Effective Go

Freax

贡献于2013-08-14

字数:65736 关键词: Go语言编程 Go

简介 Go is a new language. Although it borrows ideas from existing languages, it has unusual properties that make effective Go programs different in character from programs written in its relatives. A straightforward translation of a C++ or Java program into Go is unlikely to produce a satisfactory result—Java programs are written in Java, not Go. On the other hand, thinking about the problem from a Go perspective could produce a successful but quite different program. In other words, to write Go well, it's important to understand its properties and idioms. It's also important to know the established conventions for programming in Go, such as naming, formatting, program construction, and so on, so that programs you write will be easy for other Go programmers to understand. Go是一个新的语言。虽然它从其他语言中借鉴了一些特性,但是Go语言的编程方式和其他是 有本质却别的。如果只是简单的将C++或Java等代码翻译为Go代码是不可能得到最优的Go代码的。 java程序员用java的思维方式编程,并不是Go的思维方式。如果采用go的思维方式,一个问题 可能有完全不同的解决方法。因此,如果要真正的用好Go语言,理解它的语言特性和设计思想是 很重要的。另外,还要知道Go语言的变成风格,例如命名方式、格式化、程序结构等等,采用通用 的方式也便于和其他的Go程序员交流。 This document gives tips for writing clear, idiomatic Go code. It augments the language specification and the tutorial, both of which you should read first. 该文档对于如何编写清晰优雅的Go程序给出一些建议。它是Go语法说明 和Go语言入门教程的补充。 例子 The Go package sources are intended to serve not only as the core library but also as examples of how to use the language. If you have a question about how to approach a problem or how something might be implemented, they can provide answers, ideas and background. Go源代码不仅包含了核心库的实现,还有很多如何使用语言的例子。 如果在使用go的过程中遇到问题,或者想了解某些库的内部工作机制,可以直接参考源代码找到答案。 格式化 Formatting issues are the most contentious but the least consequential. People can adapt to different formatting styles but it's better if they don't have to, and less time is devoted to the topic if everyone adheres to the same style. The problem is how to approach this Utopia without a long prescriptive style guide. 格式化是一个最有争议的问题。虽然人可以适应各种不同的风格,不过如果大家都遵循一个默认统一的 风格是最理想的。当然,这也是一个仁者见仁、智者见智的问题,不可能有一个终极的理想答案。 With Go we take an unusual approach and let the machine take care of most formatting issues. A program, gofmt, reads a Go program and emits the source in a standard style of indentation and vertical alignment, retaining and if necessary reformatting comments. If you want to know how to handle some new layout situation, run gofmt; if the answer doesn't seem right, fix the program (or file a bug), don't work around it. 对于Go语言,我们采用不同的处理方法:让机器处理绝大部分的格式化工作。工具程序 gofmt可以根据需要将Go代码格式自动格式化为统一的风格。如果你想 了解格式化后代码的缩进方式,你可以直接运行gofmt,然后查看输出结果。 As an example, there's no need to spend time lining up the comments on the fields of a structure. Gofmt will do that for you. Given the declaration 下面是一个例子,我们没有必要花时间手工调整类型中成员注释的对齐方式。Gofmt 可以自动将注释对齐。下面是结果的定义: type T struct { name string // name of the object value int // its value} gofmt will line up the columns: gofmt处理后的结果: type T struct { name string // name of the object value int // its value} All code in the libraries has been formatted with gofmt. Go语言库中的所有代码都是用gofmt工具格式化的。 Some formatting details remain. Very briefly, 格式化的一些细节: Indentation 缩进 We use tabs for indentation and gofmt emits them by default. Use spaces only if you must. 我们使用tab缩进,gofmt也是默认用tab缩进。当然,也可以指定空白缩进。 Line length 每行长度 Go has no line length limit. Don't worry about overflowing a punched card. If a line feels too long, wrap it and indent with an extra tab. Go语言代码每行长度没有限制。不用担心一行的代码太长超出显式范围,gofmt会自动 处理太长的行。 Parentheses 括弧 Go needs fewer parentheses: control structures (if, for, switch) do not require parentheses in their syntax. Also, the operator precedence hierarchy is shorter and clearer, so x<<8 + y<<16 means what the spacing implies. Go语言很少使用括弧:对于控制结构(if,for,switch) 括弧也不是必须的。而且Go中表达式中运算符的优先级比较简洁,例如下面代码: x<<8 + y<<16 意思是x和y移位后相加。 注释 Go provides C-style / / block comments and C++-style // line comments. Line comments are the norm; block comments appear mostly as package comments and are also useful to disable large swaths of code. Go支持C语言风格的/ /块注释,也支持C++风格的//行注释。 当然,行注释更通用,块注释主要用于针对包的详细说明或者屏蔽大块的代码。 The program—and web server—godoc processes Go source files to extract documentation about the contents of the package. Comments that appear before top-level declarations, with no intervening newlines, are extracted along with the declaration to serve as explanatory text for the item. The nature and style of these comments determines the quality of the documentation godoc produces. Every package should have a package comment, a block comment preceding the package clause. For multi-file packages, the package comment only needs to be present in one file, and any one will do. The package comment should introduce the package and provide information relevant to the package as a whole. It will appear first on the godoc page and should set up the detailed documentation that follows. /* The regexp package implements a simple library for regular expressions. The syntax of the regular expressions accepted is: regexp: concatenation { '|' concatenation } concatenation: { closure } closure: term [ '*' | '+' | '?' ] term: '^' '$' '.' character '[' [ '^' ] character-ranges ']' '(' regexp ')' */package regexp If the package is simple, the package comment can be brief. // The path package implements utility routines for// manipulating slash-separated filename paths. Comments do not need extra formatting such as banners of stars. The generated output may not even be presented in a fixed-width font, so don't depend on spacing for alignment—godoc, like gofmt, takes care of that. Finally, the comments are uninterpreted plain text, so HTML and other annotations such as this will reproduce verbatim and should not be used. Inside a package, any comment immediately preceding a top-level declaration serves as a doc comment for that declaration. Every exported (capitalized) name in a program should have a doc comment. Doc comments work best as complete English sentences, which allow a wide variety of automated presentations. The first sentence should be a one-sentence summary that starts with the name being declared. // Compile parses a regular expression and returns, if successful, a Regexp// object that can be used to match against text. func Compile(str string) (regexp *Regexp, error os.Error) { Go's declaration syntax allows grouping of declarations. A single doc comment can introduce a group of related constants or variables. Since the whole declaration is presented, such a comment can often be perfunctory. // Error codes returned by failures to parse an expression.var ( ErrInternal = os.NewError("internal error") ErrUnmatchedLpar = os.NewError("unmatched '('") ErrUnmatchedRpar = os.NewError("unmatched ')'") ...) Even for private names, grouping can also indicate relationships between items, such as the fact that a set of variables is protected by a mutex. var ( countLock sync.Mutex inputCount uint32 outputCount uint32 errorCount uint32) 命名 Names are as important in Go as in any other language. In some cases they even have semantic effect: for instance, the visibility of a name outside a package is determined by whether its first character is upper case. It's therefore worth spending a little time talking about naming conventions in Go programs. 包的命名 When a package is imported, the package name becomes an accessor for the contents. After import "bytes" the importing package can talk about bytes.Buffer. It's helpful if everyone using the package can use the same name to refer to its contents, which implies that the package name should be good: short, concise, evocative. By convention, packages are given lower case, single-word names; there should be no need for underscores or mixedCaps. Err on the side of brevity, since everyone using your package will be typing that name. And don't worry about collisions a priori. The package name is only the default name for imports; it need not be unique across all source code, and in the rare case of a collision the importing package can choose a different name to use locally. In any case, confusion is rare because the file name in the import determines just which package is being used. Another convention is that the package name is the base name of its source directory; the package in src/pkg/container/vector is imported as "container/vector" but has name vector, not container_vector and not containerVector. The importer of a package will use the name to refer to its contents (the import . notation is intended mostly for tests and other unusual situations), so exported names in the package can use that fact to avoid stutter. For instance, the buffered reader type in the bufio package is called Reader, not BufReader, because users see it as bufio.Reader, which is a clear, concise name. Moreover, because imported entities are always addressed with their package name, bufio.Reader does not conflict with io.Reader. Similarly, the function to make new instances of ring.Ring—which is the definition of a constructor in Go—would normally be called NewRing, but since Ring is the only type exported by the package, and since the package is called ring, it's called just New. Clients of the package see that as ring.New. Use the package structure to help you choose good names. Another short example is once.Do; once.Do(setup) reads well and would not be improved by writing once.DoOrWaitUntilDone(setup). Long names don't automatically make things more readable. If the name represents something intricate or subtle, it's usually better to write a helpful doc comment than to attempt to put all the information into the name. 