函数 - 《Effective Go (实效 GO 编程) 中英双语版》

Functions
函数

Functions

函数

Multiple return values

多值返回

One of Go’s unusual features is that functions and methods can return multiple values. This form can be used to improve on a couple of clumsy idioms in C programs: in-band error returns such as -1 for EOF and modifying an argument passed by address.

Go 与众不同的特性之一就是函数和方法可返回多个值。这种形式可以改善 C 中一些笨拙的习惯：将错误值返回（例如用 -1 表示 EOF）和修改通过地址传入的实参。

In C, a write error is signaled by a negative count with the error code secreted away in a volatile location. In Go, Write can return a count and an error: “Yes, you wrote some bytes but not all of them because you filled the device”. The signature of the Write method on files from package os is:

在 C 中，写入操作发生的错误会用一个负数标记，而错误码会隐藏在某个不确定的位置。而在 Go 中，Write 会返回写入的字节数以及一个错误： “是的，您写入了一些字节，但并未全部写入，因为设备已满”。在 os 包中，File.Write 的签名为：

func (file *File) Write(b []byte) (n int, err error)

and as the documentation says, it returns the number of bytes written and a non-nil error when n != len(b). This is a common style; see the section on error handling for more examples.

正如文档所述，它返回写入的字节数，并在 n != len(b) 时返回一个非 nil 的 error 错误值。这是一种常见的编码风格，更多示例见错误处理一节。

A similar approach obviates the need to pass a pointer to a return value to simulate a reference parameter. Here’s a simple-minded function to grab a number from a position in a byte slice, returning the number and the next position.

我们可以采用一种简单的方法。来避免为模拟引用参数而传入指针。以下简单的函数可从字节数组中的特定位置获取其值，并返回该数值和下一个位置。

func nextInt(b []byte, i int) (int, int) {
    for ; i < len(b) && !isDigit(b[i]); i++ {
    }
    x := 0
    for ; i < len(b) && isDigit(b[i]); i++ {
        x = x*10 + int(b[i]) - '0'
    }
    return x, i
}

You could use it to scan the numbers in an input slice b like this:

你可以像下面这样，通过它扫描输入的切片 b 来获取数字。

    for i := 0; i < len(b); {
        x, i = nextInt(b, i)
        fmt.Println(x)
    }

Named result parameters

可命名结果形参

The return or result “parameters” of a Go function can be given names and used as regular variables, just like the incoming parameters. When named, they are initialized to the zero values for their types when the function begins; if the function executes a return statement with no arguments, the current values of the result parameters are used as the returned values.

Go 函数的返回值或结果 “形参” 可被命名，并作为常规变量使用，就像传入的形参一样。命名后，一旦该函数开始执行，它们就会被初始化为与其类型相应的零值；若该函数执行了一条不带实参的 return 语句，则结果形参的当前值将被返回。

The names are not mandatory but they can make code shorter and clearer: they’re documentation. If we name the results of nextInt it becomes obvious which returned int is which.

此名称不是强制性的，但它们能使代码更加简短清晰：它们就是文档。若我们命名了 nextInt 的结果，那么它返回的 int 就值如其意了。

func nextInt(b []byte, pos int) (value, nextPos int) {

Because named results are initialized and tied to an unadorned return, they can simplify as well as clarify. Here’s a version of io.ReadFull that uses them well:

由于被命名的结果已经初始化，且已经关联至无参数的返回，它们就能让代码简单而清晰。下面的 io.ReadFull 就是个很好的例子：

func ReadFull(r Reader, buf []byte) (n int, err error) {
    for len(buf) > 0 && err == nil {
        var nr int
        nr, err = r.Read(buf)
        n += nr
        buf = buf[nr:]
    }
    return
}

Defer

Go’s defer statement schedules a function call (the deferred function) to be run immediately before the function executing the defer returns. It’s an unusual but effective way to deal with situations such as resources that must be released regardless of which path a function takes to return. The canonical examples are unlocking a mutex or closing a file.

