有没有办法打印枚举值？

Question

有没有简单的方法来格式化和打印枚举值？我希望他们有一个默认的实现std::fmt::Display,但似乎并非如此.

enum Suit {
    Heart,
    Diamond,
    Spade,
    Club
}

fn main() {
    let s: Suit = Suit::Heart;
    println!("{}", s);
}

期望的输出: Heart

错误:

error[E0277]: the trait bound `Suit: std::fmt::Display` is not satisfied
  --> src/main.rs:10:20
   |
10 |     println!("{}", s);
   |                    ^ the trait `std::fmt::Display` is not implemented for `Suit`
   |
   = note: `Suit` cannot be formatted with the default formatter; try using `:?` instead if you are using a format string
   = note: required by `std::fmt::Display::fmt`

Answer 1

您可以派生出一个实现std::format::Debug:

#[derive(Debug)]
enum Suit {
    Heart,
    Diamond,
    Spade,
    Club
}

fn main() {
    let s = Suit::Heart;
    println!("{:?}", s);
}

导出是不可能的,Display因为Display它旨在向人类显示,并且编译器无法自动决定该情况的适当样式.Debug适用于程序员,因此可以自动生成内部公开视图.

Answer 2

该Debug特性打印出的名称Enum变型.

如果您需要格式化输出,可以实现Display你Enum像这样:

use std::fmt;

enum Suit {
    Heart,
    Diamond,
    Spade,
    Club
}

impl fmt::Display for Suit {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
       match *self {
           Suit::Heart => write!(f, "?"),
           Suit::Diamond => write!(f, "?"),
           Suit::Spade => write!(f, "?"),
           Suit::Club => write!(f, "?"),
       }
    }
}

fn main() {
    let heart = Suit::Heart;
    println!("{}", heart);
}

Answer 3

如果您想自动生成Display枚举变体的实现，您可能需要使用strum crate：

#[derive(strum_macros::Display)]
enum Suit {
    Heart,
    Diamond,
    Spade,
    Club,
}

fn main() {
    let s: Suit = Suit::Heart;
    println!("{}", s); // Prints "Heart"
}

Answer 4

结合两个DK。Matilda Smeds回答了一个稍微简洁的版本：

use std::fmt;

#[derive(Debug)]
enum Suit {
    Heart,
    Diamond,
    Spade,
    Club
}

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

fn main() {
    let heart = Suit::Heart;
    println!("{}", heart);
}