接口的命名 By convention, one-method interfaces are named by the method name plus the -er suffix: Reader, Writer, Formatter etc. There are a number of such names and it's productive to honor them and the function names they capture. Read, Write, Close, Flush, String and so on have canonical signatures and meanings. To avoid confusion, don't give your method one of those names unless it has the same signature and meaning. Conversely, if your type implements a method with the same meaning as a method on a well-known type, give it the same name and signature; call your string-converter method String not ToString. MixedCaps Finally, the convention in Go is to use MixedCaps or mixedCaps rather than underscores to write multiword names. 分号 Like C, Go's formal grammar uses semicolons to terminate statements; unlike C, those semicolons do not appear in the source. Instead the lexer uses a simple rule to insert semicolons automatically as it scans, so the input text is mostly free of them. Go语言与C一样都是采用分号来结束一条语句,不一样的是,并不是所有的源码 都要使用分号。Go是采用语法解析器自动在每行末增加分号,所有你在写代码的 时候可以把分号缩略. The rule is this. If the last token before a newline is an identifier (which includes words like int and float64), a basic literal such as a number or string constant, or one of the tokens 这个规则是: 如果一个标记(token)的前一行是标识符(identifier)(就像"int"或 "float64"), 比如: 数字,一个字符串或一个标记. break continue fallthrough return ++ -- ) } the lexer always inserts a semicolon after the token. This could be summarized as, “if the newline comes after a token that could end a statement, add a semicolon”. 那么语法解析器就会在标记的后面插入分号,也就是说"在标记的后面是个换行,这说明可能是语句的结束,就增加一个分号"。 A semicolon can also be omitted immediately before a closing brace, so a statement such as 在右括号之前可以省略分号,比如: go func() { for { dst <- <-src } }() needs no semicolons. Idiomatic Go programs have semicolons only in places such as for loop clauses, to separate the initializer, condition, and continuation elements. They are also necessary to separate multiple statements on a line, should you write code that way. 不需要分号。在Go编程中只有几个地方需要增加分号, 比如: for循环 为了把初始化,条件和遍历元素分开。还有在一行中有多条语句,也需要增加分号。 One caveat. You should never put the opening brace of a control structure (if, for, switch, or select) on the next line. If you do, a semicolon will be inserted before the brace, which could cause unwanted effects. Write them like this 需要注意的是,你不能把控制语句(if, for, switch, or select)左大括号单独方在一行, 如果你这样作了在大括号之前将要插入一个分号,可能会造成不必要的麻烦, 要写成: if i < f() { g()} not like this 不要写成 if i < f() // wrong!{ // wrong! g()} 控制流 The control structures of Go are related to those of C but different in important ways. There is no do or while loop, only a slightly generalized for; switch is more flexible; if and switch accept an optional initialization statement like that of for; and there are new control structures including a type switch and a multiway communications multiplexer, select. The syntax is also slightly different: parentheses are not required and the bodies must always be brace-delimited. Go语言的控制结构与C的基本相同但是有些地方还是不同。Go中没有do, while这样的循环,for与switch 也非常的灵活。if和switch可以有一个初始化语句 就像for一样。还增加了一个type switch(类型选择)和多通道复用(multiway communications multiplexer)的select. 语法有一点点区别,圆括号大部分是 不需要的但是大括号必须始终括号分隔. If In Go a simple if looks like this: Go中简单的if实例: if x > 0 { return y} Mandatory braces encourage writing simple if statements on multiple lines. It's good style to do so anyway, especially when the body contains a control statement such as a return or break. 建议在写if语句的时候采用多行。这是一种非常好的编程风格, 特别是在控制结构体里面有return或break的时候. Since if and switch accept an initialization statement, it's common to see one used to set up a local variable. if和switch允许初始化声明,就可以使用本地变量(local variable). if err := file.Chmod(0664); err != nil { log.Stderr(err) return err} In the Go libraries, you'll find that when an if statement doesn't flow into the next statement—that is, the body ends in break, continue, goto, or return—the unnecessary else is omitted. 在Go的库文件中,你可能经常看到if语句不进入下一条语句是因为函数在 break,continue,goto或reurn结束, else可以省略。 f, err := os.Open(name, os.O_RDONLY, 0)if err != nil { return err} codeUsing(f) This is a example of a common situation where code must analyze a sequence of error possibilities. The code reads well if the successful flow of control runs down the page, eliminating error cases as they arise. Since error cases tend to end in return statements, the resulting code needs no else statements. 一般的代码都会考虑错误的处理,如果没有出错的情况就继续运行但是在出错的时候 函数就会返回,所以在这里不需要else语句。 f, err := os.Open(name, os.O_RDONLY, 0)if err != nil { return err} d, err := f.Stat()if err != nil { return err} codeUsing(f, d) For The Go for loop is similar to—but not the same as—C's. It unifies for and while and there is no do-while. There are three forms, only one of which has semicolons. Go中的for循环与C相似,但是也有不同的地方。Go只有for和 while, 没有do-while语句。这里有三种方式,只有一种方式 使用了分号。 // Like a C forfor init; condition; post { }// Like a C whilefor condition { }// Like a C for(;;)for { } Short declarations make it easy to declare the index variable right in the loop. sum := 0for i := 0; i < 10; i++ { sum += i} If you're looping over an array, slice, string, or map, or reading from a channel, a range clause can manage the loop for you. 如果你要遍历一个array, slice, string or map or 从通道(channel)读数 range将是你最好的选择。 var m map[string]int sum := 0for _, value := range m { // key is unused sum += value} For strings, the range does more work for you, breaking out individual Unicode characters by parsing the UTF-8 (erroneous encodings consume one byte and produce the replacement rune U+FFFD). The loop 对于字符串,range能为你更好的工作,比如解析UTF-8并单独输出Unicode字符. for pos, char := range "日本語" { fmt.Printf("character %c starts at byte position %d\n", char, pos)} prints character 日 starts at byte position 0 character 本 starts at byte position 3 character 語 starts at byte position 6 Finally, since Go has no comma operator and ++ and -- are statements not expressions, if you want to run multiple variables in a for you should use parallel assignment. 最后,Go中没有逗号运算符(comma operator)和++与--运算, 如果你想执行多个变量在for语句中,你可以使用并行参数(parallel assignment). // Reverse afor i, j := 0, len(a)-1; i < j; i, j = i+1, j-1 { a[i], a[j] = a[j], a[i]} Switch Go's switch is more general than C's. The expressions need not be constants or even integers, the cases are evaluated top to bottom until a match is found, and if the switch has no expression it switches on true. It's therefore possible—and idiomatic—to write an if-else-if-else chain as a switch. Go中的switch要比C更全面,C的表达式仅仅只有常数或整数。 每个分支(cases)从上到下进行匹配取值, 如果switch没有表达式 那么switches是真。所有才有可能使用switch替换 func unhex(c byte) byte { switch { case '0' <= c && c <= '9': return c - '0' case 'a' <= c && c <= 'f': return c - 'a' + 10 case 'A' <= c && c <= 'F': return c - 'A' + 10 } return 0} There is no automatic fall through, but cases can be presented in comma-separated lists. func shouldEscape(c byte) bool { switch c { case ' ', '?', '&', '=', '#', '+', '%': return true } return false} Here's a comparison routine for byte arrays that uses two switch statements: 这个操作数组的程序和上面的相似是通过两个switch语句。 // Compare returns an integer comparing the two byte arrays// lexicographically.