CODEWITHSUNDEEP
reflectiongoslice
Chrisport
Chrisport

Reputation: 3146

Convert interface{} to *[]int in golang

I receive an interface which is basically a slice. Now I want to convert it to a pointer to the slice. The problem is, that I have either the slice itself or a Pointer to an interface. I can easily show in a code example:

func main(){
    model := []int{1,2,3,4,5,6,7,8,10,11,133123123123}
    method(model)
}

func method(model interface{}){ 
    fmt.Println(reflect.TypeOf(model))    // this is of type []int
    fmt.Println(reflect.TypeOf(&model))   // this is of type *interface{}
}

What I need is this type:

fmt.Println(reflect.TypeOf(result))    // this should be type *[]int

I know the type only on runtime, therefore I cannot just take &(model.([]int))

Is there a way using golang reflection to receive this? the type 'int' is here actually not important, important is, that it is a Pointer to a slice. *[]interface{} would be okay either.

Edit:
To make the question more clear, I should have added: I am not interested in the data of the slice, but only in getting a pointer to a slice of same type (which can basically be empty). Therefore James Henstridge answers works perfectly.

Upvotes: 5

Views: 3576

Answers (1)

James Henstridge
James Henstridge

Reputation: 43949

Before trying to answer the question, it is worth stepping back and asking what the *[]int value you're after should point at?

Given the way method is called we can't possibly get a pointer to the model variable from the calling context, since it will only receive a copy of the slice as its argument (note that this is a copy of the slice header: the backing array is shared).

We also can't get a pointer to the copy passed as an argument since it is stored as an interface{} variable: the interface variable owns the memory used to store its dynamic value, and is free to reuse it when the a new value is assigned to it. If you could take a pointer to the dynamic value, this would break type safety if a different type is assigned.

We can obtain a *[]int pointer if we make a third copy of the slice, but it isn't clear whether that's what you'd necessarily want either:

v := reflect.New(reflect.TypeOf(model))
v.Elem().Set(reflect.ValueOf(model))
result := v.Interface()

This is essentially a type agnostic way of writing the following:

v := new([]int)
*v = model
var result interface{} = v

Now if you really wanted a pointer to the slice variable in the calling context, you will need to ensure that method is called with a pointer to the slice instead and act accordingly.

Upvotes: 6

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