如何实现枚举及其各自变体的特征？

Question

I'm trying to use enum variants to capture data which is heterogeneous in nature (has different collections of fields) but which is of the same "type" from a protocol perspective. However, I'm not sure how to implement subtype-specific methods and traits. Here is a minimal example of how I can create an enumeration of Data and I can use enum variant constructors to specify the types, but if I implement a trait on the variant, calling that function is not something I've figured out how to do.

use std::fmt;

enum Data {
    N(NData),
    S(SData),
}

struct NData {
    numeric: u32,
}

impl fmt::Display for NData {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        write!(f, "{}", self.numeric)
    }
}

struct SData {
    stringy: Vec<String>,
}

fn main() {
    let d_n: Data = Data::N(NData { numeric: 0x0 });
    let n = NData { numeric: 0xff };

    // Fails, fmt::Display not implemented for Data
    println!("{}", d_n);

    // Just fine!
    println!("{}", n);
}

Answer 1

One possible solution could be to implement your trait for the variants as well as for the enum , which as you can see here only calls the specific implementations of the variants:

use std::fmt;

struct NData {
    numeric: u32,
}

impl fmt::Display for NData {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        write!(f, "{}", self.numeric)
    }
}

struct SData {
    strings: Vec<String>,
}

impl fmt::Display for SData {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        write!(f, "{:?}", self.strings)
    }
}

enum Data {
    N(NData),
    S(SData),
}

impl fmt::Display for Data {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        match self {
            Data::N(n_data) => n_data.fmt(f),
            Data::S(s_data) => s_data.fmt(f),
        }
    }
}

fn main() {
    let n = NData { numeric: 0xff };
    let s = SData { strings: vec!["hello".to_string(), "world".to_string()] };

    println!("{}", n);
    println!("{}", s);

    let d_n = Data::N(n);
    let d_s = Data::S(s);

    println!("{}", d_n);
    println!("{}", d_s);
}

Which will produce the following output:

255
["hello", "world"]
255
["hello", "world"]