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泛型数据类型 (Generic Data Types)

Generic Data Types

我们使用泛型来为函数签名或结构体等项创建定义，然后我们可以将这些项与许多不同的具体数据类型一起使用。让我们先看看如何使用泛型定义函数、结构体、枚举和方法。然后，我们将讨论泛型如何影响代码性能。

We use generics to create definitions for items like function signatures or structs, which we can then use with many different concrete data types. Let’s first look at how to define functions, structs, enums, and methods using generics. Then, we’ll discuss how generics affect code performance.

在函数定义中 (In Function Definitions)

In Function Definitions

当定义一个使用泛型的函数时，我们将泛型放在函数签名中，即我们通常指定参数和返回值的数据类型的地方。这样做使我们的代码更灵活，并为我们的函数调用者提供更多功能，同时防止代码重复。

When defining a function that uses generics, we place the generics in the signature of the function where we would usually specify the data types of the parameters and return value. Doing so makes our code more flexible and provides more functionality to callers of our function while preventing code duplication.

继续我们的 largest 函数，示例 10-4 展示了两个函数，它们都寻找切片中的最大值。然后我们将这些函数合并为一个使用泛型的单一函数。

Continuing with our largest function, Listing 10-4 shows two functions that both find the largest value in a slice. We’ll then combine these into a single function that uses generics.

#![allow(unused)]
fn main() {
{{#rustdoc_include ../listings/ch10-generic-types-traits-and-lifetimes/listing-10-04/src/main.rs:here}}
}

largest_i32 函数是我们在示例 10-3 中提取的，用于寻找切片中的最大 i32。largest_char 函数寻找切片中的最大 char。函数体具有相同的代码，所以让我们通过在单个函数中引入一个泛型类型参数来消除重复。

The largest_i32 function is the one we extracted in Listing 10-3 that finds the largest i32 in a slice. The largest_char function finds the largest char in a slice. The function bodies have the same code, so let’s eliminate the duplication by introducing a generic type parameter in a single function.

要在新的单一函数中对类型进行参数化，我们需要为类型参数命名，就像我们为函数的数值参数命名一样。你可以使用任何标识符作为类型参数名称。但我们将使用 T，因为按照惯例，Rust 中的类型参数名称很短，通常只有一个字母，而且 Rust 的类型命名约定是 UpperCamelCase。作为 type 的缩写，T 是大多数 Rust 程序员的默认选择。

To parameterize the types in a new single function, we need to name the type parameter, just as we do for the value parameters to a function. You can use any identifier as a type parameter name. But we’ll use T because, by convention, type parameter names in Rust are short, often just one letter, and Rust’s type-naming convention is UpperCamelCase. Short for type, T is the default choice of most Rust programmers.

当我们在函数体中使用参数时，我们必须在签名中声明该参数名，以便编译器知道该名称的含义。同样，当我们在函数签名中使用类型参数名时，我们必须在使用它之前声明该类型参数名。为了定义泛型 largest 函数，我们将类型名声明放在尖括号 <> 内，放在函数名和参数列表之间，如下所示：

When we use a parameter in the body of the function, we have to declare the parameter name in the signature so that the compiler knows what that name means. Similarly, when we use a type parameter name in a function signature, we have to declare the type parameter name before we use it. To define the generic largest function, we place type name declarations inside angle brackets, <>, between the name of the function and the parameter list, like this:

fn largest<T>(list: &[T]) -> &T {

我们将此定义读作：“函数 largest 在某种类型 T 上是泛型的。”该函数有一个名为 list 的参数，它是一个 T 类型值的切片。largest 函数将返回一个指向相同类型 T 的值的引用。

We read this definition as “The function largest is generic over some type T.” This function has one parameter named list, which is a slice of values of type T. The largest function will return a reference to a value of the same type T.

示例 10-5 展示了在签名中使用泛型数据类型的合并后的 largest 函数定义。该示例还展示了我们如何使用 i32 值切片或 char 值切片来调用该函数。请注意，这段代码目前还无法编译。

Listing 10-5 shows the combined largest function definition using the generic data type in its signature. The listing also shows how we can call the function with either a slice of i32 values or char values. Note that this code won’t compile yet.

