Appending to go lang slice using reflection

Question

For some reason, it appears that adding new element to slice using reflection doesn't update slice itself. This is the code to demonstrate:

package main

import (
    "fmt"
    "reflect"
)

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
}

Playground link : https://play.golang.org/p/j3532H_mUL

Is there something I'm missing here?

Answer 1

reflect.Append works like append in that it returns a new slice value.

You are assigning this value to the value variable in the appendToSlice function, which replaces the previous reflect.Value, but does not update the original argument.

To make it more clear what's happening, take the equivalent function to your example without reflection:

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

What you need to use is the Value.Set method to update 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())
}

https://play.golang.org/p/Nhabg31Sju

Answer 2

    package main

    import "fmt"
    import "reflect"

    type Foo struct {
        Name string
    }

    func main() {
        _type := []Foo{}

        fmt.Printf("_type: v(%v) T(%T)\n", _type, _type)

        reflection := reflect.MakeSlice(reflect.SliceOf(reflect.TypeOf(_type).Elem()), 0, 0)

        reflectionValue := reflect.New(reflection.Type())
        reflectionValue.Elem().Set(reflection)

        slicePtr := reflect.ValueOf(reflectionValue.Interface())

        sliceValuePtr := slicePtr.Elem()

        sliceValuePtr.Set(reflect.Append(sliceValuePtr, reflect.ValueOf(Foo{"a"})))
        sliceValuePtr.Set(reflect.Append(sliceValuePtr, reflect.ValueOf(Foo{"b"})))
        sliceValuePtr.Set(reflect.Append(sliceValuePtr, reflect.ValueOf(Foo{"c"})))

        values := []Foo{Foo{"d"}, Foo{"e"}}

        for _, val := range values {
            sliceValuePtr.Set(reflect.Append(sliceValuePtr, reflect.ValueOf(val)))
        }

        result := sliceValuePtr.Interface()

        fmt.Printf("result: %T = (%v)\n", result, result)
    }

take a look at: https://play.golang.org/p/vXOqTVSEleO