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闭包 (Closures)

Rust 的闭包是你可以保存在变量中或作为参数传递给其他函数的匿名函数。你可以在一个地方创建闭包，然后在他处调用它，以便在不同的上下文中进行求值。与函数不同，闭包可以从定义它们的作用域中捕获值。我们将演示这些闭包特性如何实现代码重用和行为定制。

Rust’s closures are anonymous functions you can save in a variable or pass as arguments to other functions. You can create the closure in one place and then call the closure elsewhere to evaluate it in a different context. Unlike functions, closures can capture values from the scope in which they’re defined. We’ll demonstrate how these closure features allow for code reuse and behavior customization.

捕获环境 (Capturing the Environment)

我们将首先研究如何使用闭包从定义它们的环境中捕获值以备后用。场景如下：每隔一段时间，我们的 T 恤公司就会向邮寄列表中的某人赠送一件独家限量版衬衫作为促销。邮寄列表中的人可以有选择地在他们的个人资料中添加他们最喜欢的颜色。如果被选中获得免费衬衫的人设置了他们最喜欢的颜色，他们就会得到那种颜色的衬衫。如果该人没有指定最喜欢的颜色，他们将得到公司目前库存最多的颜色。

We’ll first examine how we can use closures to capture values from the environment they’re defined in for later use. Here’s the scenario: Every so often, our T-shirt company gives away an exclusive, limited-edition shirt to someone on our mailing list as a promotion. People on the mailing list can optionally add their favorite color to their profile. If the person chosen for a free shirt has their favorite color set, they get that color shirt. If the person hasn’t specified a favorite color, they get whatever color the company currently has the most of.

实现这一点有很多方法。在这个例子中，我们将使用一个名为 ShirtColor 的枚举，它具有 Red 和 Blue 变体（为了简单起见，限制了可用颜色的数量）。我们用一个 Inventory 结构体来代表公司的库存，该结构体有一个名为 shirts 的字段，包含一个代表当前库存衬衫颜色的 Vec<ShirtColor>。定义在 Inventory 上的 giveaway 方法获取获胜者可选的衬衫颜色偏好，并返回该人将获得的衬衫颜色。此设置如示例 13-1 所示。

There are many ways to implement this. For this example, we’re going to use an enum called ShirtColor that has the variants Red and Blue (limiting the number of colors available for simplicity). We represent the company’s inventory with an Inventory struct that has a field named shirts that contains a Vec<ShirtColor> representing the shirt colors currently in stock. The method giveaway defined on Inventory gets the optional shirt color preference of the free-shirt winner, and it returns the shirt color the person will get. This setup is shown in Listing 13-1.

{{#rustdoc_include ../listings/ch13-functional-features/listing-13-01/src/main.rs}}

main 中定义的 store 还有两件蓝色衬衫和一件红色衬衫可供此次限量版促销活动分发。我们为一位偏好红色衬衫的用户和一位没有任何偏好的用户调用了 giveaway 方法。

The store defined in main has two blue shirts and one red shirt remaining to distribute for this limited-edition promotion. We call the giveaway method for a user with a preference for a red shirt and a user without any preference.

同样，这段代码可以用很多方式实现，在这里，为了专注于闭包，除了使用闭包的 giveaway 方法体之外，我们坚持使用你已经学过的概念。在 giveaway 方法中，我们将用户偏好作为 Option<ShirtColor> 类型的参数，并在 user_preference 上调用 unwrap_or_else 方法。Option<T> 上的 unwrap_or_else 方法是由标准库定义的。它接收一个参数：一个不带任何参数并返回一个 T 值（与存储在 Option<T> 的 Some 变体中的类型相同，在此例中为 ShirtColor）的闭包。如果 Option<T> 是 Some 变体，unwrap_or_else 返回 Some 内部的值。如果 Option<T> 是 None 变体，unwrap_or_else 调用该闭包并返回闭包返回的值。

Again, this code could be implemented in many ways, and here, to focus on closures, we’ve stuck to concepts you’ve already learned, except for the body of the giveaway method that uses a closure. In the giveaway method, we get the user preference as a parameter of type Option<ShirtColor> and call the unwrap_or_else method on user_preference. The unwrap_or_else method on Option<T> is defined by the standard library. It takes one argument: a closure without any arguments that returns a value T (the same type stored in the Some variant of the Option<T>, in this case ShirtColor). If the Option<T> is the Some variant, unwrap_or_else returns the value from within the Some. If the Option<T> is the None variant, unwrap_or_else calls the closure and returns the value returned by the closure.

