Why Doesn't Appending to a Slice in a Go Struct Method Always Work?

Understanding Slice Manipulation in Go

In Go, when manipulating slices as properties of structs, it's crucial to consider value passing. In certain situations, appending to a slice within a method may seem to fail. Let's delve into the reasons behind this and explore a solution.

Value Passing in Go Structs

In Go, everything is passed by value, meaning that a copy is created for each passed value. As a result, any modifications made to the copy within a method will not affect the original value.

Test3: A Failed Append Attempt

Consider the following code:

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)
  }
}

When you call Test3.run(), it appears that c.all is not modified. This is because the combo() method operates on a copy of Test3, and any changes made within combo() are not propagated back to the original value.

Solution: Using Pointer Receivers

To resolve this issue, use a pointer receiver for the combo() method:

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

By using a pointer receiver, you ensure that combo() modifies the original Test3 value, and the changes are reflected when the method returns.

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