// The result will be 0 if a == b, -1 if a < b, and +1 if a > b func Compare(a, b []byte) int { for i := 0; i < len(a) && i < len(b); i++ { switch { case a[i] > b[i]: return 1 case a[i] < b[i]: return -1 } } switch { case len(a) < len(b): return -1 case len(a) > len(b): return 1 } return 0} A switch can also be used to discover the dynamic type of an interface variable. Such a type switch uses the syntax of a type assertion with the keyword type inside the parentheses. If the switch declares a variable in the expression, the variable will have the corresponding type in each clause. switch可以动态的取得接口变量的数据类型,比如: type switch就是 用关键字type插入到接口类型后面的括号来判断类型, 如果switch 在表达式中声明了一个变量,在分支上就有相应的类型。 switch t := interfaceValue.(type) {default: fmt.Printf("unexpected type %T", t) // %T prints typecase bool: fmt.Printf("boolean %t\n", t)case int: fmt.Printf("integer %d\n", t)case *bool: fmt.Printf("pointer to boolean %t\n", *t)case *int: fmt.Printf("pointer to integer %d\n", *t)} 函数 Multiple return values 多个返回值 One of Go's unusual features is that functions and methods can return multiple values. This 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. Go语言中函数和方法方法的一个有意思的特性是它们可以同时返回多个值。它可以比C语言 更简洁的处理多个返回值的情况:例如在修改一个参数的同时获取错误返回值 (-1或EOF)。 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 File.Write in package os is: 在传统的C语言中,如果写数据失败的话,会在另外一个地方保存错误标志,而且错误标志很容易被 其他函数产生的错误覆盖。在Go语言中,则可以在返回成功写入的数据数目的同时,也可以返回有意义 的错误信息:“您已经写了一些数据,但不是全部,因为设备在阻塞填充中”。对于os 包中的 File.Write函数,说明如下: 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. 在函数的文档中有函数返回值的描述:返回成功写入的数据长度,如果n != len(b),则同时返回一个非non-nil的错误信息。 这是Go语言中,处理错误的常见方式。在后面的“错误处理”一节,会有更多的描述。 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 array, returning the number and the next position. 多个返回值还可以用于模拟C语言中通过指针的方式遍历。下面的函数是从一个int数组中获取 一个数据,然后移动到下一个位置。 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 array a like this: 你还可以用这个方法来打印一个数组: for i := 0; i < len(a); { x, i = nextInt(a, i) fmt.Println(x) } Named result parameters 命名的返回值 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语言中,我们还可以给函数或方法的返回值命名,就像函数的输入参数那样。如果我们命名了返回值, 那么它们将在函数开始的时候被初始化为空。然后,在执行不带参数的return语句时, 命名的返回值变量将被用于返回。 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. 返回值命名并不强制使用,但是有时我们给名返回值命令可以产生更清晰的代码,同时它也可以用于文档。 例如,我们把nextInt的返回值命名: 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: 命名后,返回值会自动初始化,而且不需要在return中显式写出返回参数。下面的io.ReadFull 函数是另一个类似的例子: func ReadFull(r Reader, buf []byte) (n int, err os.Error) { for len(buf) > 0 && err == nil { var nr int nr, err = r.Read(buf) n += nr buf = buf[nr:len(buf)] } 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. // Contents returns the file's contents as a string. func Contents(filename string) (string, os.Error) { f, err := os.Open(filename, os.O_RDONLY, 0) 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 = bytes.Add(result, buf[0:n]) if err != nil { if err == os.EOF { break } return "", err // f will be closed if we return here. } } return string(result), nil // f will be closed if we return here.} Deferring a function like this 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. The arguments to the deferred function (which includes 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. 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: 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....} 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: 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 entering: bin b entering: ain 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 an example. Data 数据 Allocation with new() Go has two allocation primitives, 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 essentially the same as its namesakes in other languages: 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. Since the memory returned by new() is zeroed, it's helpful to arrange that the zeroed object 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. The zero-value-is-useful property works transitively. Consider this type declaration. 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 this snippet, both p and v will work correctly without further arrangement. p := new(SyncedBuffer) // type *SyncedBuffervar v SyncedBuffer // type SyncedBuffer Constructors and composite literals 构造和初始化成员 Sometimes the zero value isn't good enough and an initializing constructor is necessary, as in this example derived from package os. 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. func NewFile(fd int, name string) *File { if fd < 0 { return nil } f := File{fd, name, nil, 0} return &f} Note that 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. 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 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. 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. 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() 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 zero) value of type T, not T. The reason for the distinction is that these three types are, 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; 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, 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. These examples illustrate the difference between new() and make(). var p *[]int = new([]int) // allocates slice structure; *p == nil; rarely usefulvar v []int = make([]int, 100) // 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) Remember that make() applies only to maps, slices and channels and does not return a pointer. To obtain an explicit pointer allocate with new(). 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. 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 10int and 20int are distinct. 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. func Sum(a *[3]float) (sum float) { for _, v := range *a { sum += v } return} array := [...]float{7.0, 8.5, 9.1} x := Sum(&array) // Note the explicit address-of operator But even this style isn't idiomatic Go. Slices are. 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. Slices are reference types, which means that if you assign one slice to another, both refer to the same underlying array. For instance, 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: func (file File) Read(buf byte) (n int, err os.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 b, slice (here used as a verb) the buffer. n, err := f.Read(buf[0:32]) Such slicing is common and efficient. In fact, leaving efficiency aside for the moment, this snippet would also read the first 32 bytes of the buffer. var n int var err os.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 } 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. 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) // Copy data (could use bytes.Copy()). for i, c := range slice { newSlice[i] = c } 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. Maps 字典 Maps are a convenient and powerful built-in data structure to associate values of different types. The key can be of any type for which the equality operator is defined, such as integers, floats, strings, pointers, and interfaces (as long as the dynamic type supports equality). Structs, arrays and slices cannot be used as map keys, because equality is not defined on those types. Like slices, maps are a reference type. If you pass a map to a function that changes the contents of the map, the changes will be visible in the caller. 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. 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 except that the index doesn't need to be an integer. An attempt to fetch a map value with a key that is not present in the map will cause the program to crash, but there is a way to do so safely using a multiple assignment. var seconds intvar 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: func offset(tz string) int { if seconds, ok := timeZone[tz]; ok { return seconds } log.Stderr("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, a simple underscore (). The blank identifier can be assigned or declared with any value of any type, with the value discarded harmlessly. For testing presence in a map, use the blank identifier in place of the usual variable for the value. _, present := timeZone[tz] To delete a map entry, turn the multiple assignment around by placing an extra boolean on the right; if the boolean is false, the entry is deleted. It's safe to do this even if the key is already absent from the map. timeZone["PDT"] = 0, false // Now on Standard Time 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. 