我们指定闭包表达式 || self.most_stocked() 作为 unwrap_or_else 的参数。这是一个本身不带参数的闭包（如果闭包带有参数，它们将出现在两条垂直线之间）。闭包体调用了 self.most_stocked()。我们在这里定义了闭包，如果需要结果，unwrap_or_else 的实现稍后将对该闭包进行求值。

We specify the closure expression || self.most_stocked() as the argument to unwrap_or_else. This is a closure that takes no parameters itself (if the closure had parameters, they would appear between the two vertical pipes). The body of the closure calls self.most_stocked(). We’re defining the closure here, and the implementation of unwrap_or_else will evaluate the closure later if the result is needed.

运行这段代码会打印以下内容：

Running this code prints the following:

{{#include ../listings/ch13-functional-features/listing-13-01/output.txt}}

这里一个有趣的方面是我们传递了一个闭包，它在当前的 Inventory 实例上调用 self.most_stocked()。标准库不需要了解我们定义的 Inventory 或 ShirtColor 类型，也不需要了解我们在这个场景中想要使用的逻辑。闭包捕获了对 self Inventory 实例的一个不可变引用，并将其与我们指定的代码一起传递给 unwrap_or_else 方法。另一方面，函数无法以这种方式捕获其环境。

One interesting aspect here is that we’ve passed a closure that calls self.most_stocked() on the current Inventory instance. The standard library didn’t need to know anything about the Inventory or ShirtColor types we defined, or the logic we want to use in this scenario. The closure captures an immutable reference to the self Inventory instance and passes it with the code we specify to the unwrap_or_else method. Functions, on the other hand, are not able to capture their environment in this way.

推断并标注闭包类型 (Inferring and Annotating Closure Types)

函数和闭包之间还有更多的区别。闭包通常不要求你像 fn 函数那样标注参数或返回值的类型。函数要求类型标注是因为类型是暴露给用户的显式接口的一部分。严谨地定义此接口对于确保每个人都同意函数使用和返回什么类型的值非常重要。另一方面，闭包不用于这样的暴露接口：它们存储在变量中，并且在使用时不需要命名它们并将其暴露给库的用户。

There are more differences between functions and closures. Closures don’t usually require you to annotate the types of the parameters or the return value like fn functions do. Type annotations are required on functions because the submitters of the types are part of an explicit interface exposed to your users. Defining this interface rigidly is important for ensuring that everyone agrees on what types of values a function uses and returns. Closures, on the other hand, aren’t used in an exposed interface like this: They’re stored in variables, and they’re used without naming them and exposing them to users of our library.

闭包通常很短，并且仅在狭窄的语境中相关，而不是在任何任意场景中。在这些有限的语境中，编译器可以推断参数的类型和返回类型，类似于它推断大多数变量类型的方式（在极少数情况下，编译器也需要闭包类型标注）。

Closures are typically short and relevant only within a narrow context rather than in any arbitrary scenario. Within these limited contexts, the compiler can infer the types of the parameters and the return type, similar to how it’s able to infer the types of most variables (there are rare cases where the compiler needs closure type annotations too).

与变量一样，如果我们想要增加显式性和清晰度，我们可以添加类型标注，代价是比严格必要的更冗长。为一个闭包标注类型看起来像示例 13-2 中的定义。在这个例子中，我们定义了一个闭包并将其存储在一个变量中，而不是像在示例 13-1 中那样在将其作为参数传递的地方定义闭包。

As with variables, we can add type annotations if we want to increase explicitness and clarity at the cost of being more verbose than is strictly necessary. Annotating the types for a closure would look like the definition shown in Listing 13-2. In this example, we’re defining a closure and storing it in a variable rather than defining the closure in the spot we pass it as an argument, as we did in Listing 13-1.