{{#rustdoc_include ../listings/ch10-generic-types-traits-and-lifetimes/listing-10-05/src/main.rs}}

如果我们现在编译这段代码，我们会得到这个错误：

If we compile this code right now, we’ll get this error:

{{#include ../listings/ch10-generic-types-traits-and-lifetimes/listing-10-05/output.txt}}

帮助文本提到了 std::cmp::PartialOrd，这是一个“特征 (trait)”，我们将在下一节讨论特征。目前，只需知道此错误指出 largest 的主体并不适用于 T 可能具有的所有类型。因为我们想在主体中比较 T 类型的值，所以我们只能使用其值可以排序的类型。为了启用比较，标准库提供了 std::cmp::PartialOrd 特征，你可以在类型上实现该特征（有关此特征的更多信息，请参阅附录 C）。要修复示例 10-5，我们可以遵循帮助文本的建议，将对 T 有效的类型限制为仅那些实现了 PartialOrd 的类型。这样示例就可以编译了，因为标准库在 i32 和 char 上都实现了 PartialOrd。

The help text mentions std::cmp::PartialOrd, which is a trait, and we’re going to talk about traits in the next section. For now, know that this error states that the body of largest won’t work for all possible types that T could be. Because we want to compare values of type T in the body, we can only use types whose values can be ordered. To enable comparisons, the standard library has the std::cmp::PartialOrd trait that you can implement on types (see Appendix C for more on this trait). To fix Listing 10-5, we can follow the help text’s suggestion and restrict the types valid for T to only those that implement PartialOrd. The listing will then compile, because the standard library implements PartialOrd on both i32 and char.

在结构体定义中 (In Struct Definitions)

In Struct Definitions

我们也可以使用 <> 语法定义结构体，以便在一个或多个字段中使用泛型类型参数。示例 10-6 定义了一个 Point<T> 结构体，用于持有任何类型的 x 和 y 坐标值。

We can also define structs to use a generic type parameter in one or more fields using the <> syntax. Listing 10-6 defines a Point<T> struct to hold x and y coordinate values of any type.

#![allow(unused)]
fn main() {
{{#rustdoc_include ../listings/ch10-generic-types-traits-and-lifetimes/listing-10-06/src/main.rs}}
}

在结构体定义中使用泛型的语法与函数定义中使用的语法类似。首先，我们在紧跟结构体名称后的尖括号内声明类型参数的名称。然后，我们在通常指定具体数据类型的结构体定义中使用该泛型类型。

The syntax for using generics in struct definitions is similar to that used in function definitions. First, we declare the name of the type parameter inside angle brackets just after the name of the struct. Then, we use the generic type in the struct definition where we would otherwise specify concrete data types.

请注意，因为我们只使用了一种泛型类型来定义 Point<T>，所以这个定义说明 Point<T> 结构体在某种类型 T 上是泛型的，并且字段 x 和 y “都是”该相同类型，无论该类型是什么。如果我们创建一个具有不同类型值的 Point<T> 实例，如示例 10-7 所示，我们的代码将无法编译。

Note that because we’ve used only one generic type to define Point<T>, this definition says that the Point<T> struct is generic over some type T, and the fields x and y are both that same type, whatever that type may be. If we create an instance of a Point<T> that has values of different types, as in Listing 10-7, our code won’t compile.

{{#rustdoc_include ../listings/ch10-generic-types-traits-and-lifetimes/listing-10-07/src/main.rs}}

在这个例子中，当我们把整数值 5 分配给 x 时，我们让编译器知道对于 Point<T> 的这个实例，泛型类型 T 将是一个整数。然后，当我们为 y 指定 4.0 时（我们已将其定义为与 x 具有相同的类型），我们将得到一个如下所示的类型不匹配错误：

In this example, when we assign the integer value 5 to x, we let the compiler know that the generic type T will be an integer for this instance of Point<T>. Then, when we specify 4.0 for y, which we’ve defined to have the same type as x, we’ll get a type mismatch error like this:

{{#include ../listings/ch10-generic-types-traits-and-lifetimes/listing-10-07/output.txt}}

要定义一个 x 和 y 都是泛型但可以具有不同类型的 Point 结构体，我们可以使用多个泛型类型参数。例如，在示例 10-8 中，我们将 Point 的定义更改为在类型 T 和 U 上是泛型的，其中 x 的类型为 T，y 的类型为 U。

To define a Point struct where x and y are both generics but could have different types, we can use multiple generic type parameters. For example, in Listing 10-8, we change the definition of Point to be generic over types T and U where x is of type T and y is of type U.

