I am reading the docs for the sort stdlib package and the sample code reads like this:
type ByAge []Person
func (a ByAge) Len() int { return len(a) }
func (a ByAge) Swap(i, j int) { a[i], a[j] = a[j], a[i] }
func (a ByAge) Less(i, j int) bool { return a[i].Age < a[j].Age }
As I've learnt, function that mutate a type T needs to use *T as its method receiver. In the case of Len, Swap and Less why does it work ? Or am I misunderstanding the difference between using T vs *T as method receivers ?
Go has three reference types:
Every instance of these types holds a pointer to the actual data internally. This means that when you pass a value of one of these types the value is copied like every other value but the internal pointer still points to the same value.
Quick example (run on play):
func dumpFirst(s []int) {
fmt.Printf("address of slice var: %p, address of element: %p
", &s, &s[0])
}
s1 := []int{1, 2, 3}
s2 := s1
dumpFirst(s1)
dumpFirst(s2)
will print something like:
address of slice var: 0x1052e110, address of element: 0x1052e100
address of slice var: 0x1052e120, address of element: 0x1052e100
You can see: the address of the slice variable changes but the address of the first element in that slice remains the same.
I just had a minor epiphany regarding this exact same question.
As has already been explained, a typed slice (not a pointer to a slice) can implement the sort.Interface interface; part of the reason for this is that, even though the slice is being copied, one of its fields is a pointer to an array, so any modification of that backing array will be reflected in the original slice.
Normally, though, this isn't enough of a justification for a bare slice to be an acceptable receiver. It's generally incorrect to try to modify a struct as a receiver of a method, because any append() calls will change the slice copy's length without modifying the original slice's headers. Slice modification may even trigger the initialization of a new backing array, completely disconnecting the copied receiver from the original slice.
By the very nature of sorting, however, this isn't a problem in the sort.Sort case. The only array-modifying operation it exercises is Swap, meaning the array's required memory will remain the same, so the slice will not change size, and therefore there will be no changes to the slice's actual values (starting index, length, and array pointer).
I'm sure this was obvious to a lot of people, but it just dawned on me, and I thought it might be useful to others wondering why sort plays nicely with bare slices.