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 ln version also inserts a blank between arguments if neither is a string and appends a newline to the output. In this example each line produces the same output. fmt.Printf("Hello %d\n", 23) fmt.Fprint(os.Stdout, "Hello ", 23, "\n") fmt.Println(fmt.Sprint("Hello ", 23)) As mentioned in the tutorial, 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. 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. var x uint64 = 1<<64 - 1 fmt.Printf("%d %x; %d %x\n", x, x, int64(x), int64(x)) prints 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, structs, and maps. Here is a print statement for the time zone map defined in the previous section. fmt.Printf("%v\n", timeZone) // or just fmt.Println(timeZone) which gives output 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. type T struct { a int b float 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 &{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. Also, %x works on strings and arrays of bytes as well as on integers, generating a long hexadecimal string, and with a space in the format (% x) it puts spaces between the bytes. Another handy format is %T, which prints the type of a value. fmt.Printf("%T\n", timeZone) prints map[string] int If you want to control the default format for a custom type, all that's required is to define a method String() string on the type. For our simple type T, that might look like this. 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 7/-2.35/"abc\tdef" Our String() method is able to call Sprintf because the print routines are fully reentrant and can be used recursively. We can even go one step further and pass a print routine's arguments directly to another such routine. The signature of Printf uses the ... type for its final argument to specify that an arbitrary number of parameters can appear after the format. func Printf(format string, v ...) (n int, errno os.Error) { Within the function Printf, v is a variable that can be passed, for instance, to another print routine. Here is the implementation of the function log.Stderr we used above. It passes its arguments directly to fmt.Sprintln for the actual formatting. // Stderr is a helper function for easy logging to stderr. It is analogous to Fprint(os.Stderr). func Stderr(v ...) { stderr.Output(2, fmt.Sprintln(v)) // Output takes parameters (int, string)} There's even more to printing than we've covered here. See the godoc documentation for package fmt for the details. Initialization 初始化 Although it doesn't look superficially very different from initialization in C or C++, initialization in Go is more powerful. Complex structures can be built during initialization and the ordering issues between initialized objects in different packages are handled correctly. Constants 常量初始化 Constants in Go are just that—constant. They are created at compile time, even when defined as locals in functions, and can only be numbers, strings or booleans. Because of the compile-time restriction, the expressions that define them must be constant expressions, evaluatable by the compiler. For instance, 1<<3 is a constant expression, while math.Sin(math.Pi/4) is not because the function call to math.Sin needs to happen at run time. In Go, enumerated constants are created using the iota enumerator. Since iota can be part of an expression and expressions can be implicitly repeated, it is easy to build intricate sets of values. type ByteSize float64const ( _ = iota // ignore first value by assigning to blank identifier KB ByteSize = 1<<(10*iota) MB GB TB PB EB ZB YB) The ability to attach a method such as String to a type makes it possible for such values to format themselves automatically for printing, even as part of a general type. func (b ByteSize) String() string { switch { case b >= YB: return fmt.Sprintf("%.2fYB", b/YB) case b >= ZB: return fmt.Sprintf("%.2fZB", b/ZB) case b >= EB: return fmt.Sprintf("%.2fEB", b/EB) case b >= PB: return fmt.Sprintf("%.2fPB", b/PB) case b >= TB: return fmt.Sprintf("%.2fTB", b/TB) case b >= GB: return fmt.Sprintf("%.2fGB", b/GB) case b >= MB: return fmt.Sprintf("%.2fMB", b/MB) case b >= KB: return fmt.Sprintf("%.2fKB", b/KB) } return fmt.Sprintf("%.2fB", b)} The expression YB prints as 1.00YB, while ByteSize(1e13) prints as 9.09TB. Variables 变量初始化 Variables can be initialized just like constants but the initializer can be a general expression computed at run time. var ( HOME = os.Getenv("HOME") USER = os.Getenv("USER") GOROOT = os.Getenv("GOROOT")) The init function init函数 Finally, each source file can define its own init() function to set up whatever state is required. The only restriction is that, although goroutines can be launched during initialization, they will not begin execution until it completes; initialization always runs as a single thread of execution. And finally means finally: init() is called after all the variable declarations in the package have evaluated their initializers, and those are evaluated only after all the imported packages have been initialized. Besides initializations that cannot be expressed as declarations, a common use of init() functions is to verify or repair correctness of the program state before real execution begins. func init() { if USER == "" { log.Exit("$USER not set") } if HOME == "" { HOME = "/usr/" + USER } if GOROOT == "" { GOROOT = HOME + "/go" } // GOROOT may be overridden by --goroot flag on command line. flag.StringVar(&GOROOT, "goroot", GOROOT, "Go root directory")} Methods 方法 Pointers vs. Values 指针vs值 Methods can be defined for any named type that is not a pointer or an interface; the receiver does not have to be a struct. In the discussion of slices above, we wrote an Append function. We can define it as a method on slices instead. To do this, we first declare a named type to which we can bind the method, and then make the receiver for the method a value of that type. type ByteSlice []byte func (slice ByteSlice) Append(data []byte) []byte { // Body exactly the same as above} This still requires the method to return the updated slice. We can eliminate that clumsiness by redefining the method to take a pointer to a ByteSlice as its receiver, so the method can overwrite the caller's slice. func (p *ByteSlice) Append(data []byte) { slice := *p // Body as above, without the return. *p = slice} In fact, we can do even better. If we modify our function so it looks like a standard Write method, like this, func (p *ByteSlice) Write(data []byte) (n int, err os.Error) { slice := *p // Again as above. *p = slice return len(data), nil} then the type ByteSlice satisfies the standard interface io.Writer, which is handy. For instance, we can print into one. var b ByteSlice fmt.Fprintf(&b, "This hour has %d days\n", 7) We pass the address of a ByteSlice because only ByteSlice satisfies io.Writer. The rule about pointers vs. values for receivers is that value methods can be invoked on pointers and values, but pointer methods can only be invoked on pointers. This is because pointer methods can modify the receiver; invoking them on a copy of the value would cause those modifications to be discarded. By the way, the idea of using Write on a slice of bytes is implemented by bytes.Buffer. Interfaces and other types 接口和其他类型 Interfaces 接口 Interfaces in Go provide a way to specify the behavior of an object: if something can do this, then it can be used here. We've seen a couple of simple examples already; custom printers can be implemented by a String method while Fprintf can generate output to anything with a Write method. Interfaces with only one or two methods are common in Go code, and are usually given a name derived from the method, such as io.Writer for something that implements Write. Go中的接口提供了一类对象的抽象。我们在前面已经看到了关于接口的一些例子。 我们可以给新定义的对象实现一个String方法,这样就可以用 Fprintf输出该类型的值。同样,Fprintf可以将 结果输出到任意实现了Write方法的对象。接口一般只包含一类方法, 并且以ed后缀的方式命名,例如io.Writer接口对应Write 方法实现。 A type can implement multiple interfaces. For instance, a collection can be sorted by the routines in package sort if it implements sort.Interface, which contains Len(), Less(i, j int) bool, and Swap(i, j int), and it could also have a custom formatter. In this contrived example Sequence satisfies both. 一种类型可以实现多个接口。例如,如果想支持sort包中的排序 方法,那么只需要实现sort.Interface接口的Len()、 Less()、Swap(i, j int)方法就可以了。下面的例子 实现了sort.Interface接口的同时,还定制了输出函数。 