地图值指针添加到切片的行为

Question

func TestMapValuePointer2(t *testing.T) {
    fmt.Println("Test Map Value Pointer 2")
    m := map[string]int{"rsc": 3711, "r": 2138, "gri": 1908, "adg": 912}
    n := len(m)
    array := make([]*int, n)
    i := 0
    for _, v := range m {
        array[i] = &v
        fmt.Printf("Add to array: %d
", v)
        i++
    }
    for _, k := range array {
        fmt.Printf("Value: %d
", *k)
    }
}

The output is not:

Value: 912
Value: 3711
Value: 2138
Value: 1908

Instead, the output maybe like:

Value: 912
Value: 912
Value: 912
Value: 912

Can someone explain why the results are like the above?

Answer 1

Quoting from this doc:

[...] each iteration of the loop uses the same instance of the variable v, so each closure shares that single variable [...]

In other words, the loop variable v is reused in all iterations, so your assigning the same pointer to all your slice elements.

Variables declared inside the loop won't be recycled like that, so this will work as you would expect:

for _, v := range m {
    vv := v
    array[i] = &vv
    fmt.Printf("Add to array: %d
", v)
    i++
}

Btw, you haven't explained why you want to use *int as the type of the values. All this would be simpler if you used simply int values.