How to set an interface{} parameter by reference?

I have a function that has a parameter with the type interface{}. This parameter represents my template data. So on each page it stores different data types (mostly structs). I want to append some data to this parameter's data, but it's an interface{} type and I can't do it.

This is what I tried:

func LoadTemplate(templateData interface) {
  appendCustomData(&templateData)
  ... //other functionality that is not relevant
}

func appendCustomData(dst interface{}) {
	// ValueOf to enter reflect-land
	dstPtrValue := reflect.ValueOf(dst)
	// need the type to create a value
	dstPtrType := dstPtrValue.Type()
	// *T -> T, crashes if not a ptr
	dstType := dstPtrType.Elem()
	// the *dst in *dst = zero
	dstValue := reflect.Indirect(dstPtrValue)
	// the zero in *dst = zero
	zeroValue := reflect.Zero(dstType)
	// the = in *dst = 0

	v := reflect.ValueOf(dst).Elem().Elem().FieldByName("HeaderCSS")
	if v.IsValid() {
		v = reflect.ValueOf("new header css value")
	}

	reflect.ValueOf(dst).Elem().Elem().FieldByName("HeaderCSS").Set(reflect.ValueOf(v))

	//dstValue.Set(zeroValue)
	fmt.Println("new dstValue: ", dstValue)
}

I can successfully get the "HeaderCSS" value. But I can't replace it with another value. What am I doing wrong?

My templateData looks like this:

I have a generic struct:

type TemplateData struct {
	FooterJS template.HTML
	HeaderJS template.HTML
	HeaderCSS template.HTML
    //and some more
}

and I have another struct, such as:

type pageStruct struct {
    TemplateData //extends the previous struct
    Form template.HTML
    // and some other maps/string
}

I send this second struct as templateData argument.

Right now I get this error:

> "reflect.Value.Set using unaddressable value" at the line: reflect.ValueOf(dst).Elem().Elem().FieldByName("HeaderCSS").Set(reflect.ValueOf(v))

The code from above is inspired from this answer: https://stackoverflow.com/a/26824071/1564840

I want to be able to append/edit values from this interface. Any idea how can I do it? Thanks.

Don't pass a pointer to interface. Instead the interface{} value should contain the pointer. And simply just hand over this interface{} value:

func LoadTemplate(templateData interface) {
  appendCustomData(templateData)
  ... //other functionality that is not relevant
}

Even if you can't use a more concrete type than interface{} (because you must allow multiple types), you can still use type assertion, it will be "super" easy:

func appendCustomData(d interface{}) {
	if ps, ok := d.(*pageStruct); ok {
		ps.TemplateData.HeaderCSS += "+new"
	}

}

Try this one on the Go Playground.

If you must or want to use reflection, this is how appendCustomData() can be implemented:

type Data struct {
	Name  string
	Age   int
	Marks []int
}

func appendCustomData(d interface{}) {
	v := reflect.ValueOf(d).Elem()

	if f := v.FieldByName("Name"); f.IsValid() {
		f.SetString(f.Interface().(string) + "2")
	}

	if f := v.FieldByName("Age"); f.IsValid() {
		f.SetInt(f.Int() + 2)
	}

	if f := v.FieldByName("Marks"); f.IsValid() {
		f.Set(reflect.ValueOf(append(f.Interface().([]int), 2)))
	}

	if f := v.FieldByName("Invalid"); f.IsValid() {
		f.Set(reflect.ValueOf(append(f.Interface().([]int), 2)))
	}
}

Testing it:

d := &Data{
	Name:  "Bob",
	Age:   22,
	Marks: []int{5, 4, 3},
}
fmt.Printf("%+v\n", d)
appendCustomData(d)
fmt.Printf("%+v\n", d)

Output (try it on the Go Playground):

&{Name:Bob Age:22 Marks:[5 4 3]}
&{Name:Bob2 Age:24 Marks:[5 4 3 2]}

Update:

To answer your edited question: there is no difference when the value passed is a struct that embeds another struct. But the value wrapped in the interface{} still must be a pointer.

Example appendCustomData() that appends content to pageStruct.TemplateData.HeaderCSS:

func appendCustomData(d interface{}) {
	v := reflect.ValueOf(d).Elem()

	if f := v.FieldByName("TemplateData"); f.IsValid() {
		if f = f.FieldByName("HeaderCSS"); f.IsValid() {
			f.Set(reflect.ValueOf(f.Interface().(template.HTML) + "+new"))
		}
	}
}

Testing it:

ps := &pageStruct{
	TemplateData: TemplateData{
		HeaderCSS: template.HTML("old"),
	},
}
fmt.Printf("%+v\n", ps)
appendCustomData(ps)
fmt.Printf("%+v\n", ps)

Output (try it on the Go Playground):

&{TemplateData:{FooterJS: HeaderJS: HeaderCSS:old} Form:}
&{TemplateData:{FooterJS: HeaderJS: HeaderCSS:old+new} Form:}