Why Doesn\'t Reflection\'s `Append` Function Update the Original Go Slice?

Append Function in Go Reflection

In Go, appending an element to a slice using reflection does not seem to update the original slice. This is illustrated in the following code:

func appendToSlice(arrPtr interface{}) {
    valuePtr := reflect.ValueOf(arrPtr)
    value := valuePtr.Elem()
    value = reflect.Append(value, reflect.ValueOf(55))

    fmt.Println(value.Len()) // prints 1
}

func main() {
    arr := []int{}
    appendToSlice(&arr)
    fmt.Println(len(arr)) // prints 0
}

Why doesn't it work?

The root cause lies in the behavior of reflect.Append. Like the conventional append function, it returns a new slice value. In this case, the value assigned to the value variable is replaced with this new slice, effectively creating a copy.

To illustrate, let's modify the example to highlight what's happening:

func appendToSlice(arrPtr *[]int) {
    value := *arrPtr
    value = append(value, 55)
    fmt.Println(len(value))
}

func main() {
    arr := []int{}
    appendToSlice(&arr)
    fmt.Println(len(arr))
}

This modified code produces the same output even though we're not using reflection.

Solution

To correctly append to a slice using reflection, we need to use the Value.Set method to modify the original value:

func appendToSlice(arrPtr interface{}) {
    valuePtr := reflect.ValueOf(arrPtr)
    value := valuePtr.Elem()

    value.Set(reflect.Append(value, reflect.ValueOf(55)))

    fmt.Println(value.Len())
}

This approach passes the newly created slice to value.Set, updating the original slice.

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