枚举

enum 关键字允许创建具有几个 不同变体的类型：

#[derive(Debug)]
enum Direction {
    Left,
    Right,
}

#[derive(Debug)]
enum PlayerMove {
    Pass,                        // Simple variant
    Run(Direction),              // Tuple variant
    Teleport { x: u32, y: u32 }, // Struct variant
}

fn main() {
    let m: PlayerMove = PlayerMove::Run(Direction::Left);
    println!("On this turn: {:?}", m);
}
This slide should take about 5 minutes.

关键点：

  • Enumerations allow you to collect a set of values under one type.
  • Direction is a type with variants. There are two values of Direction: Direction::Left and Direction::Right.
  • PlayerMove is a type with three variants. In addition to the payloads, Rust will store a discriminant so that it knows at runtime which variant is in a PlayerMove value.
  • This might be a good time to compare structs and enums:
    • In both, you can have a simple version without fields (unit struct) or one with different types of fields (variant payloads).
    • You could even implement the different variants of an enum with separate structs but then they wouldn’t be the same type as they would if they were all defined in an enum.
  • Rust 使用最小的空间来存储判标识。

    • 如有必要，它会存储所需最小大小的整数

    • 如果允许的变体值未涵盖所有位模式，则它将使用无效的位模式对判别标识进行编码（“小众优化”）。例如，Option<&u8> 存储的要么是指向整数的指针，要么是 None 变体的 NULL 值。

    • You can control the discriminant if needed (e.g., for compatibility with C):

      #[repr(u32)]
enum Bar {
    A, // 0
    B = 10000,
    C, // 10001
}

fn main() {
    println!("A: {}", Bar::A as u32);
    println!("B: {}", Bar::B as u32);
    println!("C: {}", Bar::C as u32);
}

      Without repr, the discriminant type takes 2 bytes, because 10001 fits 2 bytes.

探索更多

Rust 具有多种优化措施，可以减少枚举占用的空间。

  • Null pointer optimization: For some types, Rust guarantees that size_of::<T>() equals size_of::<Option<T>>().

    Example code if you want to show how the bitwise representation may look like in practice. It’s important to note that the compiler provides no guarantees regarding this representation, therefore this is totally unsafe.

    use std::mem::transmute;

macro_rules! dbg_bits {
    ($e:expr, $bit_type:ty) => {
        println!("- {}: {:#x}", stringify!($e), transmute::<_, $bit_type>($e));
    };
}

fn main() {
    unsafe {
        println!("bool:");
        dbg_bits!(false, u8);
        dbg_bits!(true, u8);

        println!("Option<bool>:");
        dbg_bits!(None::<bool>, u8);
        dbg_bits!(Some(false), u8);
        dbg_bits!(Some(true), u8);

        println!("Option<Option<bool>>:");
        dbg_bits!(Some(Some(false)), u8);
        dbg_bits!(Some(Some(true)), u8);
        dbg_bits!(Some(None::<bool>), u8);
        dbg_bits!(None::<Option<bool>>, u8);

        println!("Option<&i32>:");
        dbg_bits!(None::<&i32>, usize);
        dbg_bits!(Some(&0i32), usize);
    }
}