如何有条件地检查枚举是一种变体还是另一种变体？

Question

我有一个有两个变体的枚举：

enum DatabaseType {
    Memory,
    RocksDB,
}

我需要什么才能在检查参数是 DatabaseType::Memory 还是 DatabaseType::RocksDB 的函数中创建条件 if？

fn initialize(datastore: DatabaseType) -> Result<V, E> {
    if /* Memory */ {
        //..........
    } else if /* RocksDB */ {
        //..........
    }
}

Answer 1

首先，返回并重新阅读免费的官方 Rust 书籍：The Rust Programming Language, specifically the chapter on enums。

`match`

fn initialize(datastore: DatabaseType) {
    match datastore {
        DatabaseType::Memory => {
            // ...
        }
        DatabaseType::RocksDB => {
            // ...
        }
    }
}

`if let`

fn initialize(datastore: DatabaseType) {
    if let DatabaseType::Memory = datastore {
        // ...
    } else {
        // ...
    }
}

`==`

#[derive(PartialEq)]
enum DatabaseType {
    Memory,
    RocksDB,
}

fn initialize(datastore: DatabaseType) {
    if DatabaseType::Memory == datastore {
        // ...
    } else {
        // ...
    }
}

`matches!`

自 Rust 1.42.0 起可用

fn initialize(datastore: DatabaseType) {
    if matches!(datastore, DatabaseType::Memory) {
        // ...
    } else {
        // ...
    }
}

另请参阅：

How to compare enum without pattern matching
https://doc.rust-lang.org/std/macro.matches.html

Answer 2

// A simple example that runs in rust 1.58:
enum Aap {
    Noot(i32, char),
    Mies(String, f64),
}

fn main() {
    let aap: Aap = Aap::Noot(42, 'q');
    let noot: Aap = Aap::Mies(String::from("noot"), 422.0);
    println!("{}", doe(aap));
    println!("{}", doe(noot));
}

fn doe(a: Aap) -> i32 {
    match a {
        Aap::Noot(i, _) => i,
        Aap::Mies(_, f) => f as i32,
    }
}

如何有条件地检查枚举是一种变体还是另一种变体？

How do I conditionally check if an enum is one variant or another?

enums

conditional

rust

`match`

`if let`

`==`

`matches!`