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{}?

Answer 1

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.