Data

数据

Allocation with new

new 分配

Go has two allocation primitives, the built-in functions new and make. They do different things and apply to different types, which can be confusing, but the rules are simple. Let's talk about new first. It's a built-in function that allocates memory, but unlike its namesakes in some other languages it does not initialize the memory, it only zeros it. That is, new(T) allocates zeroed storage for a new item of type T and returns its address, a value of type *T. In Go terminology, it returns a pointer to a newly allocated zero value of type T.

Go 提供了两种分配原语,即内建函数 new 和 make。 它们所做的事情不同,所应用的类型也不同。它们可能会引起混淆,但规则却很简单。 让我们先来看看 new。这是个用来分配内存的内建函数, 但与其它语言中的同名函数不同,它不会初始化内存,只会将内存置零。 也就是说,new(T) 会为类型为 T 的新项分配已置零的内存空间, 并返回它的地址,也就是一个类型为 *T 的值。用 Go 的术语来说,它返回一个指针, 该指针指向新分配的,类型为 T 的零值。

Since the memory returned by new is zeroed, it's helpful to arrange when designing your data structures that the zero value of each type can be used without further initialization. This means a user of the data structure can create one with new and get right to work. For example, the documentation for bytes.Buffer states that "the zero value for Buffer is an empty buffer ready to use." Similarly, sync.Mutex does not have an explicit constructor or Init method. Instead, the zero value for a sync.Mutex is defined to be an unlocked mutex.

既然 new 返回的内存已置零,那么当你设计数据结构时, 每种类型的零值就不必进一步初始化了,这意味着该数据结构的使用者只需用 new 创建一个新的对象就能正常工作。例如,bytes.Buffer 的文档中提到 “零值的 Buffer 就是已准备就绪的缓冲区。" 同样,sync.Mutex 并没有显式的构造函数或 Init 方法, 而是零值的 sync.Mutex 就已经被定义为已解锁的互斥锁了。

The zero-value-is-useful property works transitively. Consider this type declaration.

“零值属性” 是传递性的。考虑以下类型声明。

type SyncedBuffer struct {
    lock    sync.Mutex
    buffer  bytes.Buffer
}

Values of type SyncedBuffer are also ready to use immediately upon allocation or just declaration. In the next snippet, both p and v will work correctly without further arrangement.

SyncedBuffer 类型的值也是在声明时就分配好内存就绪了。后续代码中, p 和 v 无需进一步处理即可正确工作。

p := new(SyncedBuffer)  // type *SyncedBuffer
var v SyncedBuffer      // type  SyncedBuffer

Constructors and composite literals

构造函数与复合字面量

Sometimes the zero value isn't good enough and an initializing constructor is necessary, as in this example derived from package os.

有时零值还不够好,这时就需要一个初始化构造函数,如来自 os 包中的这段代码所示。

func NewFile(fd int, name string) *File {
    if fd < 0 {
        return nil
    }
    f := new(File)
    f.fd = fd
    f.name = name
    f.dirinfo = nil
    f.nepipe = 0
    return f
}

There's a lot of boiler plate in there. We can simplify it using a composite literal, which is an expression that creates a new instance each time it is evaluated.

这里显得代码过于冗长。我们可通过复合字面量来简化它, 该表达式在每次求值时都会创建新的实例。

func NewFile(fd int, name string) *File {
    if fd < 0 {
        return nil
    }
    f := File{fd, name, nil, 0}
    return &f
}

Note that, unlike in C, it's perfectly OK to return the address of a local variable; the storage associated with the variable survives after the function returns. In fact, taking the address of a composite literal allocates a fresh instance each time it is evaluated, so we can combine these last two lines.

请注意,返回一个局部变量的地址完全没有问题,这点与 C 不同。该局部变量对应的数据 在函数返回后依然有效。实际上,每当获取一个复合字面量的地址时,都将为一个新的实例分配内存, 因此我们可以将上面的最后两行代码合并:

    return &File{fd, name, nil, 0}

The fields of a composite literal are laid out in order and must all be present. However, by labeling the elements explicitly as field:value pairs, the initializers can appear in any order, with the missing ones left as their respective zero values. Thus we could say

复合字面量的字段必须按顺序全部列出。但如果以 字段: 值 对的形式明确地标出元素,初始化字段时就可以按任何顺序出现,未给出的字段值将赋予零值。 因此,我们可以用如下形式:

    return &File{fd: fd, name: name}

As a limiting case, if a composite literal contains no fields at all, it creates a zero value for the type. The expressions new(File) and &File{} are equivalent.