#![allow(unused)]
fn main() {
{{#rustdoc_include ../listings/ch10-generic-types-traits-and-lifetimes/listing-10-08/src/main.rs}}
}

现在显示的所有 Point 实例都是允许的！你可以在定义中使用任意数量的泛型类型参数，但使用超过几个会使你的代码难以阅读。如果你发现代码中需要大量的泛型类型，这可能表明你的代码需要重构成更小的部分。

Now all the instances of Point shown are allowed! You can use as many generic type parameters in a definition as you want, but using more than a few makes your code hard to read. If you’re finding you need lots of generic types in your code, it could indicate that your code needs restructuring into smaller pieces.

在枚举定义中 (In Enum Definitions)

In Enum Definitions

就像我们对结构体所做的那样，我们可以定义枚举以便在其变体中持有泛型数据类型。让我们再看看我们在第 6 章中使用的标准库提供的 Option<T> 枚举：

As we did with structs, we can define enums to hold generic data types in their variants. Let’s take another look at the Option<T> enum that the standard library provides, which we used in Chapter 6:

#![allow(unused)]
fn main() {
enum Option<T> {
    Some(T),
    None,
}
}

现在这个定义对你来说应该更有意义了。如你所见，Option<T> 枚举在类型 T 上是泛型的，具有两个变体：Some（持有一个 T 类型的值）和不持有任何值的 None 变体。通过使用 Option<T> 枚举，我们可以表达可选值这一抽象概念，并且由于 Option<T> 是泛型的，无论可选值的类型是什么，我们都可以使用这种抽象。

This definition should now make more sense to you. As you can see, the Option<T> enum is generic over type T and has two variants: Some, which holds one value of type T, and a None variant that doesn’t hold any value. By using the Option<T> enum, we can express the abstract concept of an optional value, and because Option<T> is generic, we can use this abstraction no matter what the type of the optional value is.

枚举也可以使用多种泛型类型。我们在第 9 章中使用的 Result 枚举的定义就是一个例子：

Enums can use multiple generic types as well. The definition of the Result enum that we used in Chapter 9 is one example:

#![allow(unused)]
fn main() {
enum Result<T, E> {
    Ok(T),
    Err(E),
}
}

Result 枚举在两种类型 T 和 E 上是泛型的，具有两个变体：Ok（持有 T 类型的值）和 Err（持有 E 类型的值）。这个定义使得在任何操作可能成功（返回某种类型 T 的值）或失败（返回某种类型 E 的错误）的地方，使用 Result 枚举都很方便。事实上，这就是我们在示例 9-3 中用于打开文件的内容，其中当文件成功打开时，T 被填充为类型 std::fs::File；当打开文件出现问题时，E 被填充为类型 std::io::Error。

The Result enum is generic over two types, T and E, and has two variants: Ok, which holds a value of type T, and Err, which holds a value of type E. This definition makes it convenient to use the Result enum anywhere we have an operation that might succeed (return a value of some type T) or fail (return an error of some type E). In fact, this is what we used to open a file in Listing 9-3, where T was filled in with the type std::fs::File when the file was opened successfully and E was filled in with the type std::io::Error when there were problems opening the file.

当你发现代码中存在多个仅在所持有值的类型上有所不同的结构体或枚举定义时，你可以通过使用泛型类型来避免重复。

When you recognize situations in your code with multiple struct or enum definitions that differ only in the types of the values they hold, you can avoid duplication by using generic types instead.

在方法定义中 (In Method Definitions)

In Method Definitions

我们可以在结构体和枚举上实现方法（正如我们在第 5 章中所做的），并且也可以在它们的定义中使用泛型类型。示例 10-9 展示了我们在示例 10-6 中定义的 Point<T> 结构体，并在其上实现了一个名为 x 的方法。

We can implement methods on structs and enums (as we did in Chapter 5) and use generic types in their definitions too. Listing 10-9 shows the Point<T> struct we defined in Listing 10-6 with a method named x implemented on it.