Go 的 defer 语句用于预设一个函数调用（即推迟执行函数），该函数会在执行 defer 的函数返回之前立即执行。它显得非比寻常，但却是处理一些事情的有效方式，例如无论以何种路径返回，都必须释放资源的函数。典型的例子就是解锁互斥和关闭文件。

// Contents returns the file's contents as a string.
func Contents(filename string) (string, error) {
    f, err := os.Open(filename)
    if err != nil {
        return "", err
    }
    defer f.Close()  // f.Close will run when we're finished.
    var result []byte
    buf := make([]byte, 100)
    for {
        n, err := f.Read(buf[0:])
        result = append(result, buf[0:n]...) // append is discussed later.
        if err != nil {
            if err == io.EOF {
                break
            }
            return "", err  // f will be closed if we return here.
        }
    }
    return string(result), nil // f will be closed if we return here.
}

// Contents 将文件的内容作为字符串返回。
func Contents(filename string) (string, error) {
    f, err := os.Open(filename)
    if err != nil {
        return "", err
    }
    defer f.Close()  // f.Close 会在我们结束后运行。
    var result []byte
    buf := make([]byte, 100)
    for {
        n, err := f.Read(buf[0:])
        result = append(result, buf[0:n]...) // append 将在后面讨论。
        if err != nil {
            if err == io.EOF {
                break
            }
            return "", err  // 我们在这里返回后，f 就会被关闭。
        }
    }
    return string(result), nil // 我们在这里返回后，f 就会被关闭。
}

Deferring a call to a function such as Close has two advantages. First, it guarantees that you will never forget to close the file, a mistake that’s easy to make if you later edit the function to add a new return path. Second, it means that the close sits near the open, which is much clearer than placing it at the end of the function.

推迟诸如 Close 之类的函数调用有两点好处：第一，它能确保你不会忘记关闭文件。如果你以后又为该函数添加了新的返回路径时，这种情况往往就会发生。第二，它意味着 “关闭” 离 “打开” 很近，这总比将它放在函数结尾处要清晰明了。

The arguments to the deferred function (which include the receiver if the function is a method) are evaluated when the defer executes, not when the call executes. Besides avoiding worries about variables changing values as the function executes, this means that a single deferred call site can defer multiple function executions. Here’s a silly example.

被推迟函数的实参（如果该函数为方法则还包括接收者）在推迟执行时就会求值，而不是在调用执行时才求值。这样不仅无需担心变量值在函数执行时被改变，同时还意味着单个已推迟的调用可推迟多个函数的执行。下面是个简单的例子。

for i := 0; i < 5; i++ {
    defer fmt.Printf("%d ", i)
}

Deferred functions are executed in LIFO order, so this code will cause 4 3 2 1 0 to be printed when the function returns. A more plausible example is a simple way to trace function execution through the program. We could write a couple of simple tracing routines like this:

被推迟的函数按照后进先出（LIFO）的顺序执行，因此以上代码在函数返回时会打印 4 3 2 1 0。一个更具实际意义的例子是通过一种简单的方法，用程序来跟踪函数的执行。我们可以编写一对简单的跟踪例程：

func trace(s string)   { fmt.Println("entering:", s) }
func untrace(s string) { fmt.Println("leaving:", s) }
// Use them like this:
func a() {
    trace("a")
    defer untrace("a")
    // do something....
}

func trace(s string)   { fmt.Println("entering:", s) }
func untrace(s string) { fmt.Println("leaving:", s) }
// 像这样使用它们：
func a() {
    trace("a")
    defer untrace("a")
    // 做一些事情....
}

We can do better by exploiting the fact that arguments to deferred functions are evaluated when the defer executes. The tracing routine can set up the argument to the untracing routine. This example:

我们可以充分利用这个特点，即被推迟函数的实参在 defer 执行时才会被求值。跟踪例程可针对反跟踪例程设置实参。以下例子：

func trace(s string) string {
    fmt.Println("entering:", s)
    return s
}
func un(s string) {
    fmt.Println("leaving:", s)
}
func a() {
    defer un(trace("a"))
    fmt.Println("in a")
}
func b() {
    defer un(trace("b"))
    fmt.Println("in b")
    a()
}
func main() {
    b()
}

prints

会打印

entering: b
in b
entering: a
in a
leaving: a
leaving: b

For programmers accustomed to block-level resource management from other languages, defer may seem peculiar, but its most interesting and powerful applications come precisely from the fact that it’s not block-based but function-based. In the section on panic and recover we’ll see another example of its possibilities.

对于习惯其它语言中块级资源管理的程序员，defer 似乎有点怪异，但它最有趣而强大的应用恰恰来自于其基于函数而非块的特点。在 panic 和 recover 这两节中，我们将看到关于它可能性的其它例子。