少数情况下,若复合字面量不包括任何字段,它将创建该类型的零值。表达式 new(File) 和 &File{} 是等价的。

Composite literals can also be created for arrays, slices, and maps, with the field labels being indices or map keys as appropriate. In these examples, the initializations work regardless of the values of Enone, Eio, and Einval, as long as they are distinct.

复合字面量同样可用于创建数组、切片以及映射,字段标签是索引还是映射键则视情况而定。 在下例初始化过程中,无论 Enone、Eio 和 Einval 的值是什么,只要它们的标签不同就行。

a := [...]string   {Enone: "no error", Eio: "Eio", Einval: "invalid argument"}
s := []string      {Enone: "no error", Eio: "Eio", Einval: "invalid argument"}
m := map[int]string{Enone: "no error", Eio: "Eio", Einval: "invalid argument"}

Allocation with make

make 分配

Back to allocation. The built-in function make(T, args) serves a purpose different from new(T). It creates slices, maps, and channels only, and it returns an initialized (not zeroed) value of type T (not *T). The reason for the distinction is that these three types represent, under the covers, references to data structures that must be initialized before use. A slice, for example, is a three-item descriptor containing a pointer to the data (inside an array), the length, and the capacity, and until those items are initialized, the slice is nil. For slices, maps, and channels, make initializes the internal data structure and prepares the value for use. For instance,

再回到内存分配上来。内建函数 make(T, args) 的目的不同于 new(T)。它只用于创建切片、映射和信道,并返回类型为 T(而非 *T)的一个已初始化 (而非置零)的值。 出现这种用差异的原因在于,这三种类型本质上为引用数据类型,它们在使用前必须初始化。 例如,切片是一个具有三项内容的描述符,包含一个指向(数组内部)数据的指针、长度以及容量, 在这三项被初始化之前,该切片为 nil。对于切片、映射和信道,make 用于初始化其内部的数据结构并准备好将要使用的值。例如,

make([]int, 10, 100)

allocates an array of 100 ints and then creates a slice structure with length 10 and a capacity of 100 pointing at the first 10 elements of the array. (When making a slice, the capacity can be omitted; see the section on slices for more information.) In contrast, new([]int) returns a pointer to a newly allocated, zeroed slice structure, that is, a pointer to a nil slice value.

会分配一个具有 100 个 int 的数组空间,接着创建一个长度为 10, 容量为 100 并指向该数组中前 10 个元素的切片结构。(生成切片时,其容量可以省略,更多信息见切片一节。) 与此相反,new([]int) 会返回一个指向新分配的,已置零的切片结构, 即一个指向 nil 切片值的指针。

These examples illustrate the difference between new and make.

下面的例子阐明了 new 和 make 之间的区别:

var p *[]int = new([]int)       // allocates slice structure; *p == nil; rarely useful
var v  []int = make([]int, 100) // the slice v now refers to a new array of 100 ints

// Unnecessarily complex:
var p *[]int = new([]int)
*p = make([]int, 100, 100)

// Idiomatic:
v := make([]int, 100)
var p *[]int = new([]int)       // 分配切片结构;*p == nil;基本没用
var v  []int = make([]int, 100) // 切片 v 现在引用了一个具有 100 个 int 元素的新数组

// 没必要的复杂:
var p *[]int = new([]int)
*p = make([]int, 100, 100)

// 习惯用法:
v := make([]int, 100)

Remember that make applies only to maps, slices and channels and does not return a pointer. To obtain an explicit pointer allocate with new or take the address of a variable explicitly.

请记住,make 只适用于映射、切片和信道且不返回指针。若要获得明确的指针, 请使用 new 分配内存或显式地获取一个变量的地址。

Arrays

数组

Arrays are useful when planning the detailed layout of memory and sometimes can help avoid allocation, but primarily they are a building block for slices, the subject of the next section. To lay the foundation for that topic, here are a few words about arrays.

在详细规划内存布局时,数组是非常有用的,有时还能避免过多的内存分配, 但它们主要用作切片的构件。这是下一节的主题了,不过要先说上几句来为它做铺垫。

There are major differences between the ways arrays work in Go and C. In Go,

  • Arrays are values. Assigning one array to another copies all the elements.
  • In particular, if you pass an array to a function, it will receive a copy of the array, not a pointer to it.
  • The size of an array is part of its type. The types [10]int and [20]int are distinct.