#![allow(unused)]
fn main() {
{{#rustdoc_include ../listings/ch10-generic-types-traits-and-lifetimes/listing-10-09/src/main.rs}}
}

在这里，我们在 Point<T> 上定义了一个名为 x 的方法，它返回一个指向字段 x 中数据的引用。

Here, we’ve defined a method named x on Point<T> that returns a reference to the data in the field x.

请注意，我们必须在 impl 之后立即声明 T，以便我们可以使用 T 来指定我们是在类型 Point<T> 上实现方法。通过在 impl 之后将 T 声明为泛型类型，Rust 可以识别出 Point 尖括号内的类型是一个泛型类型，而不是一个具体类型。我们可以为这个泛型参数选择一个与结构体定义中声明的泛型参数不同的名称，但使用相同的名称是惯例。如果你在一个声明了泛型类型的 impl 块中编写方法，该方法将在该类型的任何实例上定义，而不管最终替代泛型类型的是哪种具体类型。

Note that we have to declare T just after impl so that we can use T to specify that we’re implementing methods on the type Point<T>. By declaring T as a generic type after impl, Rust can identify that the type in the angle brackets in Point is a generic type rather than a concrete type. We could have chosen a different name for this generic parameter than the generic parameter declared in the struct definition, but using the same name is conventional. If you write a method within an impl that declares a generic type, that method will be defined on any instance of the type, no matter what concrete type ends up substituting for the generic type.

我们也可以在为类型定义方法时指定对泛型类型的约束。例如，我们可以仅在 Point<f32> 实例上实现方法，而不是在具有任何泛型类型的 Point<T> 实例上实现。在示例 10-10 中，我们使用了具体类型 f32，这意味着我们不在 impl 之后声明任何类型。

We can also specify constraints on generic types when defining methods on the type. We could, for example, implement methods only on Point<f32> instances rather than on Point<T> instances with any generic type. In Listing 10-10, we use the concrete type f32, meaning we don’t declare any types after impl.

#![allow(unused)]
fn main() {
{{#rustdoc_include ../listings/ch10-generic-types-traits-and-lifetimes/listing-10-10/src/main.rs:here}}
}

这段代码意味着类型 Point<f32> 将拥有一个 distance_from_origin 方法；而 T 不是 f32 类型的其他 Point<T> 实例将不会定义此方法。该方法测量我们的点与坐标 (0.0, 0.0) 处的点之间的距离，并使用了仅对浮点类型可用的数学运算。

This code means the type Point<f32> will have a distance_from_origin method; other instances of Point<T> where T is not of type f32 will not have this method defined. The method measures how far our point is from the point at coordinates (0.0, 0.0) and uses mathematical operations that are available only for floating-point types.

结构体定义中的泛型类型参数并不总是与你在该相同结构体的方法签名中使用的参数相同。示例 10-11 为 Point 结构体使用了泛型类型 X1 和 Y1，并为 mixup 方法签名使用了 X2 和 Y2，以使示例更清晰。该方法创建一个新的 Point 实例，其 x 值来自 self 的 Point（类型为 X1），而 y 值来自传入的 Point（类型为 Y2）。

Generic type parameters in a struct definition aren’t always the same as those you use in that same struct’s method signatures. Listing 10-11 uses the generic types X1 and Y1 for the Point struct and X2 and Y2 for the mixup method signature to make the example clearer. The method creates a new Point instance with the x value from the self Point (of type X1) and the y value from the passed-in Point (of type Y2).

#![allow(unused)]
fn main() {
{{#rustdoc_include ../listings/ch10-generic-types-traits-and-lifetimes/listing-10-11/src/main.rs}}
}

在 main 中，我们定义了一个 Point，其 x 为 i32（值为 5），y 为 f64（值为 10.4）。变量 p2 是一个 Point 结构体，其 x 为字符串切片（值为 "Hello"），y 为 char（值为 c）。在 p1 上调用 mixup 并传入参数 p2 得到了 p3，它将拥有一个 i32 类型的 x，因为 x 来自 p1。变量 p3 将拥有一个 char 类型的 y，因为 y 来自 p2。println! 宏调用将打印 p3.x = 5, p3.y = c。

In main, we’ve defined a Point that has an i32 for x (with value 5) and an f64 for y (with value 10.4). The p2 variable is a Point struct that has a string slice for x (with value "Hello") and a char for y (with value c). Calling mixup on p1 with the argument p2 gives us p3, which will have an i32 for x because x came from p1. The p3 variable will have a char for y because y came from p2. The println! macro call will print p3.x = 5, p3.y = c.