type Sequence []int// Methods required by sort.Interface. func (s Sequence) Len() int { return len(s)} func (s Sequence) Less(i, j int) bool { return s[i] < s[j]} func (s Sequence) Swap(i, j int) { s[i], s[j] = s[j], s[i]}// Method for printing - sorts the elements before printing. func (s Sequence) String() string { sort.Sort(s) str := "[" for i, elem := range s { if i > 0 { str += " " } str += fmt.Sprint(elem) } return str + "]"} Conversions The String method of Sequence is recreating the work that Sprint already does for slices. We can share the effort if we convert the Sequence to a plain int before calling Sprint. func (s Sequence) String() string { sort.Sort(s) return fmt.Sprint([]int(s))} The conversion causes s to be treated as an ordinary slice and therefore receive the default formatting. Without the conversion, Sprint would find the String method of Sequence and recur indefinitely. Because the two types (Sequence and int) are the same if we ignore the type name, it's legal to convert between them. The conversion doesn't create a new value, it just temporarily acts as though the existing value has a new type. (There are other legal conversions, such as from integer to float, that do create a new value.) It's an idiom in Go programs to convert the type of an expression to access a different set of methods. As an example, we could use the existing type sort.IntArray to reduce the entire example to this: type Sequence []int// Method for printing - sorts the elements before printing func (s Sequence) String() string { sort.IntArray(s).Sort() return fmt.Sprint([]int(s))} Now, instead of having Sequence implement multiple interfaces (sorting and printing), we're using the ability of a data item to be converted to multiple types (Sequence, sort.IntArray and int), each of which does some part of the job. That's more unusual in practice but can be effective. Generality If a type exists only to implement an interface and has no exported methods beyond that interface, there is no need to export the type itself. Exporting just the interface makes it clear that it's the behavior that matters, not the implementation, and that other implementations with different properties can mirror the behavior of the original type. It also avoids the need to repeat the documentation on every instance of a common method. In such cases, the constructor should return an interface value rather than the implementing type. As an example, in the hash libraries both crc32.NewIEEE() and adler32.New() return the interface type hash.Hash32. Substituting the CRC-32 algorithm for Adler-32 in a Go program requires only changing the constructor call; the rest of the code is unaffected by the change of algorithm. A similar approach allows the streaming cipher algorithms in the crypto/block package to be separated from the block ciphers they chain together. By analogy with the bufio package, they wrap a Cipher interface and return hash.Hash, io.Reader, or io.Writer interface values, not specific implementations. The interface to crypto/block includes: type Cipher interface { BlockSize() int Encrypt(src, dst []byte) Decrypt(src, dst []byte)}// NewECBDecrypter returns a reader that reads data// from r and decrypts it using c in electronic codebook (ECB) mode. func NewECBDecrypter(c Cipher, r io.Reader) io.Reader// NewCBCDecrypter returns a reader that reads data// from r and decrypts it using c in cipher block chaining (CBC) mode// with the initialization vector iv. func NewCBCDecrypter(c Cipher, iv []byte, r io.Reader) io.Reader NewECBDecrypter and NewCBCReader apply not just to one specific encryption algorithm and data source but to any implementation of the Cipher interface and any io.Reader. Because they return io.Reader interface values, replacing ECB encryption with CBC encryption is a localized change. The constructor calls must be edited, but because the surrounding code must treat the result only as an io.Reader, it won't notice the difference. Interfaces and methods 接口和方法 Since almost anything can have methods attached, almost anything can satisfy an interface. One illustrative example is in the http package, which defines the Handler interface. Any object that implements Handler can serve HTTP requests. type Handler interface { ServeHTTP(*Conn, *Request)} For brevity, let's ignore POSTs and assume HTTP requests are always GETs; that simplification does not affect the way the handlers are set up. Here's a trivial but complete implementation of a handler to count the number of times the page is visited. // Simple counter server. type Counter struct { n int} func (ctr *Counter) ServeHTTP(c *http.Conn, req *http.Request) { ctr.n++ fmt.Fprintf(c, "counter = %d\n", ctr.n)} (Keeping with our theme, note how Fprintf can print to an HTTP connection.) For reference, here's how to attach such a server to a node on the URL tree. import "http"... ctr := new(Counter) http.Handle("/counter", ctr) But why make Counter a struct? An integer is all that's needed. (The receiver needs to be a pointer so the increment is visible to the caller.) // Simpler counter server. type Counter int func (ctr *Counter) ServeHTTP(c *http.Conn, req *http.Request) { *ctr++ fmt.Fprintf(c, "counter = %d\n", *ctr)} What if your program has some internal state that needs to be notified that a page has been visited? Tie a channel to the web page. // A channel that sends a notification on each visit.// (Probably want the channel to be buffered.) type Chan chan *http.Request func (ch Chan) ServeHTTP(c *http.Conn, req *http.Request) { ch <- req fmt.Fprint(c, "notification sent")} Finally, let's say we wanted to present on /args the arguments used when invoking the server binary. It's easy to write a function to print the arguments. func ArgServer() { for i, s := range os.Args { fmt.Println(s) }} How do we turn that into an HTTP server? We could make ArgServer a method of some type whose value we ignore, but there's a cleaner way. Since we can define a method for any type except pointers and interfaces, we can write a method for a function. The http package contains this code: // The HandlerFunc type is an adapter to allow the use of// ordinary functions as HTTP handlers. If f is a function// with the appropriate signature, HandlerFunc(f) is a// Handler object that calls f. type HandlerFunc func(*Conn, *Request)// ServeHTTP calls f(c, req). func (f HandlerFunc) ServeHTTP(c *Conn, req *Request) { f(c, req)} HandlerFunc is a type with a method, ServeHTTP, so values of that type can serve HTTP requests. Look at the implementation of the method: the receiver is a function, f, and the method calls f. That may seem odd but it's not that different from, say, the receiver being a channel and the method sending on the channel. To make ArgServer into an HTTP server, we first modify it to have the right signature. // Argument server. func ArgServer(c *http.Conn, req *http.Request) { for i, s := range os.Args { fmt.Fprintln(c, s) }} ArgServer now has same signature as HandlerFunc, so it can be converted to that type to access its methods, just as we converted Sequence to IntArray to access IntArray.Sort. The code to set it up is concise: http.Handle("/args", http.HandlerFunc(ArgServer)) When someone visits the page /args, the handler installed at that page has value ArgServer and type HandlerFunc. The HTTP server will invoke the method ServeHTTP of that type, with ArgServer as the receiver, which will in turn call ArgServer (via the invocation f(c, req) inside HandlerFunc.ServeHTTP). The arguments will then be displayed. In this section we have made an HTTP server from a struct, an integer, a channel, and a function, all because interfaces are just sets of methods, which can be defined for (almost) any type. Embedding 组合 Go does not provide the typical, type-driven notion of subclassing, but it does have the ability to “borrow” pieces of an implementation by embedding types within a struct or interface. Interface embedding is very simple. We've mentioned the io.Reader and io.Writer interfaces before; here are their definitions. type Reader interface { Read(p []byte) (n int, err os.Error)} type Writer interface { Write(p []byte) (n int, err os.Error)} The io package also exports several other interfaces that specify objects that can implement several such methods. For instance, there is io.ReadWriter, an interface containing both Read and Write. We could specify io.ReadWriter by listing the two methods explicitly, but it's easier and more evocative to embed the two interfaces to form the new one, like this: // ReadWrite is the interface that groups the basic Read and Write methods. type ReadWriter interface { Reader Writer} This says just what it looks like: A ReadWriter can do what a Reader does and what a Writer does; it is a union of the embedded interfaces (which must be disjoint sets of methods). Only interfaces can be embedded within interfaces. The same basic idea applies to structs, but with more far-reaching implications. The bufio package has two struct types, bufio.Reader and bufio.Writer, each of which of course implements the analogous interfaces from package io. And bufio also implements a buffered reader/writer, which it does by combining a reader and a writer into one struct using embedding: it lists the types within the struct but does not give them field names. // ReadWriter stores pointers to a Reader and a Writer.