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. This document gives tips for writing clear, idiomatic Go code. It augments the language specification,
the Tour of Go, and How to Write Go Code, all of which you should read first. Note added January, 2022: This document was written for Go's release in 2009, and has not been updated significantly since. Although it is a good guide to understand how to use the language itself, thanks to the stability of the language, it says little about the libraries and nothing about significant
changes to the Go ecosystem since it was written, such as the build system, testing, modules, and polymorphism. There are no plans to update it, as so much has happened and a large and growing set of documents, blogs, and books do a fine job of describing modern Go usage. Effective Go continues to be useful, but the reader should understand it is far from a complete guide. See issue 28782 for context. The Go package sources are intended to serve not only as the core library but also as examples of how to use the language. Moreover, many of the packages contain working, self-contained executable examples you can run directly from the golang.org web site, such as this one (if necessary, click on the word "Example" to
open it up). If you have a question about how to approach a problem or how something might be implemented, the documentation, code and examples in the library can provide answers, ideas and background. 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. With Go we take an unusual approach and let the machine take care of most formatting issues. The As an example, there's no need to spend time lining up the comments on the fields of a structure. All Go code in the standard packages has been formatted with Some formatting details remain. Very briefly: Go provides C-style Comments that appear before top-level declarations, with no intervening newlines, are considered to document the declaration itself. These “doc comments” are the primary
documentation for a given Go package or command. For more about doc comments, see “Go Doc Comments”. Names are as important in Go as in any other language. They even have semantic effect: 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. When a package is imported, the package name becomes an accessor for the contents. After the importing package can talk about Another convention is that the package name is the base
name of its source directory; the package in The importer of a package will use the name to refer to its contents, so exported names in the package can use that fact to avoid repetition. (Don't use the Another short example is Go doesn't provide automatic support for getters and setters. There's nothing wrong with providing getters and setters
yourself, and it's often appropriate to do so, but it's neither idiomatic nor necessary to put By convention, one-method interfaces are named by the method name plus an -er suffix or similar modification to construct an agent noun: There are a number of such names and it's productive to honor them and the function names they capture. Finally, the convention in Go is to use Like C, Go's formal grammar uses semicolons to terminate
statements, but unlike in 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. The rule is this. If the last token before a newline is an identifier (which includes words like 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, insert a semicolon”. A semicolon can also be omitted immediately before a closing brace, so a statement such as needs no semicolons. Idiomatic Go programs have semicolons only in places such as One consequence of
the semicolon insertion rules is that you cannot put the opening brace of a control structure ( not like this The control structures of Go are related to those of C but differ in important ways. There is no In Go
a simple Mandatory braces encourage writing simple Since In the Go libraries, you'll find that when an This is an example of a common situation where code must guard against a sequence of error conditions. 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 An aside: The last example in the previous section demonstrates a detail of how the This statement declares two variables, which looks as if it declares In a This unusual property is pure pragmatism, making it easy to use a single § It's worth noting here that in Go the scope of function parameters and return values is the same as the function body, even though they appear lexically outside the
braces that enclose the body. The Go Short declarations make it easy to declare the index variable right in the loop. If you're looping over an array, slice, string, or map, or reading from a channel, a If you only need
the first item in the range (the key or index), drop the second: If you only need the second item in the range (the value), use the blank identifier, an underscore, to discard the first: The blank identifier has many uses, as described in a later section. For strings, the prints Finally, Go has no comma operator and Go's There is no automatic fall through, but cases can be