这个例子的目的是演示一种情况，其中一些泛型参数是在 impl 处声明的，而另一些是在方法定义处声明的。在这里，泛型参数 X1 和 Y1 在 impl 之后声明，因为它们与结构体定义相对应。泛型参数 X2 和 Y2 在 fn mixup 之后声明，因为它们仅与该方法相关。

The purpose of this example is to demonstrate a situation in which some generic parameters are declared with impl and some are declared with the method definition. Here, the generic parameters X1 and Y1 are declared after impl because they go with the struct definition. The generic parameters X2 and Y2 are declared after fn mixup because they’re only relevant to the method.

使用泛型的代码的性能 (Performance of Code Using Generics)

Performance of Code Using Generics

你可能想知道使用泛型类型参数是否会产生运行时开销。好消息是，使用泛型类型不会使你的程序比使用具体类型运行时更慢。

You might be wondering whether there is a runtime cost when using generic type parameters. The good news is that using generic types won’t make your program run any slower than it would with concrete types.

Rust 通过在编译时对使用泛型的代码执行单态化来实现这一点。“单态化 (Monomorphization)”是通过填充编译时使用的具体类型来将泛型代码转换为特定代码的过程。在此过程中，编译器执行的操作与我们在示例 10-5 中创建泛型函数所采取的步骤相反：编译器查找所有调用泛型代码的地方，并为调用泛型代码时使用的具体类型生成代码。

Rust accomplishes this by performing monomorphization of the code using generics at compile time. Monomorphization is the process of turning generic code into specific code by filling in the concrete types that are used when compiled. In this process, the compiler does the opposite of the steps we used to create the generic function in Listing 10-5: The compiler looks at all the places where generic code is called and generates code for the concrete types the generic code is called with.

让我们通过使用标准库的泛型 Option<T> 枚举来看看这是如何工作的：

Let’s look at how this works by using the standard library’s generic Option<T> enum:

#![allow(unused)]
fn main() {
let integer = Some(5);
let float = Some(5.0);
}

当 Rust 编译这段代码时，它会执行单态化。在此过程中，编译器读取已在 Option<T> 实例中使用的值，并识别出两种 Option<T>：一种是 i32，另一种是 f64。因此，它将 Option<T> 的泛型定义展开为针对 i32 和 f64 特化的两个定义，从而用特定的定义替换泛型定义。

When Rust compiles this code, it performs monomorphization. During that process, the compiler reads the values that have been used in Option<T> instances and identifies two kinds of Option<T>: One is i32 and the other is f64. As such, it expands the generic definition of Option<T> into two definitions specialized to i32 and f64, thereby replacing the generic definition with the specific ones.

代码的单态化版本看起来类似于以下内容（为了说明，编译器使用了与我们这里使用的不同的名称）：

The monomorphized version of the code looks similar to the following (the compiler uses different names than what we’re using here for illustration):

文件名: src/main.rs

enum Option_i32 {
    Some(i32),
    None,
}

enum Option_f64 {
    Some(f64),
    None,
}

fn main() {
    let integer = Option_i32::Some(5);
    let float = Option_f64::Some(5.0);
}

泛型 Option<T> 被编译器创建的特定定义所取代。因为 Rust 将泛型代码编译为在每个实例中指定类型的代码，所以我们不需要为使用泛型支付任何运行时开销。当代码运行时，它的表现就像我们手工复制了每个定义一样。单态化的过程使得 Rust 的泛型在运行时极其高效。

The generic Option<T> is replaced with the specific definitions created by the compiler. Because Rust compiles generic code into code that specifies the type in each instance, we pay no runtime cost for using generics. When the code runs, it performs just as it would if we had duplicated each definition by hand. The process of monomorphization makes Rust’s generics extremely efficient at runtime.