Set a pointer to an empty interface

We are using the openapi-generator to generate a go-gin-server. This generates models containing properties of type *interface{} eg.

type Material struct {
	Id *interface{} `json:"id,omitempty"`
	Reference *interface{} `json:"reference,omitempty"`
}

If I have an instance of this struct, with nil pointers, how can these be set? I have tried the following:

theReturnId := "abc123"
material.Id = &theReturnId

This gives a compilation error of:

>cannot use &theReturnId (value of type *string) as *interface{} value in assignment: *string does not implement *interface{} (type interface{} is pointer to interface, not interface)

theReturnId := "abc123"
*material.Id = theReturnId

This gives a runtime error that the pointer is nil.

I have tried a bunch of other things but to no avail. What am I missing here? Thanks!

You almost never need a pointer to an interface. You should be passing interfaces as values, but the underlying data though can still be a pointer.

You need to reconsider your design/code generation technique to not use such an approach as it is not idiomatic Go.

If you still want to use it, use a typed interface{} variable and take its address. The way you are doing in your example is incorrect as theReturnId is a string type taking its address would mean *string type, which cannot be assigned to *interface{} type directly as Go is a strongly typed language

package main

import "fmt"

type Material struct {
	Id        *interface{} `json:"id,omitempty"`
	Reference *interface{} `json:"reference,omitempty"`
}

func main() {
	newMaterial := Material{}
	var foobar interface{} = "foobar"
	newMaterial.Id = &foobar
	fmt.Printf("%T\n", newMaterial.Id)
}

> theReturnId := "abc123"
> *material.Id = theReturnId
> This gives a runtime error that the pointer is nil.

Because the field Id is set to its zero value, i.e.: nil, and dereferencing it (*material.Id) results in a run-time error. You can write a constructor for Material that initializes its *interface{} fields:

func NewMaterial() Material {
    return Material {
        new(interface{}),
        new(interface{}),
    }
}

Now, you can safely dereference the field Id:

material := NewMaterial()
theReturnId := "abc123"
*material.Id = theReturnId