presented in comma-separated lists. Although they are not nearly as common in Go as some other C-like languages, Of course, the To close this section, here's a comparison routine for byte slices that uses two 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 One of Go's unusual features is that functions and methods can return multiple values. This form can be used to improve on a couple of clumsy idioms in C programs: in-band error returns such as In C, a write error is signaled by a negative count with the error code secreted away in a volatile location. In Go, 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 A similar approach obviates the need to pass a pointer to a return value to simulate a reference parameter. Here's a simple-minded function to grab a number from a position in a byte slice, returning the number and the next position. func nextInt(b []byte, i int) (int, int) { for ; i < len(b) && !isDigit(b[i]); i++ { } x := 0 for ; i < len(b) && isDigit(b[i]); i++ { x = x*10 + int(b[i]) - '0' } return x, i } You could use it to scan the numbers in an input slice for i := 0; i < len(b); { x, i = nextInt(b, i) fmt.Println(x) } Named result parameters 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 The names are not mandatory but they can make code shorter and clearer: they're documentation. If we name the results of 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 func ReadFull(r Reader, buf []byte) (n int, err error) { for len(buf) > 0 && err == nil { var nr int nr, err = r.Read(buf) n += nr buf = buf[nr:] } return } Defer Go's // Contents returns the file's contents as a string. func Contents(filename string) (string, error) { f, err := os.Open(filename) if err != nil { return "", err } defer f.Close() // f.Close will run when we're finished. var result []byte buf := make([]byte, 100) for { n, err := f.Read(buf[0:]) result = append(result, buf[0:n]...) // append is discussed later. if err != nil { if err == io.EOF { break } return "", err // f will be closed if we return here. } } return string(result), nil // f will be closed if we return here. } Deferring a call to a function such as The arguments to the deferred function (which include the receiver if the function is a method) are evaluated when the defer executes, not when the call executes. Besides avoiding worries about variables changing values as the function executes, this means that a single deferred call site can defer multiple function executions. Here's a silly example. for i := 0; i < 5; i++ { defer fmt.Printf("%d ", i) } Deferred functions are executed in LIFO order, so this code will cause 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 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: b in b entering: a in a leaving: a leaving: b For programmers accustomed to block-level resource management from other languages, DataAllocation with new Go has two allocation primitives, the built-in functions Since the memory returned by The zero-value-is-useful property works transitively. Consider this type declaration. type SyncedBuffer struct { lock sync.Mutex buffer bytes.Buffer } Values of type p := new(SyncedBuffer) // type *SyncedBuffer var v SyncedBuffer // type SyncedBuffer Constructors and composite literals Sometimes the zero value isn't good enough and an initializing constructor is necessary, as in this example derived from package 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, 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. 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 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 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 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([]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, These examples illustrate the
difference between 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) Remember that ArraysArrays 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,
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]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 But even this style isn't idiomatic Go. Use slices instead. SlicesSlices 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 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 func (f *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 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. var n int var err error for i := 0; i < 32; i++ { nbytes, e := f.Read(buf[i:i+1]) // Read one byte. n += nbytes if nbytes == 0 || e != nil { err = e break } } 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 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)] copy(slice[l:], data) return slice } We must return the slice afterwards because, although
The idea of appending to a slice is so useful it's captured by the Two-dimensional slicesGo'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: type Transform [3][3]float64 // A 3x3 array, really an array of arrays. type LinesOfText [][]byte // A slice of byte slices. 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 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: // 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) } And now as one allocation, sliced into lines: // 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:] } MapsMaps 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. 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 and slices except that the index doesn't need to be an integer. 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 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") } Sometimes you need to
distinguish a missing entry from a zero value. Is there an entry for var seconds int var ok bool seconds, ok = timeZone[tz] For obvious reasons this is called the “comma ok” idiom. In this example, if 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 ( _, present := timeZone[tz] To delete a map entry, use the delete(timeZone, "PDT") // Now on Standard Time Printing Formatted printing in
Go uses a style similar to C's You don't need to provide a format string. For each of 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
Here things start to diverge from C. First, the numeric formats such as 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 fmt.Printf("%v\n", timeZone) // or just fmt.Println(timeZone) which gives output: map[CST:-21600 EST:-18000 MST:-25200 PST:-28800 UTC:0] For maps, When printing a struct, the modified format 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 &{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, "EST":-18000, "MST":-25200, "PST":-28800, "UTC":0} (Note the ampersands.)