以下为数组在 Go 和 C 中的主要区别。在 Go 中,

  • 数组是值。将一个数组赋予另一个数组会复制其所有元素。
  • 特别地,若将某个数组传入某个函数,它将接收到该数组的一份副本而非指针。
  • 数组的大小是其类型的一部分。类型 [10]int 和 [20]int 是不同的。

The value property can be useful but also expensive; if you want C-like behavior and efficiency, you can pass a pointer to the array.

数组为值的属性很有用,但代价高昂;若你想要 C 那样的行为和效率,你可以传递一个指向该数组的指针。

func Sum(a *[3]float64) (sum float64) {
    for _, v := range *a {
        sum += v
    }
    return
}

array := [...]float64{7.0, 8.5, 9.1}
x := Sum(&array)  // Note the explicit address-of operator
func Sum(a *[3]float64) (sum float64) {
    for _, v := range *a {
        sum += v
    }
    return
}

array := [...]float64{7.0, 8.5, 9.1}
x := Sum(&array)  // 注意显式的取址操作

But even this style isn't idiomatic Go. Use slices instead.

但这并不是 Go 的习惯用法,切片才是。

Slices

切片

Slices wrap arrays to give a more general, powerful, and convenient interface to sequences of data. Except for items with explicit dimension such as transformation matrices, most array programming in Go is done with slices rather than simple arrays.

切片通过对数组进行封装,为数据序列提供了更通用、强大而方便的接口。 除了矩阵变换这类需要明确维度的情况外,Go 中的大部分数组编程都是通过切片来完成的。

Slices hold references to an underlying array, and if you assign one slice to another, both refer to the same array. If a function takes a slice argument, changes it makes to the elements of the slice will be visible to the caller, analogous to passing a pointer to the underlying array. A Read function can therefore accept a slice argument rather than a pointer and a count; the length within the slice sets an upper limit of how much data to read. Here is the signature of the Read method of the File type in package os:

切片保存了对底层数组的引用,若你将某个切片赋予另一个切片,它们会引用同一个数组。 若某个函数将一个切片作为参数传入,则它对该切片元素的修改对调用者而言同样可见, 这可以理解为传递了底层数组的指针。因此,Read 函数可接受一个切片实参 而非一个指针和一个计数;切片的长度决定了可读取数据的上限。以下为 os 包中 File 类型的 Read 方法签名:

func (file *File) Read(buf []byte) (n int, err error)

The method returns the number of bytes read and an error value, if any. To read into the first 32 bytes of a larger buffer buf, slice (here used as a verb) the buffer.

该方法返回读取的字节数和一个错误值(若有的话)。若要从更大的缓冲区 b 中读取前 32 个字节,只需对其进行切片即可。

    n, err := f.Read(buf[0:32])

Such slicing is common and efficient. In fact, leaving efficiency aside for the moment, the following snippet would also read the first 32 bytes of the buffer.

这种切片的方法常用且高效。若不谈效率,以下片段同样能读取该缓冲区的前 32 个字节。

    var n int
    var err error
    for i := 0; i < 32; i++ {
        nbytes, e := f.Read(buf[i:i+1])  // Read one byte.
        if nbytes == 0 || e != nil {
            err = e
            break
        }
        n += nbytes
    }
    var n int
    var err error
    for i := 0; i < 32; i++ {
        nbytes, e := f.Read(buf[i:i+1])  // 读取一个字节
        if nbytes == 0 || e != nil {
            err = e
            break
        }
        n += nbytes
    }

The length of a slice may be changed as long as it still fits within the limits of the underlying array; just assign it to a slice of itself. The capacity of a slice, accessible by the built-in function cap, reports the maximum length the slice may assume. Here is a function to append data to a slice. If the data exceeds the capacity, the slice is reallocated. The resulting slice is returned. The function uses the fact that len and cap are legal when applied to the nil slice, and return 0.