// It implements io.ReadWriter. type ReadWriter struct { *Reader // *bufio.Reader *Writer // *bufio.Writer} The embedded elements are pointers to structs and of course must be initialized to point to valid structs before they can be used. The ReadWriter struct could be written as type ReadWriter struct { reader *Reader writer *Writer} but then to promote the methods of the fields and to satisfy the io interfaces, we would also need to provide forwarding methods, like this: func (rw *ReadWriter) Read(p []byte) (n int, err os.Error) { return rw.reader.Read(p)} By embedding the structs directly, we avoid this bookkeeping. The methods of embedded types come along for free, which means that bufio.ReadWriter not only has the methods of bufio.Reader and bufio.Writer, it also satisfies all three interfaces: io.Reader, io.Writer, and io.ReadWriter. There's an important way in which embedding differs from subclassing. When we embed a type, the methods of that type become methods of the outer type, but when they are invoked the receiver of the method is the inner type, not the outer one. In our example, when the Read method of a bufio.ReadWriter is invoked, it has exactly the same effect as the forwarding method written out above; the receiver is the reader field of the ReadWriter, not the ReadWriter itself. Embedding can also be a simple convenience. This example shows an embedded field alongside a regular, named field. type Job struct { Command string *log.Logger} The Job type now has the Log, Logf and other methods of log.Logger. We could have given the Logger a field name, of course, but it's not necessary to do so. And now, once initialized, we can log to the Job: job.Log("starting now...") The Logger is a regular field of the struct and we can initialize it in the usual way with a constructor, func NewJob(command string, logger *log.Logger) *Job { return &Job{command, logger}} or with a composite literal, job := &Job{command, log.New(os.Stderr, nil, "Job: ", log.Ldate)} If we need to refer to an embedded field directly, the type name of the field, ignoring the package qualifier, serves as a field name. If we needed to access the log.Logger of a Job variable job, we would write job.Logger. This would be useful if we wanted to refine the methods of Logger. func (job *Job) Logf(format string, args ...) { job.Logger.Logf("%q: %s", job.Command, fmt.Sprintf(format, args))} Embedding types introduces the problem of name conflicts but the rules to resolve them are simple. First, a field or method X hides any other item X in a more deeply nested part of the type. If log.Logger contained a field or method called Command, the Command field of Job would dominate it. Second, if the same name appears at the same nesting level, it is usually an error; it would be erroneous to embed log.Logger if Job struct contained another field or method called Logger. However, if the duplicate name is never mentioned in the program outside the type definition, it is OK. This qualification provides some protection against changes made to types embedded from outside; there is no problem if a field is added that conflicts with another field in another subtype if neither field is ever used. Concurrency 并发 Share by communicating Concurrent programming is a large topic and there is space only for some Go-specific highlights here. Concurrent programming in many environments is made difficult by the subtleties required to implement correct access to shared variables. Go encourages a different approach in which shared values are passed around on channels and, in fact, never actively shared by separate threads of execution. Only one goroutine has access to the value at any given time. Data races cannot occur, by design. To encourage this way of thinking we have reduced it to a slogan: Do not communicate by sharing memory; instead, share memory by communicating. This approach can be taken too far. Reference counts may be best done by putting a mutex around an integer variable, for instance. But as a high-level approach, using channels to control access makes it easier to write clear, correct programs. One way to think about this model is to consider a typical single-threaded program running on one CPU. It has no need for synchronization primitives. Now run another such instance; it too needs no synchronization. Now let those two communicate; if the communication is the synchronizer, there's still no need for other synchronization. Unix pipelines, for example, fit this model perfectly. Although Go's approach to concurrency originates in Hoare's Communicating Sequential Processes (CSP), it can also be seen as a type-safe generalization of Unix pipes. Goroutines They're called goroutines because the existing terms—threads, coroutines, processes, and so on—convey inaccurate connotations. A goroutine has a simple model: it is a function executing in parallel with other goroutines in the same address space. It is lightweight, costing little more than the allocation of stack space. And the stacks start small, so they are cheap, and grow by allocating (and freeing) heap storage as required. Goroutines are multiplexed onto multiple OS threads so if one should block, such as while waiting for I/O, others continue to run. Their design hides many of the complexities of thread creation and management. Prefix a function or method call with the go keyword to run the call in a new goroutine. When the call completes, the goroutine exits, silently. (The effect is similar to the Unix shell's & notation for running a command in the background.) go list.Sort() // run list.Sort in parallel; don't wait for it. A function literal can be handy in a goroutine invocation. func Announce(message string, delay int64) { go func() { time.Sleep(delay) fmt.Println(message) }() // Note the parentheses - must call the function.} In Go, function literals are closures: the implementation makes sure the variables referred to by the function survive as long as they are active. These examples aren't too practical because the functions have no way of signaling completion. For that, we need channels. Channels Like maps, channels are a reference type and are allocated with make. If an optional integer parameter is provided, it sets the buffer size for the channel. The default is zero, for an unbuffered or synchronous channel. ci := make(chan int) // unbuffered channel of integers cj := make(chan int, 0) // unbuffered channel of integers cs := make(chan *os.File, 100) // buffered channel of pointers to Files Channels combine communication—the exchange of a value—with synchronization—guaranteeing that two calculations (goroutines) are in a known state. There are lots of nice idioms using channels. Here's one to get us started. In the previous section we launched a sort in the background. A channel can allow the launching goroutine to wait for the sort to complete. c := make(chan int) // Allocate a channel.// Start the sort in a goroutine; when it completes, signal on the channel. go func() { list.Sort() c <- 1 // Send a signal; value does not matter. }() doSomethingForAWhile()<-c // Wait for sort to finish; discard sent value. Receivers always block until there is data to receive. If the channel is unbuffered, the sender blocks until the receiver has received the value. If the channel has a buffer, the sender blocks only until the value has been copied to the buffer; if the buffer is full, this means waiting until some receiver has retrieved a value. A buffered channel can be used like a semaphore, for instance to limit throughput. In this example, incoming requests are passed to handle, which sends a value into the channel, processes the request, and then receives a value from the channel. The capacity of the channel buffer limits the number of simultaneous calls to process. var sem = make(chan int, MaxOutstanding) func handle(r *Request) { sem <- 1 // Wait for active queue to drain. process(r) // May take a long time. <-sem // Done; enable next request to run.