That quoted string format is also available through Another
handy format is 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 with the signature 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" (If you need to print values of type Our type MyString string func (m MyString) String() string { return fmt.Sprintf("MyString=%s", m) // Error: will recur forever. } It's also easy to fix: convert the argument to the basic string type, which does not have the method. type MyString string func (m MyString) String() string { return fmt.Sprintf("MyString=%s", string(m)) // OK: note conversion. } In the initialization section we'll see another technique that avoids this recursion. Another printing technique is to pass a print routine's arguments directly to another such routine. The signature of func Printf(format string, v ...interface{}) (n int, err error) { Within the function // 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) } We write There's even more to
printing than we've covered here. See the By the way, a func Min(a ...int) int { min := int(^uint(0) >> 1) // largest int for _, i := range a { if i < min { min = i } } return min } Append Now we have the missing piece we needed to explain the design of the 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 What x := []int{1,2,3} x = append(x, 4, 5, 6) fmt.Println(x) prints But what if we wanted to do what our x := []int{1,2,3} y := []int{4,5,6} x = append(x, y...) fmt.Println(x) Without that InitializationAlthough 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 among initialized objects, even among different packages, are handled correctly. Constants 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, characters (runes), strings or
booleans. Because of the compile-time restriction, the expressions that define them must be constant expressions, evaluatable by the compiler. For instance, In Go, enumerated constants are created using the type ByteSize float64 const ( _ = iota KB ByteSize = 1 << (10 * iota) MB GB TB PB EB ZB YB ) The ability to attach a method such as 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 The use here of VariablesVariables 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") gopath = os.Getenv("GOPATH") ) The init function Finally, each source file
can define its own niladic Besides initializations that cannot be expressed as declarations, a common use of func init() { if user == "" { log.Fatal("$USER not set") } if home == "" { home = "/home/" + user } if gopath == "" { gopath = home + "/go" } // gopath may be overridden by --gopath flag on command line. flag.StringVar(&gopath, "gopath", gopath, "override default GOPATH") } MethodsPointers vs. Values As we saw with In the discussion of slices above, we wrote an type ByteSlice []byte func (slice ByteSlice) Append(data []byte) []byte { // Body exactly the same as the Append function defined 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 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 func (p *ByteSlice) Write(data []byte) (n int, err error) { slice := *p // Again as above. *p = slice return len(data), nil } then the type var b ByteSlice fmt.Fprintf(&b, "This hour has %d days\n", 7) We pass the address of a This rule arises because pointer methods can modify the receiver; invoking them on a value would cause the method to receive a copy of the value, so any
modifications would be discarded. The language therefore disallows this mistake. There is a handy exception, though. When the value is addressable, the language takes care of the common case of invoking a pointer method on a value by inserting the address operator automatically. In our example, the variable By the way, the idea of using Interfaces and other typesInterfaces 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 A type can implement multiple interfaces. For instance, a collection can be sorted by the routines in package type Sequence []int 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] } func (s Sequence) Copy() Sequence { copy := make(Sequence, 0, len(s)) return append(copy, s...) } func (s Sequence) String() string { s = s.Copy() sort.Sort(s) str := "[" for i, elem := range s { if i > 0 { str += " " } str += fmt.Sprint(elem) } return str + "]" } Conversions The func (s Sequence) String() string { s = s.Copy() sort.Sort(s) return fmt.Sprint([]int(s)) } This method is another example of the conversion technique for calling 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 type Sequence []int // Method for printing - sorts the elements before printing func (s Sequence) String() string { s = s.Copy() sort.IntSlice(s).Sort() return fmt.Sprint([]int(s)) } Now, instead of having Interface conversions and type assertions Type switches are a form of conversion: they take an interface and, for each case in the
switch, in a sense convert it to the type of that case. Here's a simplified version of how the code under type Stringer interface { String() string } var value interface{} // Value provided by caller. switch str := value.(type) { case string: return str case Stringer: return str.String() } The first case finds a concrete value; the second converts the interface into another interface. It's perfectly fine to mix types this way. What if there's