#![allow(unused)]
fn main() {
{{#rustdoc_include ../listings/ch13-functional-features/listing-13-02/src/main.rs:here}}
}

添加类型标注后，闭包的语法看起来与函数的语法更加相似。为了对比，这里我们定义了一个在其参数上加 1 的函数和一个具有相同行为的闭包。我们添加了一些空格来对齐相关部分。这说明了除了使用垂直线和可选语法的数量之外，闭包语法如何与函数语法相似：

With type annotations added, the syntax of closures looks more similar to the syntax of functions. Here, we define a function that adds 1 to its parameter and a closure that has the same behavior, for comparison. We’ve added some spaces to line up the relevant parts. This illustrates how closure syntax is similar to function syntax except for the use of pipes and the amount of syntax that is optional:

fn  add_one_v1   (x: u32) -> u32 { x + 1 }
let add_one_v2 = |x: u32| -> u32 { x + 1 };
let add_one_v3 = |x|             { x + 1 };
let add_one_v4 = |x|               x + 1  ;

第一行显示了一个函数定义，第二行显示了一个带有完整标注的闭包定义。在第三行，我们从闭包定义中删除了类型标注。在第四行，我们删除了括号，因为闭包体只有一个表达式，所以括号是可选的。这些都是有效的定义，当它们被调用时将产生相同的行为。add_one_v3 和 add_one_v4 行要求对闭包进行求值才能通过编译，因为类型将从其用法中推断出来。这类似于 let v = Vec::new(); 需要类型标注或向 Vec 中插入某种类型的值才能让 Rust 推断出类型。

The first line shows a function definition and the second line shows a fully annotated closure definition. In the third line, we remove the type annotations from the closure definition. In the fourth line, we remove the brackets, which are optional because the closure body has only one expression. These are all valid definitions that will produce the same behavior when they’re called. The add_one_v3 and add_one_v4 lines require the closures to be evaluated to be able to compile because the types will be inferred from their usage. This is similar to let v = Vec::new(); needing either type annotations or values of some type to be inserted into the Vec for Rust to be able to infer the type.

对于闭包定义，编译器将为其每个参数和返回值推断一个具体的类型。例如，示例 13-3 显示了一个简单的闭包定义，它只返回它作为参数接收的值。除了用于本例目的之外，这个闭包并不是很有用。注意我们没有向定义中添加任何类型标注。因为没有类型标注，我们可以用任何类型调用该闭包，这里我们第一次使用了 String。如果我们随后尝试用一个整数调用 example_closure，我们将得到一个错误。

For closure definitions, the compiler will infer one concrete type for each of their parameters and for their return value. For instance, Listing 13-3 shows the definition of a short closure that just returns the value it receives as a parameter. This closure isn’t very useful except for the purposes of this example. Note that we haven’t added any type annotations to the definition. Because there are no type annotations, we can call the closure with any type, which we’ve done here with String the first time. If we then try to call example_closure with an integer, we’ll get an error.

{{#rustdoc_include ../listings/ch13-functional-features/listing-13-03/src/main.rs:here}}

编译器给了我们这个错误：

{{#include ../listings/ch13-functional-features/listing-13-03/output.txt}}

第一次我们用 String 值调用 example_closure 时，编译器推断出 x 的类型和闭包的返回类型为 String。这些类型随后被锁定在 example_closure 的闭包中，当我们下次尝试对同一个闭包使用不同类型时，就会得到一个类型错误。

The first time we call example_closure with the String value, the compiler infers the type of x and the return type of the closure to be String. Those types are then locked into the closure in example_closure, and we get a type error when we next try to use a different type with the same closure.

