Why Doesn\'t Reflection\'s `reflect.Append` Modify the Original Go Slice, and How Can I Fix It?

Appending to Golang Slices using Reflection: A Comprehensive Exploration

Problem Statement:

In Go, when attempting to append an element to a slice using reflection, the original slice remains unchanged. This is demonstrated in the code below:

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
}

Understanding the Behavior:

The reflect.Append function operates similarly to the built-in append function by returning a new slice value. In the provided example, the code assigns the returned value to the value variable within the appendToSlice function, effectively replacing the previous reflect.Value. However, this does not modify the original argument.

Correct Approach to Append Using Reflection:

To append an element to the original slice using reflection, the Value.Set method should be employed. Here's the modified code:

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

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

    fmt.Println(value.Len())
}

By using Value.Set, the value variable is updated with the appended slice value, which then reflects in the original slice referenced by the arrPtr argument.

Conclusion:

Appending to Go slices using reflection requires using the Value.Set method to modify the original slice. The reflect.Append function returns a new slice value, which must be assigned to the original slice using Value.Set. This ensures that the changes made within the reflection scope are reflected in the original slice.

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