} func Serve(queue chan *Request) { for { req := <-queue go handle(req) // Don't wait for handle to finish. }} Here's the same idea implemented by starting a fixed number of handle goroutines all reading from the request channel. The number of goroutines limits the number of simultaneous calls to process. This Serve function also accepts a channel on which it will be told to exit; after launching the goroutines it blocks receiving from that channel. func handle(queue chan *Request) { for r := range queue { process(r) }} func Serve(clientRequests chan *clientRequests, quit chan bool) { // Start handlers for i := 0; i < MaxOutstanding; i++ { go handle(clientRequests) } <-quit // Wait to be told to exit.} Channels of channels One of the most important properties of Go is that a channel is a first-class value that can be allocated and passed around like any other. A common use of this property is to implement safe, parallel demultiplexing. In the example in the previous section, handle was an idealized handler for a request but we didn't define the type it was handling. If that type includes a channel on which to reply, each client can provide its own path for the answer. Here's a schematic definition of type Request. type Request struct { args []int f func([]int) int resultChan chan int} The client provides a function and its arguments, as well as a channel inside the request object on which to receive the answer. func sum(a []int) (s int) { for _, v := range a { s += v } return} request := &Request{[]int{3, 4, 5}, sum, make(chan int)}// Send request clientRequests <- request// Wait for response. fmt.Printf("answer: %d\n", <-request.resultChan) On the server side, the handler function is the only thing that changes. func handle(queue chan *Request) { for req := range queue { req.resultChan <- req.f(req.args) }} There's clearly a lot more to do to make it realistic, but this code is a framework for a rate-limited, parallel, non-blocking RPC system, and there's not a mutex in sight. Parallelization Another application of these ideas is to parallelize a calculation across multiple CPU cores. If the calculation can be broken into separate pieces, it can be parallelized, with a channel to signal when each piece completes. Let's say we have an expensive operation to perform on a vector of items, and that the value of the operation on each item is independent, as in this idealized example. type Vector []float64// Apply the operation to v[i], v[i+1] ... up to v[n-1]. func (v Vector) DoSome(i, n int, u Vector, c chan int) { for ; i < n; i++ { v[i] += u.Op(v[i]) } c <- 1 // signal that this piece is done} We launch the pieces independently in a loop, one per CPU. They can complete in any order but it doesn't matter; we just count the completion signals by draining the channel after launching all the goroutines. const NCPU = 4 // number of CPU cores func (v Vector) DoAll(u Vector) { c := make(chan int, NCPU) // Buffering optional but sensible. for i := 0; i < NCPU; i++ { go v.DoSome(i*len(v)/NCPU, (i+1)*len(v)/NCPU, u, c) } // Drain the channel. for i := 0; i < NCPU; i++ { <-c // wait for one task to complete } // All done.} The current implementation of gc (6g, etc.) will not parallelize this code by default. It dedicates only a single core to user-level processing. An arbitrary number of goroutines can be blocked in system calls, but by default only one can be executing user-level code at any time. It should be smarter and one day it will be smarter, but until it is if you want CPU parallelism you must tell the run-time how many goroutines you want executing code simultaneously. There are two related ways to do this. Either run your job with environment variable GOMAXPROCS set to the number of cores to use (default 1); or import the runtime package and call runtime.GOMAXPROCS(NCPU). Again, this requirement is expected to be retired as the scheduling and run-time improve. A leaky buffer The tools of concurrent programming can even make non-concurrent ideas easier to express. Here's an example abstracted from an RPC package. The client goroutine loops receiving data from some source, perhaps a network. To avoid allocating and freeing buffers, it keeps a free list, and uses a buffered channel to represent it. If the channel is empty, a new buffer gets allocated. Once the message buffer is ready, it's sent to the server on serverChan. var freeList = make(chan *Buffer, 100)var serverChan = make(chan *Buffer) func client() { for { b, ok := <-freeList // grab a buffer if available if !ok { // if not, allocate a new one b = new(Buffer) } load(b) // read next message from the net serverChan <- b // send to server }} The server loop receives messages from the client, processes them, and returns the buffer to the free list. func server() { for { b := <-serverChan // wait for work process(b) _ = freeList <- b // reuse buffer if room }} The client's non-blocking receive from freeList obtains a buffer if one is available; otherwise the client allocates a fresh one. The server's non-blocking send on freeList puts b back on the free list unless the list is full, in which case the buffer is dropped on the floor to be reclaimed by the garbage collector. (The assignment of the send operation to the blank identifier makes it non-blocking but ignores whether the operation succeeded.) This implementation builds a leaky bucket free list in just a few lines, relying on the buffered channel and the garbage collector for bookkeeping. Errors 错误处理 Library routines must often return some sort of error indication to the caller. As mentioned earlier, Go's multivalue return makes it easy to return a detailed error description alongside the normal return value. By convention, errors have type os.Error, a simple interface. 一些函数在调用后一般会返回一些错误的标志。在Go中我们可以用返回返回多个值来 方便地处理错误标志信息。一般情况下,错误都实现了os.Error 接口。 type Error interface { String() string} A library writer is free to implement this interface with a richer model under the covers, making it possible not only to see the error but also to provide some context. For example, os.Open returns an os.PathError. 库的编写者一般会在os.Error接口的基础上扩展更多的信息,这样函数调用者 可以知道错误的更多细节。例如:os.Open返回的是os.PathError 类型错误(里面已经包含最基本的错误接口)。 // PathError records an error and the operation and// file path that caused it. type PathError struct { Op string // "open", "unlink", etc. Path string // The associated file. Error Error // Returned by the system call.} func (e *PathError) String() string { return e.Op + " " + e.Path + ": " + e.Error.String()} PathError's String generates a string like this: PathError生成的String错误信息如下: open /etc/passwx: no such file or directory Such an error, which includes the problematic file name, the operation, and the operating system error it triggered, is useful even if printed far from the call that caused it; it is much more informative than the plain "no such file or directory". 这个错误信息包含了要操作的文件名,对文件的具体操作,以及操作系统返回的错误信息。 这样肯定比简单输出"no such file or directory"错误信息更有价值。 Callers that care about the precise error details can use a type switch or a type assertion to look for specific errors and extract details. For PathErrors this might include examining the internal Error field for recoverable failures. 如果函数调用者想获取错误的全部细节,那么需要将错误结果从基本的类型动态转换到 更具体的错误类型。例如:下面的代码将Error转换为PathErrors 类型,因为后者的错误细细更加丰富。 for try := 0; try < 2; try++ { file, err = os.Open(filename, os.O_RDONLY, 0) if err == nil { return } if e, ok := err.