捕获引用或移动所有权 (Capturing References or Moving Ownership)

闭包可以以三种方式从其环境中捕获值，这直接对应于函数接收参数的三种方式：不可变借用、可变借用和获取所有权。闭包将根据函数体对捕获的值执行什么操作来决定使用哪种方式。

Closures can capture values from their environment in three ways, which directly map to the three ways a function can take a parameter: borrowing immutably, borrowing mutably, and taking ownership. The closure will decide which of these to use based on what the body of the function does with the captured values.

在示例 13-4 中，我们定义了一个捕获对名为 list 的向量的不可变引用的闭包，因为它只需要一个不可变引用来打印该值。

In Listing 13-4, we define a closure that captures an immutable reference to the vector named list because it only needs an immutable reference to print the value.

#![allow(unused)]
fn main() {
{{#rustdoc_include ../listings/ch13-functional-features/listing-13-04/src/main.rs}}
}

这个例子还说明了一个变量可以绑定到一个闭包定义，并且我们稍后可以通过使用变量名和圆括号来调用该闭包，就像变量名是一个函数名一样。

This example also illustrates that a variable can bind to a closure definition, and we can later call the closure by using the variable name and parentheses as if the variable name were a function name.

因为我们可以同时拥有多个对 list 的不可变引用，所以 list 在闭包定义之前、闭包定义之后但在闭包被调用之前、以及在闭包被调用之后，仍然是可以访问的。这段代码可以编译、运行并打印：

Because we can have multiple immutable references to list at the same time, list is still accessible from the code before the closure definition, after the closure definition but before the closure is called, and after the closure is called. This code compiles, runs, and prints:

{{#include ../listings/ch13-functional-features/listing-13-04/output.txt}}

接下来，在示例 13-5 中，我们更改了闭包体，使其向 list 向量中添加一个元素。闭包现在捕获了一个可变引用。

Next, in Listing 13-5, we change the closure body so that it adds an element to the list vector. The closure now captures a mutable reference.

#![allow(unused)]
fn main() {
{{#rustdoc_include ../listings/ch13-functional-features/listing-13-05/src/main.rs}}
}

这段代码可以编译、运行并打印：

{{#include ../listings/ch13-functional-features/listing-13-05/output.txt}}

注意，在 borrows_mutably 闭包的定义和调用之间不再有 println! ：当 borrows_mutably 被定义时，它捕获了对 list 的一个可变引用。我们在闭包被调用后就不再使用该闭包，所以可变借用结束了。在闭包定义和闭包调用之间，不允许进行用于打印的不可变借用，因为当存在可变借用时不允许有其他借用。尝试在那里添加一个 println! 看看你会得到什么错误消息！

Note that there’s no longer a println! between the definition and the call of the borrows_mutably closure: When borrows_mutably is defined, it captures a mutable reference to list. We don’t use the closure again after the closure is called, so the mutable borrow ends. Between the closure definition and the closure call, an immutable borrow to print isn’t allowed, because no other borrows are allowed when there’s a mutable borrow. Try adding a println! there to see what error message you get!

如果你想强制闭包获取它在环境中所使用值的所有权，即使闭包体并不严格需要所有权，你可以在参数列表前使用 move 关键字。

If you want to force the closure to take ownership of the values it uses in the environment even though the body of the closure doesn’t strictly need ownership, you can use the move keyword before the parameter list.

这种技术主要在将闭包传递给新线程以移动数据使其归新线程所有时很有用。我们将在第 16 章讨论并发时详细讨论线程以及你为什么想要使用它们，但现在，让我们简要地探索一下使用需要 move 关键字的闭包来启动一个新线程。示例 13-6 显示了修改后的示例 13-4，在新线程而不是主线程中打印向量。

This technique is mostly useful when passing a closure to a new thread to move the data so that it’s owned by the new thread. We’ll discuss threads and why you would want to use them in detail in Chapter 16 when we talk about concurrency, but for now, let’s briefly explore spawning a new thread using a closure that needs the move keyword. Listing 13-6 shows Listing 13-4 modified to print the vector in a new thread rather than in the main thread.