只要切片不超出底层数组的限制,它的长度就是可变的,只需将它赋予其自身的切片即可。 切片的容量可通过内建函数 cap 获得,它将给出该切片可取得的最大长度。 以下是将数据追加到切片的函数。若数据超出其容量,则会重新分配该切片。返回值即为所得的切片。 该函数中所使用的 len 和 cap 在应用于 nil 切片时是合法的,它会返回 0.

func Append(slice, data[]byte) []byte {
    l := len(slice)
    if l + len(data) > cap(slice) {  // reallocate
        // Allocate double what's needed, for future growth.
        newSlice := make([]byte, (l+len(data))*2)
        // The copy function is predeclared and works for any slice type.
        copy(newSlice, slice)
        slice = newSlice
    }
    slice = slice[0:l+len(data)]
    for i, c := range data {
        slice[l+i] = c
    }
    return slice
}
func Append(slice, data[]byte) []byte {
    l := len(slice)
    if l + len(data) > cap(slice) {  // 重新分配
        // 为了后面的增长,需分配两份。
        newSlice := make([]byte, (l+len(data))*2)
        // copy 函数是预声明的,且可用于任何切片类型。
        copy(newSlice, slice)
        slice = newSlice
    }
    slice = slice[0:l+len(data)]
    for i, c := range data {
        slice[l+i] = c
    }
    return slice
}

We must return the slice afterwards because, although Append can modify the elements of slice, the slice itself (the run-time data structure holding the pointer, length, and capacity) is passed by value.

最终我们必须返回切片,因为尽管 Append 可修改 slice 的元素,但切片自身(其运行时数据结构包含指针、长度和容量)是通过值传递的。

The idea of appending to a slice is so useful it's captured by the append built-in function. To understand that function's design, though, we need a little more information, so we'll return to it later.

向切片追加东西的想法非常有用,因此有专门的内建函数 append。 要理解该函数的设计,我们还需要一些额外的信息,我们将稍后再介绍它。

Two-dimensional slices

二维切片

Go's arrays and slices are one-dimensional. To create the equivalent of a 2D array or slice, it is necessary to define an array-of-arrays or slice-of-slices, like this:

Go 的数组和切片都是一维的。要创建等价的二维数组或切片,就必须定义一个数组的数组, 或切片的切片,就像这样:

type Transform [3][3]float64  // A 3x3 array, really an array of arrays.
type LinesOfText [][]byte     // A slice of byte slices.
type Transform [3][3]float64  // 一个 3x3 的数组,其实是包含多个数组的一个数组。
type LinesOfText [][]byte     // 包含多个字节切片的一个切片。

Because slices are variable-length, it is possible to have each inner slice be a different length. That can be a common situation, as in our LinesOfText example: each line has an independent length.

由于切片长度是可变的,因此其内部可能拥有多个不同长度的切片。在我们的 LinesOfText 例子中,这是种常见的情况:每行都有其自己的长度。

text := LinesOfText{
    []byte("Now is the time"),
    []byte("for all good gophers"),
    []byte("to bring some fun to the party."),
}

Sometimes it's necessary to allocate a 2D slice, a situation that can arise when processing scan lines of pixels, for instance. There are two ways to achieve this. One is to allocate each slice independently; the other is to allocate a single array and point the individual slices into it. Which to use depends on your application. If the slices might grow or shrink, they should be allocated independently to avoid overwriting the next line; if not, it can be more efficient to construct the object with a single allocation. For reference, here are sketches of the two methods. First, a line at a time:

有时必须分配一个二维数组,例如在处理像素的扫描行时,这种情况就会发生。 我们有两种方式来达到这个目的。一种就是独立地分配每一个切片;而另一种就是只分配一个数组, 将各个切片都指向它。采用哪种方式取决于你的应用。若切片会增长或收缩, 就应该通过独立分配来避免覆盖下一行;若不会,用单次分配来构造对象会更加高效。 以下是这两种方法的大概代码,仅供参考。首先是一次一行的:

// Allocate the top-level slice.
picture := make([][]uint8, YSize) // One row per unit of y.
// Loop over the rows, allocating the slice for each row.
for i := range picture {
    picture[i] = make([]uint8, XSize)
}
// 分配顶层切片。
picture := make([][]uint8, YSize) // 每 y 个单元一行。
// 遍历行,为每一行都分配切片
for i := range picture {
    picture[i] = make([]uint8, XSize)
}

And now as one allocation, sliced into lines:

现在是一次分配,对行进行切片:

// Allocate the top-level slice, the same as before.
picture := make([][]uint8, YSize) // One row per unit of y.
// Allocate one large slice to hold all the pixels.
pixels := make([]uint8, XSize*YSize) // Has type []uint8 even though picture is [][]uint8.
// Loop over the rows, slicing each row from the front of the remaining pixels slice.
for i := range picture {
    picture[i], pixels = pixels[:XSize], pixels[XSize:]
}
// 分配顶层切片,和前面一样。
picture := make([][]uint8, YSize) // 每 y 个单元一行。
// 分配一个大的切片来保存所有像素
pixels := make([]uint8, XSize*YSize) // 拥有类型 []uint8,尽管图片是 [][]uint8.
// 遍历行,从剩余像素切片的前面切出每行来。
for i := range picture {
    picture[i], pixels = pixels[:XSize], pixels[XSize:]
}

Maps

映射

Maps are a convenient and powerful built-in data structure that associate values of one type (the key) with values of another type (the element or value) The key can be of any type for which the equality operator is defined, such as integers, floating point and complex numbers, strings, pointers, interfaces (as long as the dynamic type supports equality), structs and arrays. Slices cannot be used as map keys, because equality is not defined on them. Like slices, maps hold references to an underlying data structure. If you pass a map to a function that changes the contents of the map, the changes will be visible in the caller.

映射是方便而强大的内建数据结构,它可以关联不同类型的值。其键可以是任何相等性操作符支持的类型, 如整数、浮点数、复数、字符串、指针、接口(只要其动态类型支持相等性判断)、结构以及数组。 切片不能用作映射键,因为它们的相等性还未定义。与切片一样,映射也是引用类型。 若将映射传入函数中,并更改了该映射的内容,则此修改对调用者同样可见。

Maps can be constructed using the usual composite literal syntax with colon-separated key-value pairs, so it's easy to build them during initialization.

映射可使用一般的复合字面语法进行构建,其键 - 值对使用逗号分隔,因此可在初始化时很容易地构建它们。

var timeZone = map[string]int{
    "UTC":  0*60*60,
    "EST": -5*60*60,
    "CST": -6*60*60,
    "MST": -7*60*60,
    "PST": -8*60*60,
}

Assigning and fetching map values looks syntactically just like doing the same for arrays and slices except that the index doesn't need to be an integer.

赋值和获取映射值的语法类似于数组,不同的是映射的索引不必为整数。

offset := timeZone["EST"]

An attempt to fetch a map value with a key that is not present in the map will return the zero value for the type of the entries in the map. For instance, if the map contains integers, looking up a non-existent key will return 0. A set can be implemented as a map with value type bool. Set the map entry to true to put the value in the set, and then test it by simple indexing.

若试图通过映射中不存在的键来取值,就会返回与该映射中项的类型对应的零值。 例如,若某个映射包含整数,当查找一个不存在的键时会返回 0。 集合可实现成一个值类型为 bool 的映射。将该映射中的项置为 true 可将该值放入集合中,此后通过简单的索引操作即可判断是否存在。

attended := map[string]bool{
    "Ann": true,
    "Joe": true,
    ...
}

if attended[person] { // will be false if person is not in the map
    fmt.Println(person, "was at the meeting")
}
attended := map[string]bool{
    "Ann": true,
    "Joe": true,
    ...
}

if attended[person] { // 若某人不在此映射中,则为 false
    fmt.Println(person, "正在开会")
}

Sometimes you need to distinguish a missing entry from a zero value. Is there an entry for "UTC" or is that the empty string because it's not in the map at all? You can discriminate with a form of multiple assignment.

有时你需要区分某项是不存在还是其值为零值。如对于一个值本应为零的 "UTC" 条目,也可能是由于不存在该项而得到零值。你可以使用多重赋值的形式来分辨这种情况。

var seconds int
var ok bool
seconds, ok = timeZone[tz]

For obvious reasons this is called the “comma ok” idiom. In this example, if tz is present, seconds will be set appropriately and ok will be true; if not, seconds will be set to zero and ok will be false. Here's a function that puts it together with a nice error report:

显然,我们可称之为 “逗号 ok” 惯用法。在下面的例子中,若 tz 存在, seconds 就会被赋予适当的值,且 ok 会被置为 true; 若不存在,seconds 则会被置为零,而 ok 会被置为 false。

func offset(tz string) int {
    if seconds, ok := timeZone[tz]; ok {
        return seconds
    }
    log.Println("unknown time zone:", tz)
    return 0
}

To test for presence in the map without worrying about the actual value, you can use the blank identifier (_) in place of the usual variable for the value.