only one type we care about? If we know the value holds a value.(typeName) and the result is a new value with the static type str := value.(string) But if it turns out that the value does not contain a string, the program will crash with a run-time error. To guard against that, use the "comma, ok" idiom to test, safely, whether the value is a string: str, ok := value.(string) if ok { fmt.Printf("string value is: %q\n", str) } else { fmt.Printf("value is not a string\n") } If the type assertion fails, As an illustration of the capability, here's an if str, ok := value.(string); ok { return str } else if str, ok := value.(Stringer); ok { return str.String() } GeneralityIf a type exists only to implement an interface and will never have exported methods beyond that interface, there is no need to export the type itself. Exporting just the interface makes it clear the value has no interesting behavior beyond what is described in the interface. 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 A similar
approach allows the streaming cipher algorithms in the various The type Block interface { BlockSize() int Encrypt(dst, src []byte) Decrypt(dst, src []byte) } type Stream interface { XORKeyStream(dst, src []byte) } Here's the definition of the counter mode (CTR) stream, which turns a block cipher into a streaming cipher; notice that the block cipher's details are abstracted away: // NewCTR returns a Stream that encrypts/decrypts using the given Block in // counter mode. The length of iv must be the same as the Block's block size. func NewCTR(block Block, iv []byte) Stream Interfaces and methods Since almost anything can have methods attached, almost anything can satisfy an interface. One illustrative example is in the type Handler interface { ServeHTTP(ResponseWriter, *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 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(w http.ResponseWriter, req *http.Request) { ctr.n++ fmt.Fprintf(w, "counter = %d\n", ctr.n) } (Keeping with our theme, note how For reference, here's how to attach such a server to a node on the URL tree. import "net/http" ... ctr := new(Counter) http.Handle("/counter", ctr) But why make // Simpler counter server. type Counter int func (ctr *Counter) ServeHTTP(w http.ResponseWriter, req *http.Request) { *ctr++ fmt.Fprintf(w, "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(w http.ResponseWriter, req *http.Request) { ch <- req fmt.Fprint(w, "notification sent") } Finally, let's say we wanted to present on func ArgServer() { fmt.Println(os.Args) } How do we turn that into an HTTP server? We could make // 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(ResponseWriter, *Request) // ServeHTTP calls f(w, req). func (f HandlerFunc) ServeHTTP(w ResponseWriter, req *Request) { f(w, req) } To make
// Argument server. func ArgServer(w http.ResponseWriter, req *http.Request) { fmt.Fprintln(w, os.Args) } http.Handle("/args", http.HandlerFunc(ArgServer)) When someone visits the page 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. The blank identifier We've mentioned the blank identifier a couple of times now, in the context of
The blank identifier in multiple assignment The use of a blank identifier in a If an assignment requires multiple values on the left side, but one of the values will not be used by the program, a blank identifier on the left-hand-side of the assignment avoids the need to create a dummy variable and makes it clear that the value is to be discarded. For instance, when calling a function that returns a value and an error, but only the error is important, use the blank identifier to discard the irrelevant value. if _, err := os.Stat(path); os.IsNotExist(err) { fmt.Printf("%s does not exist\n", path) } Occasionally you'll see code that discards the error value in order to ignore the error; this is terrible practice. Always check error returns; they're provided for a reason. // Bad! This code will crash if path does not exist. fi, _ := os.Stat(path) if fi.IsDir() { fmt.Printf("%s is a directory\n", path) } Unused imports and variablesIt is an error to import a package or to declare a variable without using it. Unused imports bloat the program and slow compilation, while a variable that is initialized but not used is at least a wasted computation and perhaps indicative of a larger bug. When a program is under active development, however, unused imports and variables often arise and it can be annoying to delete them just to have the compilation proceed, only to have them be needed again later. The blank identifier provides a workaround. This half-written program has two unused imports ( package main import ( "fmt" "io" "log" "os" ) func main() { fd, err := os.Open("test.go") if err != nil { log.Fatal(err) } } To silence complaints about the unused imports, use a blank identifier to refer to a symbol from the imported package. Similarly, assigning the unused variable package main import ( "fmt" "io" "log" "os" ) var _ = fmt.Printf var _ io.Reader func main() { fd, err := os.Open("test.go") if err != nil { log.Fatal(err) } _ = fd } By convention, the global declarations to silence import errors should come right after the imports and be commented, both to make them easy to find and as a reminder to clean things up later. Import for side effect An unused import like import _ "net/http/pprof" This form of import makes clear that the package is being imported for its side effects, because there is no other possible use of the package: in this file, it doesn't have a name. (If it did, and we didn't use that name, the compiler would reject the program.) Interface checks As we saw in the discussion of interfaces above, a type need not declare explicitly that it implements an interface. Instead, a type implements the interface just by implementing the interface's methods. In practice, most interface conversions are static and
