字符串指针

Question

Consider the following program (http://play.golang.org/p/IbAstvudtE):

package main

import (
    "fmt"
)

func changeStringValueNotOK(dest *string, src string) {
    dest = &src
}

func changeStringValueOK(dest *string, src string) {
    *dest = src
}

func main() {

    a := "Hello"
    b := "World"

    changeStringValueNotOK(&a, b)
    fmt.Println(a) // still "Hello"

    changeStringValueOK(&a, b)
    fmt.Println(a) // now "World"
}

My goal is to call a function and change the value of string. Works fine with the second function, but not with the first.

Question: what is the meaning of *dest = src compared to dest = &src? I guess the former is "the contents of dest is now src" and the latter is "change the dest variable so that it now points to the address of src" which discards the previous value, but not the contents of a. But even if I am right, I don't understand how the *dest = src works.

I hope my question isn't too fuzzy.

Answer 1

*dest = src

is: Set the value pointed at by dest to the value in src. So it's effective.

dest = &src

is: Set the value of dest to the address of src. As dest is a formal parameter of changeStringValueNotOK the change (to the pointer only, not to the pointee) is visible only locally. As the changed value is not really used, it's total effect is a no operation.