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

Appending to Go Lang Slice Using Reflection: Understanding the Mechanics

When attempting to append a new element to a slice using reflection, it is expected that the original slice will be updated accordingly. However, it may be observed that this does not occur, as demonstrated in the following code:

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 Reason

The key to understanding this behavior lies in recognizing how reflect.Append operates. Similar to the append function in the standard library, reflect.Append creates a new slice value instead of modifying the original. In the code snippet above, value is assigned the newly created slice, which replaces the original value within the function but does not affect the original argument outside the function.

Correcting the Approach

To modify the original slice using reflection, the Value.Set method should be employed. This method allows for updating the original value. The corrected version of the appendToSlice function would be:

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

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

    fmt.Println(value.Len())
}

This ensures that the original slice is modified as expected, and the following output is obtained:

1
1

Conclusion

When using reflection to modify slices, it is essential to consider that reflect.Append creates a new slice rather than modifying the original. By utilizing the Value.Set method, one can effectively modify the original slice using reflection, ensuring that the expected outcome is achieved.

The above is the detailed content of Why Doesn\'t Reflect.Append Modify the Original Go Slice, and How Can I Fix It?. For more information, please follow other related articles on the PHP Chinese website!