golang之method

method

Go does not have classes. However, you can define methods on types.

package main

import (
    "fmt"
    "math"
)

type Vertex struct {
    X, Y float64
}

func (v Vertex) Abs() float64 {
    return math.Sqrt(v.X*v.X + v.Y*v.Y)
}

func (v *Vertex) Scale(f float64) {
    v.X = v.X * f
    v.Y = v.Y * f
}

func (v Vertex) ScaleValue(f float64) {
    v.X = v.X * f
    v.Y = v.Y * f
}

func main() {
    v := Vertex{3, 4}
    fmt.Println(v.Abs())
    v.Scale(2)
    fmt.Println(v.Abs())
    v.ScaleValue(2)
    fmt.Println(v.Abs())

    p := &v
    p.Scale(2)
    fmt.Println(p.Abs())
}

输出如下：

5
10
10
20

三个注意点：

1. Methods with pointer receivers can modify the value to which the receiver points 。

2. methods with pointer receivers take either a value or a pointer as the receiver when they are called:

var v Vertex
v.Scale(5)  // OK
p := &v
p.Scale(10) // OK

3. methods with value receivers take either a value or a pointer as the receiver when they are called:

var v Vertex
fmt.Println(v.Abs()) // OK
p := &v
fmt.Println(p.Abs()) // OK

interface

package main

import (
    "fmt"
)

type I interface {
    M()
}

type T struct {
    S string
}

func (t *T) M() {
    fmt.Println(t.S)
}

func main() {
    var i I
    t := T{"hello"}
    i = &t
    i.M()

    v := T{"world"}
    i = v
    i.M()
}

main函数后半段代码会报错：cannot use v (type T) as type I in assignment: T does not implement I (M method has pointer receiver)

如果把M的receiver类型改成value：

func (t T) M() {
    fmt.Println(t.S)
}

main函数的代码将正常运行。