为什么指针在此Go程序中丢失其值

Question

I don't understand why the pointer s is nil even after the input() method initialised it. Any idea?

package main

import "fmt"


type ps string

func(s *ps)input(){
    x := ps("a")
    s = &x
}
func(s *ps)output(){

}

func main() {
    var v *ps
    v.input()
    if v == nil{
        fmt.Println("v shouldn't be nil")
    }
}

Playground http://play.golang.org/p/jU2hoMP7TS

Answer 1

You need two things--main needs to allocate space for a ps that input can write into, which you can do by replacing var v *ps with v := new(ps). The string will be "", but it doesn't matter what it is, just that there's space set aside in memory for a string header that input can write to. As Momer said, otherwise the pointer's nil and your program panics trying to dereference it.

And in order to assign through a pointer, input needs to use *s = x. Since *s is, informally, "get what s points to", you can read that as "change what s points to to x". Usually the automatic ref/deref behavior around the dot operator and method calls saves you from that, but when you assign through a pointer type or do other operations (arithmetic, indexing, etc.) the dereference needs to be there in the code.