将struct属性（切片）传递给从中删除元素的函数时的奇怪行为

Question

I've started learning Go these days and got stuck in trying to pass a struct property's value (a slice) to a function. Apparently it's being passed as a reference (or it holds a pointer to its slice) and changes made inside the function affect it.

Here is my code, in which testFunction is supposed to receive a slice, remove its first 3 elements and print the updated values, but without affecting it externally:

package main

import (
    "fmt"
)

type testStruct struct {
    testArray []float64
}

var test = testStruct {
    testArray: []float64{10,20,30,40,50},
}

func main() {
    fmt.Println(test.testArray)
    testFunction(test.testArray)
    fmt.Println(test.testArray)
}

func testFunction(array []float64) {
    for i:=0; i<3; i++ {
        array = removeFrom(array, 0)
    }
    fmt.Println(array)
}

func removeFrom(array []float64, index int) []float64 {
    return append(array[:index], array[index+1:]...)
}

That outputs:

[10 20 30 40 50]
[40 50]
[40 50 50 50 50]

My question is: what is causing the third fmt.Println to print this strange result?

Playground: https://play.golang.org/p/G8W3H085In

p.s.: This code is only an example. It's not my goal to remove the first elements of something. I just wanna know what is causing this strange behaviour.

Answer 1

Usually we don't know whether a given call to append will cause a reallocation, so we can't assume that the original slice refers to the same array as the resulting slice, nor that it refers to a different one.

To use slices correctly, it's important to remember that although the elements of the underlying array are indirect, the slice's pointer, length and capacity are not.

As a result, it's usual to assign the result of a call to append to the same slice variable:

array = append(array, ...)

So to sum up, to receive the desired result always remember to assign the append function to a new or the same slice variable.

Here is the corrected and working code:

package main

import (
    "fmt"
)

type testStruct struct {
    testArray []float64
}

var test = testStruct {
    testArray: []float64{10,20,30,40,50},
}

func main() {
    fmt.Println(test.testArray)
    a := testFunction(test.testArray)
    fmt.Println(a)
}

func testFunction(array []float64)[]float64 {
    for i:=0; i<3; i++ {
        array = removeFrom(array, 0)
    }
    fmt.Println(array)
    return array
}

func removeFrom(array []float64, index int) []float64 {
    return append(array[:index], array[index+1:]...)
}

Check it the working code on <kbd>Go Playground</kbd>.

---

Another solution is to pass the array argument via pointer reference:

func testFunction(array *[]float64) {
    for i:=0; i<3; i++ {
        *array = removeFrom(*array, 0)
    }
    fmt.Println(*array)
}

<kbd>Go Playground</kbd>