therefore checked at compile time. For example, passing an Some interface checks do happen at run-time, though. One instance is in the m, ok := val.(json.Marshaler) If it's necessary only to ask whether a type implements an interface, without actually using the interface itself, perhaps as part of an error check, use the blank identifier to ignore the type-asserted value: if _, ok := val.(json.Marshaler); ok { fmt.Printf("value %v of type %T implements json.Marshaler\n", val, val) } One place this situation arises is when it is necessary to guarantee within the package implementing the
type that it actually satisfies the interface. If a type—for example, var _ json.Marshaler = (*RawMessage)(nil) In this declaration, the assignment involving a conversion of a The appearance of the blank identifier in this construct indicates that the declaration exists only for the type checking, not to create a variable. Don't do this for every type that satisfies an interface, though. By convention, such declarations are only used when there are no static conversions already present in the code, which is a rare event. EmbeddingGo 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
type Reader interface { Read(p []byte) (n int, err error) } type Writer interface { Write(p []byte) (n int, err error) } The // ReadWriter is the interface that combines the Reader and Writer interfaces. type ReadWriter interface { Reader Writer } This says just what
it looks like: A The same basic idea applies to structs, but with more far-reaching implications. The // 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 type ReadWriter struct { reader *Reader writer *Writer } but then to promote the methods of the fields and to satisfy the func (rw *ReadWriter) Read(p []byte) (n int, err 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 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 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.Println("starting now...") The func NewJob(command string, logger *log.Logger) *Job { return &Job{command, logger} } or with a composite literal, job := &Job{command, log.New(os.Stderr, "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, as it did in the func (job *Job) Printf(format string, args ...interface{}) { job.Logger.Printf("%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 Second, if the same name appears at the same nesting level, it is usually an error; it would be erroneous to embed ConcurrencyShare by communicatingConcurrent 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. GoroutinesThey'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 concurrently 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 list.Sort() // run list.Sort concurrently; don't wait for it. A function literal can be handy in a goroutine invocation. func Announce(message string, delay time.Duration) { 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 allocated with 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 Unbuffered 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 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. } } Once This design has a problem, though: func Serve(queue chan *Request) { for req := range queue { sem <- 1 go func() { process(req) // Buggy; see explanation below. <-sem }() } } The bug is that in a Go func Serve(queue chan *Request) { for req := range queue { sem <- 1 go func(req *Request) { process(req) <-sem }(req) } } Compare this version with the previous to see the difference in how the closure is declared and run. Another solution is just to create a new variable with the same name, as in this example: func Serve(queue chan *Request) { for req := range queue { req := req // Create new instance of req for the goroutine. sem <- 1 go func() { process(req) <-sem }() } } It may seem odd to write req := req but it's legal and idiomatic in Go to do this. You get a fresh version of the variable with the same name, deliberately shadowing the loop variable locally but unique to each goroutine. Going back to the general problem of writing the server, another approach that manages resources well
is to start a fixed number of func handle(queue chan *Request) { for r := range queue { process(r) } } func Serve(clientRequests chan *Request, quit chan bool) { // Start handlers for i := 0; i < MaxOutstanding; i++ { go handle(clientRequests) } <-quit // Wait to be told to exit. } Channels of channelsOne 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, 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. ParallelizationAnother application of these ideas is to parallelize a calculation across multiple CPU cores. If the calculation can be broken into separate pieces that can execute independently, 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 numCPU = 4 // number of CPU cores func (v Vector) DoAll(u Vector) { c := make(chan int, numCPU) // Buffering optional but sensible. for i := 0; i < numCPU; i++ { go v.DoSome(i*len(v)/numCPU, (i+1)*len(v)/numCPU, u, c) } // Drain the channel. for i := 0; i < numCPU; i++ { <-c // wait for one task to complete } // All done. } Rather than create a constant value for numCPU, we can ask the runtime what value is appropriate. The function var numCPU = runtime.NumCPU() There is also a function var numCPU = runtime.GOMAXPROCS(0) Be sure not to confuse the ideas of concurrency—structuring a program as independently executing components—and parallelism—executing calculations in parallel for efficiency on multiple CPUs. Although the concurrency features of Go can make some problems easy to structure as parallel computations, Go is a concurrent language, not a parallel one, and not all parallelization problems fit Go's model. For a discussion of the distinction, see the talk cited in this blog post. 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 var freeList = make(chan *Buffer, 100) var serverChan = make(chan *Buffer) func client() { for { var b *Buffer // Grab a buffer if available; allocate if not. select { case b = <-freeList: // Got one; nothing more to do. default: // None free, so allocate a new one. b = new(Buffer) } load(b) // Read next message from the net. serverChan <- b // Send to server. } } The server loop receives each message from the client, processes it, and returns the buffer to the free list. func server() { for { b := <-serverChan // Wait for work. process(b) // Reuse buffer if there's room. select { case freeList <- b: // Buffer on free list; nothing more to do. default: // Free list full, just carry on. } } } The client attempts to retrieve a buffer from Errors 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. It is good style to use this feature to provide detailed error information. For example, as we'll see, By convention, errors have
