return array of pointers from an array in Go

Question

Just so you know, I am quite new to Go.

I have been trying to make a function like this:

func PointersOf(slice []AnyType) []*AnyType{
    //create an slice of pointers to the elements of the slice parameter
}

It is like doing &slice[idx] for all elements in the slice, but I am having trouble with how to type my parameters and return type, as well as how to create the slice itself.

This method needs to work for slices of built-in types, as well as for slices of structs and slices of pointers to built-in types/structs

After invoking this function it would be preferable if I don't have to cast the pointer slice

Edit: The reason why I need this method is to have a generic way to use the elements of an array in a for ... range loop, in stead of using copies of that element. Consider:

type SomeStruct struct {
    x int
}

func main() {
    strSlice := make([]SomeStruct, 5)
    for _, elem := range strSlice {
        elem.x = 5
    }
}

This Doesn't work, because elem is a copy of the strSlice element.

type SomeStruct struct {
    x int
}

func main() {
    strSlice := make([]SomeStruct, 5)
    for _, elem := range PointersOf(strSlice) {
        (*elem).x = 5
    }
}

This however should work, since you only copy the pointer that points to an element in the original array.

You can't; Go types are invariant, and there are no generics. You create a slice of the type you need when you need it. Can you give an example why you would need this? — Mr_Pink
– Mr_Pink, Commented Jan 6, 2016 at 14:15
Perhaps you should first try to write such a function and make it work for a slice of integers only, for example. Then your AnyType should be an empty interface (interface{}) i guess, in case you want to support different types. Also can you show us a bit code, so that we can specifically help with what you're having problems. — arainone
– arainone, Commented Jan 6, 2016 at 14:20
I am trying to make this because the for ... range returns a copy of the elements in the given slice. If I want to change my elements I'd normally need to use a normal for loop and fetch the element from the index e = &slice[idx]. It would however be easier if I could get the pointer array and use the elements by reference — Rik Schaaf
– Rik Schaaf, Commented Jan 6, 2016 at 14:21
@coolcat007: this is why for ... range returns an index as the first parameter, so you can index into your slice instead of making a copy. play.golang.org/p/f5ZY3-npaB — Mr_Pink
– Mr_Pink, Commented Jan 6, 2016 at 14:29
Go strives to never have hidden complexity, and your conversion requires an extra allocation and an O(n) copy. If you need to have a pointer to a value, you should always start with a pointer, i.e. your slice should be []*SomeStruct from the start. — Mr_Pink
– Mr_Pink, Commented Jan 6, 2016 at 14:34

score 3 · Accepted Answer · 2016-01-06 21:00:29Z

3

Use the following code to loop through a slice of structs setting a field. There's no need to create a slice of pointers.

type SomeStruct struct {
  x int
}

func main() {
  strSlice := make([]SomeStruct, 5)
  for i := range strSlice {
    strSlice[i].x = 5
  }
}

playground example

Here's the proposed PointersOf function:

func PointersOf(v interface{}) interface{} {
  in := reflect.ValueOf(v)
  out := reflect.MakeSlice(reflect.SliceOf(reflect.PtrTo(in.Type().Elem())), in.Len(), in.Len())
  for i := 0; i < in.Len(); i++ {
    out.Index(i).Set(in.Index(i).Addr())
  }
  return out.Interface()
}

and here's how to use it:

for _, elem := range PointersOf(strSlice).([]*SomeStruct) {
    elem.x = 5
}

playground example

edited Jan 6, 2016 at 21:00

answered Jan 6, 2016 at 15:51

user5728991

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