Golang学习笔记:语言规范二

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类型转换

形式为 T(x), T是一种类型,x是目标类型表达式。示例

*Point(p) // same as *(Point(p))(*Point)(p) // p is converted to *Point<-chan int(c) // same as <-(chan int(c))(<-chan int)(c) // c is converted to <-chan intfunc()(x) // function signature func() x(func())(x) // x is converted to func()(func() int)(x) // x is converted to func() intfunc() int(x) // x is converted to func() int (unambiguous)

常量转换

uint(iota) // iota value of type uintfloat32(2.718281828) // 2.718281828 of type float32complex128(1) // 1.0 + 0.0i of type complex128float32(0.49999999) // 0.5 of type float32string('x') // "x" of type stringstring(0x266c) // "♬" of type stringMyString("foo" + "bar") // "foobar" of type MyStringstring([]byte{'a'}) // not a constant: []byte{'a'} is not a constant(*int)(nil) // not a constant: nil is not a constant, *int is not a boolean, numeric, or string typeint(1.2) // illegal: 1.2 cannot be represented as an intstring(65.0) // illegal: 65.0 is not an integer constant

字符串转换 转换一个无符号或有符号的整数位字符串时,其结果为一个utf-8形式的字符串。

string('a') // "a"string(-1) // "/ufffd" == "/xef/xbf/xbd"string(0xf8) // "/u00f8" == "ø" == "/xc3/xb8"type MyString stringMyString(0x65e5) // "/u65e5" == "日" == "/xe6/x97/xa5"

转换byte类型的slice为字符串

string([]byte{'h', 'e', 'l', 'l', '/xc3', '/xb8'}) // "hellø"string([]byte{}) // ""string([]byte(nil)) // ""type MyBytes []bytestring(MyBytes{'h', 'e', 'l', 'l', '/xc3', '/xb8'}) // "hellø"

转换rune类型的slice为字符串

string([]rune{0x767d, 0x9d6c, 0x7fd4}) // "/u767d/u9d6c/u7fd4" == "白鵬翔"string([]rune{}) // ""string([]rune(nil)) // ""type MyRunes []runestring(MyRunes{0x767d, 0x9d6c, 0x7fd4}) // "/u767d/u9d6c/u7fd4" == "白鵬翔"[]byte("hellø") // []byte{'h', 'e', 'l', 'l', '/xc3', '/xb8'}[]byte("") // []byte{}MyBytes("hellø") // []byte{'h', 'e', 'l', 'l', '/xc3', '/xb8'}[]rune(MyString("白鵬翔")) // []rune{0x767d, 0x9d6c, 0x7fd4}[]rune("") // []rune{}MyRunes("白鵬翔") // []rune{0x767d, 0x9d6c, 0x7fd4}常量表达式

常量表达式在编译时可只包含常量操作数和值

const a = 2 + 3.0 // a == 5.0 (untyped floating-point constant)const b = 15 / 4 // b == 3 (untyped integer constant)const c = 15 / 4.0 // c == 3.75 (untyped floating-point constant)const Θ float64 = 3/2 // Θ == 1.0 (type float64, 3/2 is integer division)const Π float64 = 3/2. // Π == 1.5 (type float64, 3/2. is float division)const d = 1 << 3.0 // d == 8 (untyped integer constant)const e = 1.0 << 3 // e == 8 (untyped integer constant)const f = int32(1) << 33 // illegal (constant 8589934592 overflows int32)const g = float64(2) >> 1 // illegal (float64(2) is a typed floating-point constant)const h = "foo" > "bar" // h == true (untyped boolean constant)const j = true // j == true (untyped boolean constant)const k = 'w' + 1 // k == 'x' (untyped rune constant)const l = "hi" // l == "hi" (untyped string constant)const m = string(k) // m == "x" (type string)const Σ = 1 - 0.707i // (untyped complex constant)const Δ = Σ + 2.0e-4 // (untyped complex constant)const Φ = iota*1i - 1/1i // (untyped complex constant)语句

标签

Error: log.Panic("error encountered")

发送语句

ch <- 3 // send value 3 to channel ch

自增自减语句

IncDec statement Assignmentx++ x += 1x-- x -= 1

If 语句

if x > max { x = max}if x := f(); x < y { return x} else if x > z { return z} else { return y}

Switch 语句 表达式switch,用fallthrough 表示继续执行下一个case

switch tag {default: s3()case 0, 1, 2, 3: s1()case 4, 5, 6, 7: s2()}switch x := f(); { // missing switch expression means "true"case x < 0: return -xdefault: return x}switch {case x < y: f1()case x < z: f2()case x == 4: f3()}

类型switch

switch i := x.(type) {case nil: printString("x is nil") // type of i is type of x (interface{})case int: printInt(i) // type of i is intcase float64: printFloat64(i) // type of i is float64case func(int) float64: printFunction(i) // type of i is func(int) float64case bool, string: printString("type is bool or string") // type of i is type of x (interface{})default: printString("don't know the type") // type of i is type of x (interface{})}

For 语句

for a < b { a *= 2}for i := 0; i < 10; i++ { f(i)}var testdata *struct { a *[7]int}for i, _ := range testdata.a { // testdata.a is never evaluated; len(testdata.a) is constant // i ranges from 0 to 6 f(i)}var a [10]stringfor i, s := range a { // type of i is int // type of s is string // s == a[i] g(i, s)}var key stringvar val interface {} // value type of m is assignable to valm := map[string]int{"mon":0, "tue":1, "wed":2, "thu":3, "fri":4, "sat":5, "sun":6}for key, val = range m { h(key, val)}// key == last map key encountered in iteration// val == map[key]var ch chan Work = producer()for w := range ch { doWork(w)}// empty a channelfor range ch {}

Go 语句 go语句表示启动一条独立的线程或goroutine

GoStmt = "go" Expressiongo Server()go func(ch chan<- bool) { for { sleep(10); ch <- true; }} (c)

Select 语句 select 语句 监听channel上的数据流动,并处理发送和接收数据的操作

var a []intvar c, c1, c2, c3, c4 chan intvar i1, i2 intselect {case i1 = <-c1: print("received ", i1, " from c1/n")case c2 <- i2: print("sent ", i2, " to c2/n")case i3, ok := (<-c3): // same as: i3, ok := <-c3 if ok { print("received ", i3, " from c3/n") } else { print("c3 is closed/n") }case a[f()] = <-c4: // same as: // case t := <-c4 // a[f()] = tdefault: print("no communication/n")}for { // send random sequence of bits to c select { case c <- 0: // note: no statement, no fallthrough, no folding of cases case c <- 1: }}select {} // block forever

延迟执行语句

DeferStmt = "defer" Expression .lock(l)defer unlock(l) // unlocking happens before surrounding function returns// prints 3 2 1 0 before surrounding function returnsfor i := 0; i <= 3; i++ { defer fmt.Print(i)}// f returns 1func f() (result int) { defer func() { result++ }() return 0}错误类型

预定义的error接口

type error interface { Error() string}其他考虑事项

数值类型大小

type size in bytesbyte, uint8, int8 1uint16, int16 2uint32, int32, float32 4uint64, int64, float64, complex64 8complex128

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