Set an interface to nil in Golang

Question

I'm trying to set an internal value of an interface to nil something like the following :

typ := &TYP{InternalState: "filled"}
setNil(typ)

fmt.Printf("Expecting that %v to be nil", typ)

And I need to know how to implement the setNil(typ interface{}) method.

For more details see this code in play.golang.org.

Solution

The thing is you don't have an interface value. You have a pointer value, a pointer to a concrete type. That is not the same as an interface value.

If you want to change the value of a variable of any type, you have to pass a pointer to it. This also includes variables of interface type, and also variables of pointer type. This is one of those very rare cases when a pointer to interface{} makes sense (*interface{}), in fact it's inevitable.

But if your function expects an interface and you pass a non-interface value, an interface value will be created implicitly and you could only nil this implicitly created value.

So we have 2 different / distinct cases:

Function to nil an interface{}

func setNilIf(v *interface{}) {
    *v = nil
}

Using it:

var i interface{} = "Bob"
fmt.Printf("Before: %v\n", i)
setNilIf(&i)
fmt.Printf("After: %v\n", i)

Output:

Before: Bob
After: <nil>

So it works.

Function to nil a pointer; using unsafe

Actually this is your case. We want to change the value of a variable of pointer type. To accept a pointer to any type, we can use unsafe.Pointer. unsafe.Pointer is a language support, it's a special pointer type which can be converted from and to any pointer type.

We want to accept the address (pointer) of the pointer variable, which is something like **SomeType. To actually be able to assign a new value (nil) to the pointer variable, we have to dereference it (* operator). But unsafe.Pointer cannot be dereferenced, so first we have to convert it to a pointer to pointer to "something", but since we only want to assign nil (which is the same to all pointer types regardless of the type of the pointed value), the "something" doesn't matter, I will just use int, and so I will convert the unsafe.Pointer pointer value to **int.

func setNilPtr(p unsafe.Pointer) {
    *(**int)(p) = nil
}

Using it:

typ := &TYP{InternalState: "filled"}
fmt.Printf("Before: %v\n", typ)
setNilPtr(unsafe.Pointer(&typ))
fmt.Printf("After: %v\n", typ)

Output:

Before: &{filled}
After: <nil>

So this one also works. There is still another way using reflection:

Function to nil a pointer; using reflect

You can also nil a pointer using reflection only (reflect package). We still have to pass the address of the variable of pointer type. Note that in this case the type of the parameter will simply be interface{}. And it will contain a dynamic type like **SomeType. Since pointers have zero value nil, we can obtain such a value with reflect.Zero() which we will set using Value.Set():

func setNilPtr2(i interface{}) {
    v := reflect.ValueOf(i)
    v.Elem().Set(reflect.Zero(v.Elem().Type()))
}

Using it:

typ2 := &TYP{InternalState: "filled"}
fmt.Printf("Before: %v\n", typ2)
setNilPtr2(&typ2)
fmt.Printf("After: %v\n", typ2)

Output:

Before: &{filled}
After: <nil>

So this one also works. Try these on the Go Playground.

---

But seriously: if you want to nil something, assign nil to it. Do not complicate things unnecessarily.

i = nil
typ = nil

Comments

[Mohsen]

Thanks icza , but I want avoid my client to know about unsafe.Pointer , and I don't want my client pass the type as *interface{}. so could you make this work ?=> play.golang.org/p/KhR4-BHMPW

[icza]

@Mohsen See edited answer. I added a 3rd way, using only reflection inside the function. You have to pass the address of the variable, you can't avoid that, but this one does not use unsafe.Pointer.

[Mohsen]

:Thx very much icza, but I have to , take a look : play.golang.org/p/aPslX_vgiJ (why doesn't go have generic!) :-)