type type error interface { Error() 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. As mentioned, alongside the usual // 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. Err error // Returned by the system call. } func (e *PathError) Error() string { return e.Op + " " + e.Path + ": " + e.Err.Error() } 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". When feasible, error strings should identify their origin, such as by having a prefix naming the operation or package that generated the error. For example, in package 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 for try := 0; try < 2; try++ { file, err = os.Create(filename) if err == nil { return } if e, ok := err.(*os.PathError); ok && e.Err == syscall.ENOSPC { deleteTempFiles() // Recover some space. continue } return } The second Panic The usual way to report an error to a caller is to return an For this purpose, there is a built-in function // A toy implementation of cube root using Newton's method. func CubeRoot(x float64) float64 { z := x/3 // Arbitrary initial 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 var user = os.Getenv("USER") func init() { if user == "" { panic("no value for $USER") } } Recover When A call to One application of func server(workChan <-chan *Work) { for work := range workChan { go safelyDo(work) } } func safelyDo(work *Work) { defer func() { if err := recover(); err != nil { log.Println("work failed:", err) } }() do(work) } In this example, if Because With our recovery pattern in place, the // Error is the type of a parse error; it satisfies the error interface. type Error string func (e Error) Error() 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 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 With error handling in place, the if pos == 0 { re.error("'*' illegal at start of expression") } Useful though this pattern is, it should be used only within a package. By the way, this re-panic idiom changes the panic value if an actual error occurs. However, both the original and new failures will be presented in the crash report, so the root cause of the problem will still be visible. Thus this simple re-panic approach is usually sufficient—it's a crash after all—but if you want to display only the original value, you can write a little more code to filter unexpected problems and re-panic with the original error. That's left as an exercise for the reader. A web server 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
at Here's the complete program. An explanation follows. package main import ( "flag" "html/template" "log" "net/http" ) var addr = flag.String("addr", ":1718", "http service address") var templ = template.Must(template.New("qr").Parse(templateStr)) func main() { flag.Parse() http.Handle("/", http.HandlerFunc(QR)) err := http.ListenAndServe(*addr, nil) if err != nil { log.Fatal("ListenAndServe:", err) } } func QR(w http.ResponseWriter, req *http.Request) { templ.Execute(w, req.FormValue("s")) } const templateStr = ` <html> <head> <title>QR Link Generator</title> </head> <body> {{if .}} <img src="http://chart.apis.google.com/chart?chs=300x300&cht=qr&choe=UTF-8&chl={{.}}" /> <br> {{.}} <br> <br> {{end}} <form action="/" name=f method="GET"> <input maxLength=1024 size=70 name=s value="" title="Text to QR Encode"> <input type=submit value="Show QR" name=qr> </form> </body> </html> ` The pieces up to The The template package The two snippets 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. And there you have it: a useful web server 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. Is it a compile error if two methods differ only in return type in the same class?* It is a compile error if two methods differ only in return type in the same class. * A private method cannot be overridden. If a method defined in a subclass is private in its superclass, the two methods are completely unrelated.
What modifier should you use so that a class in a different package Cannot access the class but its subclasses in any package can access it?The protected modifier specifies that the member can only be accessed within its own package (as with package-private) and, in addition, by a subclass of its class in another package.
Which describes the errors caused by a program and external circumstances which can be caught and handled?Exception describes errors caused by your program and external circumstances. These errors can be caught and handled by your program.
Which of the following keywords is used for creating objects in Java a create B Instanceof c new d return?The Java new keyword is used to create an instance of the class. In other words, it instantiates a class by allocating memory for a new object and returning a reference to that memory. We can also use the new keyword to create the array object.
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