Why doesn't appending to a slice within a Golang struct method work unless a pointer receiver is used?

Why can't I append to a slice that's the property of a struct in Golang?

In Golang, everything is passed by value, meaning that a copy of an object is created whenever it's passed as an argument to a function. This can lead to unexpected behavior when trying to append to a slice that's a property of a struct.

The Problem: Test3

Consider the following example:

package main

import "fmt"

type Test3 struct {
    all []int
}

func (c Test3) run() []int {
    c.combo()
    return c.all
}

func (c Test3) combo() {
    for i := 0; i < 2; i++ {
        c.all = append(c.all, i)
        fmt.Println("Test3 step", i+1, c.all)
    }
}

func main() {
    test3 := &Test3{}
    fmt.Println("Test3 final:", test3.run())
}

When you call test3.run(), it creates a copy of test3 and passes it to run. The run method then calls combo, which appends two values to c.all. However, when run returns, the changes to c.all are lost because the copy of test3 is discarded. This results in an empty slice being returned, as seen in the output:

Test3 step 1 [0]
Test3 step 2 [0 1]
Test3 final: []

The Solution: Using a Pointer Receiver

To fix this issue, you need to use a pointer receiver for the combo method. This tells the compiler to pass a pointer to the Test3 value instead of a copy.

func (c *Test3) combo() {
    for i := 0; i < 2; i++ {
        c.all = append(c.all, i)
        fmt.Println("Test3 step", i+1, c.all)
    }
}

With the pointer receiver, the changes made to c.all in combo are directly applied to the original Test3 value, since it's being passed by reference. This results in the correct output:

Test3 step 1 [0]
Test3 step 2 [0 1]
Test3 final: [0 1]

By understanding the concept of passing by value and using pointer receivers appropriately, you can avoid unexpected behavior when working with slices in Golang structs.

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