Golang學習筆記：語言規範之聲明與範圍

這是一個建立於 的文章，其中的資訊可能已經有所發展或是發生改變。

型別宣告包括：常量、自訂類型、變數、函數、標籤、還有包聲明。
程式中的標識符都必須聲明。塊、包、檔案中的標識符不能重複。

Declaration   = ConstDecl | TypeDecl | VarDecl .TopLevelDecl  = Declaration | FunctionDecl | MethodDecl .

標籤範圍

標籤是在標籤語句中使用，常用在break、continue、goto語句中。定義了標籤但不使用是非法的。與其他標識符相比，標籤是非塊範圍，不會與其他非標籤標識符有衝突。

預宣告身份識別符

如下的標識符在golang中已保留

Types:    bool byte complex64 complex128 error float32 float64    int int8 int16 int32 int64 rune string    uint uint8 uint16 uint32 uint64 uintptrConstants:    true false iotaZero value:    nilFunctions:    append cap close complex copy delete imag len    make new panic print println real recover

可匯出方法或欄位

一個標識符可匯出讓另一個包訪問，只需要滿足

• 首字母大寫
• 在包中申明的欄位或方法

常量聲明

用const 關鍵字

ConstDecl   = "const" ( ConstSpec | "(" { ConstSpec ";" } ")" ) ConstSpec    = IdentifierList [ [ Type ] "=" ExpressionList ] IdentifierList = identifier { "," identifier } .ExpressionList = Expression { "," Expression } .

const Pi float64 = 3.14159265358979323846const zero = 0.0         // untyped floating-point constantconst (    size int64 = 1024    eof        = -1  // untyped integer constant)const a, b, c = 3, 4, "foo"  // a = 3, b = 4, c = "foo", untyped integer and string constantsconst u, v float32 = 0, 3    // u = 0.0, v = 3.0

Itoa(枚舉)

itoa表示連續的無類型的整形常量，從0開始計算，當又有itoa出現時，又重新置為0.

const (  // iota is reset to 0    c0 = iota  // c0 == 0    c1 = iota  // c1 == 1    c2 = iota  // c2 == 2)const (    a = 1 << iota  // a == 1 (iota has been reset)    b = 1 << iota  // b == 2    c = 1 << iota  // c == 4)const (    u         = iota * 42  // u == 0     (untyped integer constant)    v float64 = iota * 42  // v == 42.0  (float64 constant)    w         = iota * 42  // w == 84    (untyped integer constant))const x = iota  // x == 0 (iota has been reset)const y = iota  // y == 0 (iota has been reset)

型別宣告

用type 來定義一個新的類型

TypeDecl     = "type" ( TypeSpec | "(" { TypeSpec ";" } ")" ) .TypeSpec     = identifier Type .type IntArray [16]inttype (    Point struct{ x, y float64 }    Polar Point)type TreeNode struct {    left, right *TreeNode    value *Comparable}type Block interface {    BlockSize() int    Encrypt(src, dst []byte)    Decrypt(src, dst []byte)}

自訂類型不會繼承原有類型的方法，但介面方法或組合類別型的元素則保留原有的方法。

// A Mutex is a data type with two methods, Lock and Unlock.type Mutex struct         { /* Mutex fields */ }func (m *Mutex) Lock()    { /* Lock implementation */ }func (m *Mutex) Unlock()  { /* Unlock implementation */ }// NewMutex has the same composition as Mutex but its method set is empty.type NewMutex Mutex// The method set of the base type of PtrMutex remains unchanged,// but the method set of PtrMutex is empty.type PtrMutex *Mutex// The method set of *PrintableMutex contains the methods// Lock and Unlock bound to its anonymous field Mutex.type PrintableMutex struct {    Mutex}// MyBlock is an interface type that has the same method set as Block.type MyBlock Block

自訂類型可用於布爾、數值、或字串類型，還可為其附加方法

type TimeZone intconst (    EST TimeZone = -(5 + iota)    CST    MST    PST)func (tz TimeZone) String() string {    return fmt.Sprintf("GMT%+dh", tz)}

變數聲明

變數聲明表示建立一個或多個變數，然後為之綁定相互關聯類型，並賦之於初始值。

VarDecl     = "var" ( VarSpec | "(" { VarSpec ";" } ")" ) .VarSpec     = IdentifierList ( Type [ "=" ExpressionList ] | "=" ExpressionList ) .var i intvar U, V, W float64var k = 0var x, y float32 = -1, -2var (    i       int    u, v, s = 2.0, 3.0, "bar")var re, im = complexSqrt(-1)var _, found = entries[name]  // map lookup; only interested in "found"

如果變數在聲明但未使用，編譯器會報錯

短變數聲明

短變數聲明文法

ShortVarDecl = IdentifierList ":=" ExpressionList .i, j := 0, 10f := func() int { return 7 }ch := make(chan int)r, w := os.Pipe(fd)  // os.Pipe() returns two values_, y, _ := coord(p)  // coord() returns three values; only interested in y coordinate

但，短變數聲明只能在函數內部，但在if、for、switch語句中可聲明為臨時變數。

函式宣告

文法如下

FunctionDecl = "func" FunctionName ( Function | Signature ) FunctionName = identifier Function     = Signature FunctionBody FunctionBody = Block

方法聲明

方法是帶接受者的函數。文法如下

MethodDecl   = "func" Receiver MethodName ( Function | Signature ) Receiver     = Parameters

接收者可以示T或*T類型

func (p *Point) Length() float64 {    return math.Sqrt(p.x * p.x + p.y * p.y)}func (p *Point) Scale(factor float64) {    p.x *= factor    p.y *= factor}