Cast a struct pointer to interface pointer in Golang

Question

I have a function

func doStuff(inout *interface{}) {
   ...
}

the purpose of this function is to be able to treat a pointer of any type as input. But when I want to call it with a the pointer of a struct I have an error.

type MyStruct struct {
    f1 int
}

When calling doStuff

ms := MyStruct{1}
doStuff(&ms)

I have

test.go:38: cannot use &ms (type *MyStruct) as type **interface {} in argument to doStuff

How can I cast &ms to be compatible with *interface{}?

Solution

There is no such thing as a "pointer to an interface" (technically, you can use one, but generally you don't need it).

As seen in "what is the meaning of interface{} in golang?", interface is a container with two words of data:

• one word is used to point to a method table for the value’s underlying type,
• and the other word is used to point to the actual data being held by that value.

So remove the pointer, and doStuff will work just fine: the interface data will be &ms, your pointer:

func doStuff(inout interface{}) {
   ...
}

See this example:

ms := MyStruct{1}
doStuff(&ms)
fmt.Printf("Hello, playground: %v\n", ms)

Output:

Hello, playground: {1}

---

As newacct mentions in the comments:

Passing the pointer to the interface directly works because if MyStruct conforms to a protocol, then *MyStruct also conforms to the protocol (since a type's method set is included in its pointer type's method set).

In this case, the interface is the empty interface, so it accepts all types anyway, but still.

Comments

[newacct]

How interface variables work internally is not relevant to the question. Semantically an interface variable contains a copy of the value it was assigned from, just like any other type in Go. Underneath it's implemented with an immutable pointer to the data, but that's not visible to the programmer.

[newacct]

You should explain that passing the pointer to the interface directly works because if MyStruct conforms to a protocol, then *MyStruct also conforms to the protocol (since a type's method set is included in its pointer type's method set). In this case, the interface is the empty interface, so it accepts all types anyway, but still.

[VonC]

@newacct Good point. I have included your second comment in the answer for more visibility.

[notzippy]

It is also interesting to note that you can deal with pointers passed to functions by accessing the Type.Elem() function. This allows you to have a comparable for example reflect.TypeOf(&MyStruct{}).Elem() == reflect.TypeOf(MyStruct) See play.golang.org/p/DGb87RUZqJ