为什么通过方法对结构所做的更改不持久？

Question

I'm trying to understand why the following test code is not working as expected:

package main

import (
    "fmt"
    "strings"
)

type Test struct {
    someStrings []string
}

func (this Test) AddString(s string) {
    this.someStrings = append(this.someStrings, s)
    this.Count() // will print "1"
}

func (this Test) Count() {
    fmt.Println(len(this.someStrings))
}

func main() {
    var test Test
    test.AddString("testing")
    test.Count() // will print "0"
}

This would print:

"1"
"0"

Meaning that someStrings is apparently modified... and then it's not.

Anybody know what could be the issue?

Answer 1

The AddString method is using a value (copy) receiver. The modification is made to the copy, not the original. A pointer receiver must be used to mutate the original entity:

package main

import (
        "fmt"
)

type Test struct {
        someStrings []string
}

func (t *Test) AddString(s string) {
        t.someStrings = append(t.someStrings, s)
        t.Count() // will print "1"
}

func (t Test) Count() {
        fmt.Println(len(t.someStrings))
}

func main() {
        var test Test
        test.AddString("testing")
        test.Count() // will print "0"
}

Playground

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Output

1
1