#![allow(unused)]
fn main() {
{{#rustdoc_include ../listings/ch13-functional-features/listing-13-06/src/main.rs}}
}

我们启动了一个新线程，并将一个闭包作为参数交给该线程运行。闭包体打印出了列表。在示例 13-4 中，闭包仅使用不可变引用捕获了 list ，因为那是打印它所需的最少访问量。在这个例子中，尽管闭包体仍然只需要一个不可变引用，我们仍需要通过在闭包定义的开头放置 move 关键字来指定应该将 list 移动到闭包中。如果主线程在对新线程调用 join 之前执行了更多操作，新线程可能会在主线程其余部分完成之前完成，或者主线程可能会先完成。如果主线程保持了 list 的所有权但在新线程之前结束并丢弃了 list ，那么线程中的不可变引用将是无效的。因此，编译器要求将 list 移动到交给新线程的闭包中，以便引用有效。尝试删除 move 关键字或在闭包定义后在主线程中使用 list ，看看你会得到什么编译器错误！

We spawn a new thread, giving the thread a closure to run as an argument. The closure body prints out the list. In Listing 13-4, the closure only captured list using an immutable reference because that’s the least amount of access to list needed to print it. In this example, even though the closure body still only needs an immutable reference, we need to specify that list should be moved into the closure by putting the move keyword at the beginning of the closure definition. If the main thread performed more operations before calling join on the new thread, the new thread might finish before the rest of the main thread finishes, or the main thread might finish first. If the main thread maintained ownership of list but ended before the new thread and drops list, the immutable reference in the thread would be invalid. Therefore, the compiler requires that list be moved into the closure given to the new thread so that the reference will be valid. Try removing the move keyword or using list in the main thread after the closure is defined to see what compiler errors you get!

将捕获的值移出闭包 (Moving Captured Values Out of Closures)

一旦闭包从定义它的环境中捕获了引用或捕获了值的所有权（从而影响了什么内容被移入闭包，如果有的话），闭包体内的代码就定义了当闭包稍后被求值时这些引用或值会发生什么（从而影响了什么内容从闭包被移出，如果有的话）。

Once a closure has captured a reference or captured ownership of a value from the environment where the closure is defined (thus affecting what, if anything, is moved into the closure), the code in the body of the closure defines what happens to the references or values when the closure is evaluated later (thus affecting what, if anything, is moved out of the closure).

闭包体可以执行以下任何操作：将捕获的值移出闭包、修改捕获的值、既不移动也不修改该值，或者最初就从环境中什么都不捕获。

A closure body can do any of the following: Move a captured value out of the closure, mutate the captured value, neither move nor mutate the value, or capture nothing from the environment to begin with.

闭包捕获和处理环境中的值的方式会影响闭包实现的特征，而特征是函数和结构体指定它们可以使用哪种闭包的方式。闭包将根据闭包体处理值的方式，以累加的方式自动实现这三个 Fn 特征中的一个、两个或全部三个：

The way a closure captures and handles values from the environment affects which traits the closure implements, and traits are how functions and structs can specify what kinds of closures they can use. Closures will automatically implement one, two, or all three of these Fn traits, in an additive fashion, depending on how the closure’s body handles the values:

• FnOnce 适用于可以被调用一次的闭包。所有闭包都至少实现了这个特征，因为所有闭包都是可以被调用的。一个将捕获的值从其主体中移出的闭包将仅实现 FnOnce 而不实现其他 Fn 特征，因为它只能被调用一次。

• FnMut 适用于不从其主体中移出捕获的值、但可能会修改捕获值的闭包。这些闭包可以被调用多次。

• Fn 适用于不从其主体中移出捕获的值且不修改捕获值的闭包，以及从其环境中不捕获任何内容的闭包。这些闭包可以被调用多次而不会修改其环境，这在并发地多次调用一个闭包等情况下非常重要。

• FnOnce applies to closures that can be called once. All closures implement at least this trait because all closures can be called. A closure that moves captured values out of its body will only implement FnOnce and none of the other Fn traits because it can only be called once.

• FnMut applies to closures that don’t move captured values out of their body but might mutate the captured values. These closures can be called more than once.