(*os.PathError); ok && e.Error == os.ENOSPC { deleteTempFiles() // Recover some space. continue } return} Panic The usual way to report an error to a caller is to return an os.Error as an extra return value. The canonical Read method is a well-known instance; it returns a byte count and an os.Error. But what if the error is unrecoverable? Sometimes the program simply cannot continue. For this purpose, there is a built-in function panic that in effect creates a run-time error that will stop the program (but see the next section). The function takes a single argument of arbitrary type—often a string—to be printed as the program dies. It's also a way to indicate that something impossible has happened, such as exiting an infinite loop. In fact, the compiler recognizes a panic at the end of a function and suppresses the usual check for a return statement. // A toy implementation of cube root using Newton's method. func CubeRoot(x float64) float64 { z := x/3 // Arbitrary intitial value for i := 0; i < 1e6; i++ { prevz := z z -= (z*z*z-x) / (3*z*z) if veryClose(z, prevz) { return z } } // A million iterations has not converged; something is wrong. panic(fmt.Sprintf("CubeRoot(%g) did not converge", x))} This is only an example but real library functions should avoid panic. If the problem can be masked or worked around, it's always better to let things continue to run rather than taking down the whole program. One possible counterexample is during initialization: if the library truly cannot set itself up, it might be reasonable to panic, so to speak. var user = os.Getenv("USER") func init() { if user == "" { panic("no value for $USER") }} Recover When panic is called, including implicitly for run-time errors such indexing an array out of bounds or failing a type assertion, it immediately stops execution of the current function and begins unwinding the stack of the goroutine, running any deferred functions along the way. If that unwinding reaches the top of the goroutine's stack, the program dies. However, it is possible to use the built-in function recover to regain control of the goroutine and resume normal execution. A call to recover stops the unwinding and returns the argument passed to panic. Because the only code that runs while unwinding is inside deferred functions, recover is only useful inside deferred functions. One application of recover is to shut down a failing goroutine inside a server without killing the other executing goroutines. func server(workChan <-chan *Work) { for work := range workChan { go safelyDo(work) }} func safelyDo(work *Work) { defer func() { if err := recover(); err != nil { log.Stderr("work failed:", err) } }() do(work)} In this example, if do(work) panics, the result will be logged and the goroutine will exit cleanly without disturbing the others. There's no need to do anything else in the deferred closure; calling recover handles the condition completely. Note that with this recovery pattern in place, the do function (and anything it calls) can get out of any bad situation cleanly by calling panic. We can use that idea to simplify error handling in complex software. Let's look at an idealized excerpt from the regexp package, which reports parsing errors by calling panic with a local Error type. Here's the definition of Error, an error method, and the Compile function. // Error is the type of a parse error; it satisfies os.Error. type Error string func (e Error) String() string { return string(e)}// error is a method of *Regexp that reports parsing errors by// panicking with an Error. func (regexp *Regexp) error(err string) { panic(Error(err))}// Compile returns a parsed representation of the regular expression. func Compile(str string) (regexp *Regexp, err os.Error) { regexp = new(Regexp) // doParse will panic if there is a parse error. defer func() { if e := recover(); e != nil { regexp = nil // Clear return value. err = e.(Error) // Will re-panic if not a parse error. } }() return regexp.doParse(str), nil} If doParse panics, the recovery block will set the return value to nil—deferred functions can modify named return values. It then will then check, in the assignment to err, that the problem was a parse error by asserting that it has type Error. If it does not, the type assertion will fail, causing a run-time error that continues the stack unwinding as though nothing had interrupted it. This check means that if something unexpected happens, such as an array index out of bounds, the code will fail even though we are using panic and recover to handle user-triggered errors. With this error handling in place, the error method makes it easy to report parse errors without worrying about unwinding the parse stack by hand. Useful though this pattern is, it should be used only within a package. Parse turns its internal panic calls into os.Error values; it does not expose panics to its client. That is a good rule to follow. A web server Web服务器 Let's finish with a complete Go program, a web server. This one is actually a kind of web re-server. Google provides a service athttp://chart.apis.google.com that does automatic formatting of data into charts and graphs. It's hard to use interactively, though, because you need to put the data into the URL as a query. The program here provides a nicer interface to one form of data: given a short piece of text, it calls on the chart server to produce a QR code, a matrix of boxes that encode the text. That image can be grabbed with your cell phone's camera and interpreted as, for instance, a URL, saving you typing the URL into the phone's tiny keyboard. 现在让我们来实现一个完整的程序:一个简单的web服务器。这其实是一个转发服务器。 google的http://chart.apis.google.com 提供了一个将数据转换为图表的服务。不过那个图表的转换程序使用比较复杂,因为需要用户 自己设置各种参数。不过我们这里的程序界面要稍微友好一点:因为我们只需要获取一小段数据, 然后调用google的图表转换程序生存QR码(Quick Response缩写,二维条码),对于文本 信息下编码。二维条码图像可以用手机上的摄像机采集,然后解析得到解码后的信息。 Here's the complete program. An explanation follows. 下面是完整的程序: package mainimport ( "flag" "http" "io" "log" "strings" "template")var addr = flag.String("addr", ":1718", "http service address") // Q=17, R=18var fmap = template.FormatterMap{ "html": template.HTMLFormatter, "url+html": UrlHtmlFormatter,}var templ = template.MustParse(templateStr, fmap) func main() { flag.Parse() http.Handle("/", http.HandlerFunc(QR)) err := http.ListenAndServe(*addr, nil) if err != nil { log.Exit("ListenAndServe:", err) }} func QR(c *http.Conn, req *http.Request) { templ.Execute(req.FormValue("s"), c)} func UrlHtmlFormatter(w io.Writer, v interface{}, fmt string) { template.HTMLEscape(w, strings.Bytes(http.URLEscape(v.(string))))} const templateStr = ` QR Link Generator {.section @}
{@|html}

{.end}
` The pieces up to main should be easy to follow. The one flag sets a default HTTP port for our server. The template variable templ is where the fun happens. It builds an HTML template that will be executed by the server to display the page; more about that in a moment. main函数的开始部分比较简单。有一个flag选项用于指定HTTP服务器的监听端口。 还有一个模板变量templ,主要用于保存HTML页面的生成模板,我们稍后会讨论。 The main function parses the flags and, using the mechanism we talked about above, binds the function QR to the root path for the server. Then http.ListenAndServe is called to start the server; it blocks while the server runs. main首先分析命令行选项,然后帮定QR函数为服务器的根目录 处理函数。最后http.ListenAndServe启动服务器,并在服务器运行期间一直 阻塞。 QR just receives the request, which contains form data, and executes the template on the data in the form value named s. QR接收客户端请求,然后用表单中的s变量的值替换到模板。 The template package, inspired by json-template, is powerful; this program just touches on its capabilities. In essence, it rewrites a piece of text on the fly by substituting elements derived from data items passed to templ.Execute, in this case the form value. Within the template text (templateStr), brace-delimited pieces denote template actions. The piece from the {.section @} to {.end} executes with the value of the data item @, which is a shorthand for “the current item”, which is the form value. (When the string is empty, this piece of the template is suppressed.) template包实现了json-template。 我们的程序的简洁正是得益于template包的强大功能。本质上,在执行templ.Execute的 时候,根据需要替换调模板中的某些区域。这里的原始模板文本保存在templateStr中, 其中花括弧部分对应模板的动作。在{.section @}和{.end}之间的 以@开头的元素,在处理模板的时候会被替换。 The snippet {@|url+html} says to run the data through the formatter installed in the formatter map (fmap) under the name "url+html". That is the function UrlHtmlFormatter, which sanitizes the string for safe display on the web page. 标记{@|url+html}的意思是在格式化模板的时候,用格式化字典(fmap) 中"url+html" 关键字对应的函数的处理标签的替代文本。这里的UrlHtmlFormatter 函数,只是为了安全启见过滤包含的不合法信息。 The rest of the template string is just the HTML to show when the page loads. If this is too quick an explanation, see the documentation for the template package for a more thorough discussion. 这里的模板只是用于显式的html页面。如果觉得上面的解释比较简略的话,可以看到template包的 documentation。 And there you have it: a useful webserver in a few lines of code plus some data-driven HTML text. Go is powerful enough to make a lot happen in a few lines. 我们仅仅用很少的代码加一些HTML文本就实现了一个有意思的webserver。使用go,往往用很少的 代码就能实现强大的功能。

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