A Tour of Go-Pointer receivers 一点疑问

[lueffy]

https://tour.golang.org/methods/4

刚开始学 go，从 A Tour of Go 开始看的，求大佬帮忙看看，非常感谢

package main

import (
	"fmt"
	"math"
)

type Vertex struct {
	X, Y float64
}

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

func (v *Vertex) Scale(f float64) { //这里 receiver 的类型是 指针类型
	v.X = v.X * f
	v.Y = v.Y * f
}

func main() {
	v := Vertex{3, 4}
	v.Scale(10) //但是 v 是 Vertex 类型啊？为什么也能运行通过？
	fmt.Println(v.Abs())
}

package main

import (
	"fmt"
	"math"
)

type Vertex struct {
	X, Y float64
}

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

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

func main() {
	v := Vertex{3, 4}
	var m *Vertex
	m = &v //我理解的应该是这样的
	m.Scale(10)
	fmt.Println(v.Abs())
}

[heimeil]

编译器自动帮你做了你想到的步骤

[icexin]

为了写代码方便。否则第一个例子你得这么写(&v).Scale(10)，go 默认帮你加上了取地址符，前提是变量本身是可以取地址的。
如果你写了一个签名为 func GetVertex() Vertex 的函数，GetVertex().Scale(10)就会失败，因为返回的临时变量不可取地址。

[bef0rewind]

https://golang.org/ref/spec#Method_values

As with method calls, a reference to a non-interface method with a pointer receiver using an addressable value will automatically take the address of that value: t.Mp is equivalent to (&t).Mp.

```go
f := t.Mv; f(7) // like t.Mv(7)
f := pt.Mp; f(7) // like pt.Mp(7)
f := pt.Mv; f(7) // like (*pt).Mv(7)
f := t.Mp; f(7) // like (&t).Mp(7)
f := makeT().Mp // invalid: result of makeT() is not addressable
```

Although the examples above use non-interface types, it is also legal to create a method value from a value of interface type.

```go
var i interface { M(int) } = myVal
f := i.M; f(7) // like i.M(7)
```

[lueffy]

反过来就不行

package main

import (
"fmt"
"math"
)

type Vertex struct {
X, Y float64
}

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

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

func main() {
v := Vertex{3, 4}
Scale(&v, 10)
fmt.Println(Abs(v))
}

cannot use &v (type *Vertex) as type Vertex in argument to Scale

[liangjf]

编译器自己加了。。。

[bef0rewind]

@lueffy 你的例子错了，这个报错是说参数类型不匹配。

[lueffy]

@bef0rewind 是，评论那个是函数(参数类型不对)，不是方法(接收者类型)，我搞混淆了

[liulaomo]

@lueffy 反过来也是可以的

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 main() {
v := Vertex{3, 4}
fmt.Println((&v).Abs())
}

因为编译器给指针类型*Vertex 隐式声明了一个同名方法。https://gfw.go101.org/article/method.html#implicit-pointer-methods