• Fn applies to closures that don’t move captured values out of their body and don’t mutate captured values, as well as closures that capture nothing from their environment. These closures can be called more than once without mutating their environment, which is important in cases such as calling a closure multiple times concurrently.

让我们看看我们在示例 13-1 中使用的 Option<T> 上的 unwrap_or_else 方法的定义：

Let’s look at the definition of the unwrap_or_else method on Option<T> that we used in Listing 13-1:

impl<T> Option<T> {
    pub fn unwrap_or_else<F>(self, f: F) -> T
    where
        F: FnOnce() -> T
    {
        match self {
            Some(x) => x,
            None => f(),
        }
    }
}

回想一下，T 是代表 Option 的 Some 变体中值的类型的泛型类型。该类型 T 也是 unwrap_or_else 函数的返回类型：例如，在 Option<String> 上调用 unwrap_or_else 的代码将得到一个 String 。

Recall that T is the generic type representing the type of the value in the Some variant of an Option. That type T is also the return type of the unwrap_or_else function: Code that calls unwrap_or_else on an Option<String>, for example, will get a String.

接下来，注意 unwrap_or_else 函数具有额外的泛型类型参数 F 。F 类型是名为 f 的参数的类型，它是我们在调用 unwrap_or_else 时提供的闭包。

Next, notice that the unwrap_or_else function has the additional generic type parameter F. The F type is the type of the parameter named f, which is the closure we provide when calling unwrap_or_else.

在泛型类型 F 上指定的特征约束是 FnOnce() -> T ，这意味着 F 必须能够被调用一次、不带参数并返回一个 T 。在特征约束中使用 FnOnce 表达了 unwrap_or_else 调用 f 不会超过一次的约束。在 unwrap_or_else 的主体中，我们可以看到如果 Option 是 Some ，f 就不会被调用。如果 Option 是 None ，f 将被调用一次。因为所有闭包都实现了 FnOnce ，unwrap_or_else 接受所有三种闭包，并且尽可能的灵活。

The trait bound specified on the generic type F is FnOnce() -> T, which means F must be able to be called once, take no arguments, and return a T. Using FnOnce in the trait bound expresses the constraint that unwrap_or_else will not call f more than once. In the body of unwrap_or_else, we can see that if the Option is Some, f won’t be called. If the Option is None, f will be called once. Because all closures implement FnOnce, unwrap_or_else accepts all three kinds of closures and is as flexible as it can be.

注意：如果我们想要做的事情不需要从环境中捕获值，我们可以在需要实现 Fn 特征之一的地方使用函数名称而不是闭包。例如，在一个 Option<Vec<T>> 值上，如果值是 None ，我们可以调用 unwrap_or_else(Vec::new) 来获得一个新的空向量。编译器会自动为函数定义实现任何适用的 Fn 特征。

Note: If what we want to do doesn’t require capturing a value from the environment, we can use the name of a function rather than a closure where we need something that implements one of the Fn traits. For example, on an Option<Vec<T>> value, we could call unwrap_or_else(Vec::new) to get a new, empty vector if the value is None. The compiler automatically implements whichever of the Fn traits is applicable for a function definition.

现在让我们看看在切片上定义的标准库方法 sort_by_key ，看看它与 unwrap_or_else 有何不同，以及为什么 sort_by_key 为特征约束使用了 FnMut 而不是 FnOnce 。闭包以引用的形式获得一个参数，该引用指向正在考虑的切片中的当前项，并返回一个可以排序的 K 类型的值。当你想要根据每个项的特定属性对切片进行排序时，此函数非常有用。在示例 13-7 中，我们有一个 Rectangle 实例列表，我们使用 sort_by_key 按它们的 width 属性从小到大排序。

Now let’s look at the standard library method sort_by_key, defined on slices, to see how that differs from unwrap_or_else and why sort_by_key uses FnMut instead of FnOnce for the trait bound. The closure gets one argument in the form of a reference to the current item in the slice being considered, and it returns a value of type K that can be ordered. This function is useful when you want to sort a slice by a particular attribute of each item. In Listing 13-7, we have a list of Rectangle instances, and we use sort_by_key to order them by their width attribute from low to high.

