Why Doesn\'t Reflection\'s `reflect.Append` Modify the Original Go Slice?

Appending to Go Slice with Reflection: Uncovering the Hidden Issue

When appending new elements to a slice using Go's reflection package, an unexpected behavior may arise where the original slice remains unaffected. This is evident in the following code snippet:

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
}

Despite appending an element to the sliced value, the original slice retains its original length of zero. This puzzling discrepancy needs some explanation.

Reflection and Slice Operations

Reflection allows us to inspect and manipulate data structures at runtime, including slices. The reflect.Append function, similar to append, takes a slice value and a new element and returns a new slice value containing the updated elements. However, this operation does not modify the original slice reference.

In the provided code, the reflect.Append statement assigns a new reflect.Value to the value variable, effectively replacing the original reference to the slice. While the value itself is updated, the original arr pointer remains unchanged, hence the unaltered length of the arr slice in the main function.

Updating the Original Slice

To update the original slice, we need to use the Value.Set method. This method replaces the value at the specified index in the sliced value with a new value. In our case, we need to replace the entire slice with the new value returned by reflect.Append:

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

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

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

With this modification, the original slice is now updated as reflected in the output.

Conclusion

Appending to a slice using reflection requires a bit more nuance than the conventional append operation. The key takeaway is that reflection operations work with copies of values, and to update the original value, one must explicitly set it using the Value.Set method.

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