若仅需判断映射中是否存在某项而不关心实际的值,可使用 空白标识符_)来代替该值的一般变量。

_, present := timeZone[tz]

To delete a map entry, use the delete built-in function, whose arguments are the map and the key to be deleted. It's safe to do this even if the key is already absent from the map.

要删除映射中的某项,可使用内建函数 delete,它以映射及要被删除的键为实参。 即便对应的键不在该映射中,此操作也是安全的。

delete(timeZone, "PDT")  // Now on Standard Time
delete(timeZone, "PDT")  // 现在用标准时间

Printing

打印

Formatted printing in Go uses a style similar to C's printf family but is richer and more general. The functions live in the fmt package and have capitalized names: fmt.Printf, fmt.Fprintf, fmt.Sprintf and so on. The string functions (Sprintf etc.) return a string rather than filling in a provided buffer.

Go 采用的格式化打印风格和 C 的 printf 族类似,但却更加丰富而通用。 这些函数位于 fmt 包中,且函数名首字母均为大写:如 fmt.Printf、fmt.Fprintf,fmt.Sprintf 等。 字符串函数(Sprintf 等)会返回一个字符串,而非填充给定的缓冲区。

You don't need to provide a format string. For each of Printf, Fprintf and Sprintf there is another pair of functions, for instance Print and Println. These functions do not take a format string but instead generate a default format for each argument. The Println versions also insert a blank between arguments and append a newline to the output while the Print versions add blanks only if the operand on neither side is a string. In this example each line produces the same output.

你无需提供一个格式字符串。每个 Printf、Fprintf 和 Sprintf 都分别对应另外的函数,如 Print 与 Println。 这些函数并不接受格式字符串,而是为每个实参生成一种默认格式。Println 系列的函数还会在实参中插入空格,并在输出时追加一个换行符,而 Print 版本仅在操作数两侧都没有字符串时才添加空白。以下示例中各行产生的输出都是一样的。

fmt.Printf("Hello %d\n", 23)
fmt.Fprint(os.Stdout, "Hello ", 23, "\n")
fmt.Println("Hello", 23)
fmt.Println(fmt.Sprint("Hello ", 23))

The formatted print functions fmt.Fprint and friends take as a first argument any object that implements the io.Writer interface; the variables os.Stdout and os.Stderr are familiar instances.

fmt.Fprint 一类的格式化打印函数可接受任何实现了 io.Writer 接口的对象作为第一个实参;变量 os.Stdout 与 os.Stderr 都是人们熟知的例子。

Here things start to diverge from C. First, the numeric formats such as %d do not take flags for signedness or size; instead, the printing routines use the type of the argument to decide these properties.

从这里开始,就与 C 有些不同了。首先,像 %d 这样的数值格式并不接受表示符号或大小的标记, 打印例程会根据实参的类型来决定这些属性。

var x uint64 = 1<<64 - 1
fmt.Printf("%d %x; %d %x\n", x, x, int64(x), int64(x))

prints

将打印

18446744073709551615 ffffffffffffffff; -1 -1

If you just want the default conversion, such as decimal for integers, you can use the catchall format %v (for “value”); the result is exactly what Print and Println would produce. Moreover, that format can print any value, even arrays, slices, structs, and maps. Here is a print statement for the time zone map defined in the previous section.

若你只想要默认的转换,如使用十进制的整数,你可以使用通用的格式 %v(对应 “值”);其结果与 Print 和 Println 的输出完全相同。此外,这种格式还能打印任意值,甚至包括数组、结构体和映射。 以下是打印上一节中定义的时区映射的语句。

fmt.Printf("%v\n", timeZone)  // or just fmt.Println(timeZone)

which gives output

map[CST:-21600 PST:-28800 EST:-18000 UTC:0 MST:-25200]
fmt.Printf("%v\n", timeZone)  // 或只用 fmt.Println(timeZone)

这会输出

map[CST:-21600 PST:-28800 EST:-18000 UTC:0 MST:-25200]

For maps the keys may be output in any order, of course. When printing a struct, the modified format %+v annotates the fields of the structure with their names, and for any value the alternate format %#v prints the value in full Go syntax.