#![allow(unused)]
fn main() {
{{#rustdoc_include ../listings/ch13-functional-features/listing-13-07/src/main.rs}}
}

这段代码打印：

{{#include ../listings/ch13-functional-features/listing-13-07/output.txt}}

sort_by_key 被定义为接收一个 FnMut 闭包的原因是它会多次调用该闭包：对切片中的每个项调用一次。闭包 |r| r.width 不会从其环境中捕获、修改或移出任何内容，因此它满足特征约束要求。

The reason sort_by_key is defined to take an FnMut closure is that it calls the closure multiple times: once for each item in the slice. The closure |r| r.width doesn’t capture, mutate, or move anything out from its environment, so it meets the trait bound requirements.

相比之下，示例 13-8 显示了一个仅实现了 FnOnce 特征的闭包示例，因为它从环境中移出了一个值。编译器不允许我们在 sort_by_key 中使用这个闭包。

In contrast, Listing 13-8 shows an example of a closure that implements just the FnOnce trait, because it moves a value out of the environment. The compiler won’t let us use this closure with sort_by_key.

{{#rustdoc_include ../listings/ch13-functional-features/listing-13-08/src/main.rs}}

这是一种人为设计的、令人费解的方法（而且行不通），试图计算在对 list 进行排序时 sort_by_key 调用闭包的次数。这段代码尝试通过将 value ——一个来自闭包环境的 String ——推入 sort_operations 向量来完成计数。闭包捕获了 value ，然后通过将 value 的所有权转移到 sort_operations 向量将其移出了闭包。这个闭包只能被调用一次；尝试第二次调用它将行不通，因为 value 将不再存在于环境中，无法再次推入 sort_operations ！因此，这个闭包仅实现了 FnOnce 。当我们尝试编译这段代码时，我们会得到这个错误，即 value 无法移出闭包，因为闭包必须实现 FnMut ：

This is a contrived, convoluted way (that doesn’t work) to try to count the number of times sort_by_key calls the closure when sorting list. This code attempts to do this counting by pushing value—a String from the closure’s environment—into the sort_operations vector. The closure captures value and then moves value out of the closure by transferring ownership of value to the sort_operations vector. This closure can be called once; trying to call it a second time wouldn’t work, because value would no longer be in the environment to be pushed into sort_operations again! Therefore, this closure only implements FnOnce. When we try to compile this code, we get this error that value can’t be moved out of the closure because the closure must implement FnMut:

{{#include ../listings/ch13-functional-features/listing-13-08/output.txt}}

该错误指向了闭包体中将 value 移出环境的那一行。为了修复此问题，我们需要更改闭包体，使其不将值移出环境。在环境中保持一个计数器并在闭包体中增加其值，是计算闭包被调用次数的一种更直接的方法。示例 13-9 中的闭包可以与 sort_by_key 配合使用，因为它仅捕获了对 num_sort_operations 计数器的可变引用，因此可以被多次调用。

The error points to the line in the closure body that moves value out of the environment. To fix this, we need to change the closure body so that it doesn’t move values out of the environment. Keeping a counter in the environment and incrementing its value in the closure body is a more straightforward way to count the number of times the closure is called. The closure in Listing 13-9 works with sort_by_key because it is only capturing a mutable reference to the num_sort_operations counter and can therefore be called more than once.

#![allow(unused)]
fn main() {
{{#rustdoc_include ../listings/ch13-functional-features/listing-13-09/src/main.rs}}
}

当定义或使用利用了闭包的函数或类型时， Fn 特征非常重要。在下一节中，我们将讨论迭代器。许多迭代器方法接收闭包参数，所以请在继续时记住这些闭包细节！

The Fn traits are important when defining or using functions or types that make use of closures. In the next section, we’ll discuss iterators. Many iterator methods take closure arguments, so keep these closure details in mind as we continue!