如何指定函数采用 HashMap? [英] How do I specify that a function takes a HashMap?

问题描述

我如何指定一个非原始类型作为 Rust 函数参数——特别是一个 HashMap?例如:

How do I specify a non-primitive type as a Rust function parameter - specifically, a HashMap? For example:

use std::collections::HashMap;

// a main function that would call fibbonacci...

// Here the hashmap would be used for memoizing;
// maybe ugly, but it's a first attempt
fn fibbonacci(n: i32, cache: ??) -> i32 {
}

我试过了:

• cache: Hashmap => 类型参数数量错误:预期至少为 2，找到 0
• cache: <HashMap> => error: expected ::, found )
• 缓存:std::collections::HashMap => 类型参数的数量错误:预期至少为 2，发现为 0

• cache: Hashmap => wrong number of type arguments: expected at least 2, found 0
• cache: <HashMap> => error: expected ::, found )
• cache: std::collections::HashMap => wrong number of type arguments: expected at least 2, found 0

这是 Rust 1.0.0.beta.

This is with Rust 1.0.0.beta.

推荐答案

让我们看看这段代码的编译器错误信息:

Let's check out the compiler error message for this code:

use std::collections::HashMap;

fn fibbonacci(n: i32, cache: HashMap) -> i32 {}

fn main() {}

我们得到:

error[E0243]: wrong number of type arguments: expected at least 2, found 0
 --> src/main.rs:3:29
  |
3 | fn fibonacci(n: i32, cache: HashMap) -> i32 {}
  |                             ^^^^^^^ expected at least 2 type arguments

请注意，它直接指向问题并告诉您需要 2 个类型参数.Rust 要求函数参数和返回值完全拼写出来，此时没有类型推断.

Note that it points directly to the issue and tells you that you need 2 type arguments. Rust requires that function arguments and return values be fully spelled out, there is no type inference at this point.

我不知道你想要什么键和值，所以我假设 i32:

I don't know what you want the keys and values to be, so I'll assume i32:

fn fibonacci(n: i32, cache: HashMap<i32, i32>) -> i32 { 0 }

更详细地说，HashMap 有两个通用类型参数，简称为K和V(但请参见下面的注释).要引用HashMap 的具体类型，您需要指定K 和V 是什么.您还可以使用更多泛型类型，但将特征边界放在泛型上.这有点高级，您无需担心它即可开始使用 Rust！

More verbosely, HashMap has two generic type parameters, referred to as K and V (but see note below). To reference a concrete type of HashMap, you need to specify what K and V are. You can also use more generic types but place trait bounds on the generics. This is a bit more advanced, and you don't need to worry about it to get started with Rust!

注意 - HashMap 实际上有 3 个类型参数，但第三个有一个默认值并且不经常使用.该类型参数允许控制所使用的散列算法.

note - HashMap actually has 3 type parameters, but the third has a default value and isn't often used. That type parameter allows controlling the hashing algorithm used.

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