S1 {
        member: Box::new(String::from("abc")),
    };

    println!("{:?}", s1);

    let s2 = S2 {
        member: Box::new(String::from("abc")),
    };

    println!("{:?}", s2);
}

My desired output of this program is

S1 { member: "abc" }
S2 { member: "abc" }

Is it possible to implement Debug for a struct like S2?

(Rust version: 1.35)

回答1:

Is it possible to implement Debug for a struct like S2?

Yes you can, and this is clearly pointed out in the Book

You need to implement the Debug trait for your S2 like following:

trait MyTrait {}
impl MyTrait for usize {}
impl MyTrait for String {}

trait MyTraitWritable: MyTrait + Debug {}
impl MyTraitWritable for usize {}
impl MyTraitWritable for String {}

impl std::fmt::Debug for S2 {
    fn fmt(&self, fmt: &mut std::fmt::Formatter<'_>) -> Result<(), std::fmt::Error> {
        write!(fmt, "S2 {{ member: {:?} }}", self.member)
    }
}

Playground

回答2:

You can make S2 generic but you don't need to specify that the type should also implement Debug right there. Instead, you can specify it in the impl:

struct S2<A: Tr> {
    member: Box<A>,
}

impl<A: Tr + std::fmt::Debug> std::fmt::Debug for S2<A> {
    fn fmt(&self, fmt: &mut std::fmt::Formatter<'_>) -> Result<(), std::fmt::Error> {
        write!(fmt, "S2 {{ member: {:?} }}", self.member)?;
        Ok(())
    }
}

This way, S2 will implement Debug if the actual type does.

来源：https://stackoverflow.com/questions/56343245/how-to-implement-the-debug-trait-for-struct-that-has-a-trait-object-member

标签

rust

traits