当然,映射中的键可能按任意顺序输出。当打印结构体时,改进的格式 %+v 会为结构体的每个字段添上字段名,而另一种格式 %#v 将完全按照 Go 的语法打印值。

type T struct {
    a int
    b float64
    c string
}
t := &T{ 7, -2.35, "abc\tdef" }
fmt.Printf("%v\n", t)
fmt.Printf("%+v\n", t)
fmt.Printf("%#v\n", t)
fmt.Printf("%#v\n", timeZone)

prints

将打印

&{7 -2.35 abc   def}
&{a:7 b:-2.35 c:abc     def}
&main.T{a:7, b:-2.35, c:"abc\tdef"}
map[string] int{"CST":-21600, "PST":-28800, "EST":-18000, "UTC":0, "MST":-25200}

(Note the ampersands.) That quoted string format is also available through %q when applied to a value of type string or []byte. The alternate format %#q will use backquotes instead if possible. (The %q format also applies to integers and runes, producing a single-quoted rune constant.) Also, %x works on strings, byte arrays and byte slices as well as on integers, generating a long hexadecimal string, and with a space in the format (% x) it puts spaces between the bytes.

(请注意其中的 & 符号)当遇到 string 或 []byte 值时, 可使用 %q 产生带引号的字符串;而格式 %#q 会尽可能使用反引号。 (%q 格式也可用于整数和符文,它会产生一个带单引号的符文常量。) 此外,%x 还可用于字符串、字节数组以及整数,并生成一个很长的十六进制字符串, 而带空格的格式(% x)还会在字节之间插入空格。

Another handy format is %T, which prints the type of a value.

另一种实用的格式是 %T,它会打印某个值的类型.

fmt.Printf("%T\n", timeZone) prints

会打印

map[string] int

If you want to control the default format for a custom type, all that's required is to define a method with the signature String() string on the type. For our simple type T, that might look like this.

若你想控制自定义类型的默认格式,只需为该类型定义一个具有 String() string 签名的方法。对于我们简单的类型 T,可进行如下操作。

func (t *T) String() string {
    return fmt.Sprintf("%d/%g/%q", t.a, t.b, t.c)
}
fmt.Printf("%v\n", t)

to print in the format

会打印出如下格式:

7/-2.35/"abc\tdef"

(If you need to print values of type T as well as pointers to T, the receiver for String must be of value type; this example used a pointer because that's more efficient and idiomatic for struct types. See the section below on pointers vs. value receivers for more information.)

(如果你需要像指向 T 的指针那样打印类型 T 的值, String 的接收者就必须是值类型的;上面的例子中接收者是一个指针, 因为这对结构来说更高效而通用。更多详情见 指针 vs. 值接收者 一节.)

Our String method is able to call Sprintf because the print routines are fully reentrant and can be wrapped this way. There is one important detail to understand about this approach, however: don't construct a String method by calling Sprintf in a way that will recur into your String method indefinitely. This can happen if the Sprintf call attempts to print the receiver directly as a string, which in turn will invoke the method again. It's a common and easy mistake to make, as this example shows.

我们的 String 方法也可调用 Sprintf, 因为打印例程可以完全重入并按这种方式封装。不过要理解这种方式,还有一个重要的细节: 请勿通过调用 Sprintf 来构造 String 方法,因为它会无限递归你的的 String 方法。当 Sprintf 试图将一个接收者以字符串形式打印输出,而在此过程中反过来又调用了 Sprintf 时,这种情况就会出现。这是一个很常见的错误,如下例所示。

type MyString string

func (m MyString) String() string {
    return fmt.Sprintf("MyString=%s", m) // Error: will recur forever.
}
type MyString string

func (m MyString) String() string {
    return fmt.Sprintf("MyString=%s", m) // 错误:会无限递归
}

It's also easy to fix: convert the argument to the basic string type, which does not have the method.

要解决这个问题也很简单:将该实参转换为基本的字符串类型,它没有这个方法。

type MyString string
func (m MyString) String() string {
    return fmt.Sprintf("MyString=%s", string(m)) // OK: note conversion.
}
type MyString string
func (m MyString) String() string {
    return fmt.Sprintf("MyString=%s", string(m)) // 可以:注意转换
}

In the initialization section we'll see another technique that avoids this recursion.

初始化 一节中,我们将看到避免这种递归的另一种技术。

Another printing technique is to pass a print routine's arguments directly to another such routine. The signature of Printf uses the type ...interface{} for its final argument to specify that an arbitrary number of parameters (of arbitrary type) can appear after the format.

