Go methods for types

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。

You can define methods on struct types.

The method receiver appears in its own argument list between the func keyword and the method name.

The code snipers here just run as it looks like, so no results given. 

package mainimport (    "fmt"    "math")type Vertex struct {    X, Y float64}func (v *Vertex) Abs() float64 { //一個Vertex指標類型的接收器，也是方法調用者    return math.Sqrt(v.X*v.X + v.Y*v.Y)}func main() {    v := &Vertex{3, 4}    fmt.Println(v.Abs())}

Actually, you can define a method on any type you define in your package, not just structs.

NOTE: You cannot define a method on a type from another package, or on a basic type.

package mainimport (    "fmt"    "math")type MyFloat float64func (f MyFloat) Abs() float64 {    if f < 0 {        return float64(-f)    }    return float64(f)}func main() {    f := MyFloat(-math.Sqrt2)    fmt.Println(f.Abs())}

This is a method for float64 type, note that we do not use a pointer as a receiver.

So the main difference: first, pointer will avoid copying the value on each method call (more efficient if the value type is a large struct). Second, so that the method can modify the value that the pointer receiver points to.

區別在於：指標接收器能避免每次方法調用時進行類型的拷貝（比如一個龐大的結構體類型），另外指標接收器可以修改它所指向的具體類型執行個體的值。

package mainimport (    "fmt"    "math")type Vertex struct {    X, Y float64}func (v *Vertex) Scale(f float64) {    v.X = v.X * f    v.Y = v.Y * f}func (v *Vertex) Abs() float64 {    return math.Sqrt(v.X*v.X + v.Y*v.Y)}func main() {    v := &Vertex{3, 4}    v.Scale(5)    fmt.Println(v, v.Abs()) //如果全換成v Vertex的話，Scale操作將不會影響v.Abs()的結果。}

更有甚者，介面的實現也要靠method for types!

package mainimport (    "fmt"    "math")type Abser interface {    Abs() float64}type MyFloat float64type Vertex struct {    X, Y float64}func (f MyFloat) Abs() float64 {    if f < 0 {        return float64(-f)    }    return float64(f)}func (v *Vertex) Abs() float64 { //具體的Abs()方法實現，同時指定了調用者的類型（*Vertex）    return math.Sqrt(v.X*v.X + v.Y*v.Y)}func main() {    var a Abser    f := MyFloat(-math.Sqrt2)    v := Vertex{3, 4}    a = f  // a MyFloat 實現了 Abser    a = &v // a *Vertex 實現了 Abser    // 下面一行，v 是一個 Vertex（而不是 *Vertex）    // 所以沒有實現 Abser。    // a = v    fmt.Println(a.Abs())}