什么时候应该使用结构而不是枚举？ [英] When should we use a struct as opposed to an enum?

问题描述

结构和枚举彼此相似。

Structs and enums are similar to each other.

什么时候使用结构而不是枚举会更好（反之亦然）？有人可以举一个清晰的例子，说明使用结构胜于使用枚举吗？

When would it be better to use a struct as opposed to an enum (or vice-versa)? Can someone give a clear example where using a struct is preferable to using an enum?

推荐答案

也许是解释基本原理的最简单方法区别在于，一个枚举包含变体，一次只能拥有一个，而一个结构包含一个或多个字段，必须全部具有 all 。

Perhaps the easiest way to explain the fundamental difference is that an enum contains "variants", of which you can only ever have one at a time, whereas a struct contains one or more fields, all of which you must have.

因此，您可以使用枚举来建模类似错误代码的代码，一次只能有一个错误代码：

So you might use an enum to model something like an error code, where you can only ever have one at a time:

enum ErrorCode {
    NoDataReceived,
    CorruptedData,
    BadResponse,
}

枚举变量可以根据需要包含值。例如，我们可以像这样向 ErrorCode 添加一个案例：

Enum variants can contain values if needed. For example, we could add a case to ErrorCode like so:

enum ErrorCode {
    NoDataReceived,
    CorruptedData,
    BadResponse,
    BadHTTPCode(u16),
}

在这种情况下， ErrorCode :: BadHTTPCode 的实例始终包含 u16 。

In this case, an instance of ErrorCode::BadHTTPCode always contains a u16.

这使每个变体的行为都类似于元组结构或单元结构：

This makes each individual variant behave kind of like either a tuple struct or unit struct:

// Unit structs have no fields
struct UnitStruct;

// Tuple structs contain anonymous values.
struct TupleStruct(u16, &'static str);

但是，将它们编写为枚举变量的好处是每个<$ c $可以将c> ErrorCode 存储为 ErrorCode 类型的值，如下所示（对于不相关的结构，这是不可能的）。

However, the advantage of writing them as enum variants is that each of the cases of ErrorCode can be stored in a value of type ErrorCode, as below (this would not be possible with unrelated structs).

fn handle_error(error: ErrorCode) {
    match error {
        ErrorCode::NoDataReceived => println!("No data received."),
        ErrorCode::CorruptedData => println!("Data corrupted."),
        ErrorCode::BadResponse => println!("Bad response received from server."),
        ErrorCode::BadHTTPCode(code) => println!("Bad HTTP code received: {}", code)
    };
}

fn main() {
    handle_error(ErrorCode::NoDataReceived); // prints "No data received."
    handle_error(ErrorCode::BadHTTPCode(404)); // prints "Bad HTTP code received: 404"
}

然后您可以<$在枚举上进行c $ c> match 以确定给您哪种变体，并根据其变体执行不同的操作。

You can then match on the enum to determine which variant you've been given, and perform different actions depending on which one it is.

相比之下，我上面没有提到的第三种结构类型是最常用的-这是每个人在简单地说结构时所指的结构类型

By contrast, the third type of struct that I didn't mention above is the most commonly used - it's the type of struct that everyone is referring to when they simply say "struct".

struct Response {
    data: Option<Data>,
    response: HTTPResponse,
    error: String,
}

fn main() {
    let response = Response {
        data: Option::None,
        response: HTTPResponse::BadRequest,
        error: "Bad request".to_owned()
    }
}

请注意，在这种情况下，要创建 Response ，必须为其所有字段提供值。

Note that in that case, in order to create a Response, all of its fields must be given values.

此外，的值响应的方式已创建（即 HTTPResponse :: Something ）表示 HTTPResponse 是一个枚举。可能看起来像这样：

Also, the way that the value of response is created (i.e. HTTPResponse::Something) implies that HTTPResponse is an enum. It might look something like this:

enum HTTPResponse {
    Ok,         // 200
    BadRequest, // 400
    NotFound,   // 404
}

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