另一种打印技术就是将打印例程的实参直接传入另一个这样的例程。Printf 的签名为其最后的实参使用了 ...interface{} 类型,这样格式的后面就能出现任意数量,任意类型的形参了。

func Printf(format string, v ...interface{}) (n int, err error) {

Within the function Printf, v acts like a variable of type []interface{} but if it is passed to another variadic function, it acts like a regular list of arguments. Here is the implementation of the function log.Println we used above. It passes its arguments directly to fmt.Sprintln for the actual formatting.

在 Printf 函数中,v 看起来更像是 []interface{} 类型的变量,但如果将它传递到另一个变参函数中,它就像是常规实参列表了。 以下是我们之前用过的 log.Println 的实现。它直接将其实参传递给 fmt.Sprintln 进行实际的格式化。

// Println prints to the standard logger in the manner of fmt.Println.
func Println(v ...interface{}) {
    std.Output(2, fmt.Sprintln(v...))  // Output takes parameters (int, string)
}
// Println 通过 fmt.Println 的方式将日志打印到标准记录器。
func Println(v ...interface{}) {
    std.Output(2, fmt.Sprintln(v...))  // Output 接受形参 (int, string)
}

We write ... after v in the nested call to Sprintln to tell the compiler to treat v as a list of arguments; otherwise it would just pass v as a single slice argument.

在该 Sprintln 嵌套调用中,我们将 ... 写在 v 之后来告诉编译器将 v 视作一个实参列表,否则它会将 v 当做单一的切片实参来传递。

There's even more to printing than we've covered here. See the godoc documentation for package fmt for the details.

还有很多关于打印知识点没有提及。详情请参阅 godoc 对 fmt 包的说明文档。

By the way, a ... parameter can be of a specific type, for instance ...int for a min function that chooses the least of a list of integers:

顺便一提,... 形参可指定具体的类型,例如从整数列表中选出最小值的函数 min,其形参可为 ...int 类型。

func Min(a ...int) int {
    min := int(^uint(0) >> 1)  // largest int
    for _, i := range a {
        if i < min {
            min = i
        }
    }
    return min
}
func Min(a ...int) int {
    min := int(^uint(0) >> 1)  // 最大的 int
    for _, i := range a {
        if i < min {
            min = i
        }
    }
    return min
}

Append

追加

Now we have the missing piece we needed to explain the design of the append built-in function. The signature of append is different from our custom Append function above. Schematically, it's like this:

现在我们要对内建函数 append 的设计进行补充说明。append 函数的签名不同于前面我们自定义的 Append 函数。大致来说,它就像这样:

func append(slice []T, elements ...T) []T

where T is a placeholder for any given type. You can't actually write a function in Go where the type T is determined by the caller. That's why append is built in: it needs support from the compiler.

其中的 T 为任意给定类型的占位符。实际上,你无法在 Go 中编写一个类型 T 由调用者决定的函数。这也就是为何 append 为内建函数的原因:它需要编译器的支持。

What append does is append the elements to the end of the slice and return the result. The result needs to be returned because, as with our hand-written Append, the underlying array may change. This simple example

append 会在切片末尾追加元素并返回结果。我们必须返回结果, 原因与我们手写的 Append 一样,即底层数组可能会被改变。以下简单的例子

x := []int{1,2,3}
x = append(x, 4, 5, 6)
fmt.Println(x)

prints [1 2 3 4 5 6]. So append works a little like Printf, collecting an arbitrary number of arguments.

将打印 [1 2 3 4 5 6]。因此 append 有点像 Printf 那样,可接受任意数量的实参。

But what if we wanted to do what our Append does and append a slice to a slice? Easy: use ... at the call site, just as we did in the call to Output above. This snippet produces identical output to the one above.

但如果我们要像 Append 那样将一个切片追加到另一个切片中呢? 很简单:在调用的地方使用 ...,就像我们在上面调用 Output 那样。以下代码片段的输出与上一个相同。

x := []int{1,2,3}
y := []int{4,5,6}
x = append(x, y...)
fmt.Println(x)

Without that ..., it wouldn't compile because the types would be wrong; y is not of type int.

如果没有 ...,它就会由于类型错误而无法编译,因为 y 不是 int 类型的。

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