Guide to Go Slice

Slice is just a struct

type IntSlice struct {
    realArray *[...]int // this isnt valid Go, but think of [...] as any size array
    startIndex int
    len int
    cap int
}

func (s *IntSlice) at(index int) int {
    return s.realArray[startIndex+index]
}

Because a slice is just an internal struct that access another array, when you pass it to a function, its still referring to the same underlying array. And it will always do that unless you create a copy of that underlying array.

append() may modify the struct

Wonder why you need to assign the return value of append() to your slice variable? well thats because append() modify the struct. your slice struct is passed by value, this mean the append() func got a copy of it (copy of the struct). When you append something, the underlying array might not have enough capacity, therefore the append() func may allocate new array that has enough capacity and set the slice struct to point to that new allocated array. Because the slice struct is a copy of yours, it needs to be returned to the caller (you), and you need to assign that new struct to your slice variable

Passing slice is cheap

You never need to pass a slice pointer because slice is a small struct. Something to keep in mind is that slice will behave similar to a pointer, whether you pass it as pointer or not. Thus be careful when modifying a slice.

Key points

• Slice is just a struct that holds a pointer to an underlying array.
• Passing a struct mean passing an array pointer
• append() may modify a struct, this is why you have to reassign your variable with its return value