Invoke golang struct function gives “cannot refer

Question

I have a golang structure something like this:

type MyStruct struct {
    Id    string
}

and function:

func (m *MyStruct) id() {
   // doing something with id here
}

Also I have another structure like this:

type MyStruct2 struct {
    m *MyStruct
}

Now I have a function:

func foo(str *MyStruct2) {
    str.m.id()
}

But I'm getting error in compile time:

str.m.id undefined (cannot refer to unexported field or method mypackage.(*MyStruct)."".id

How can I call this function correctly?

Thank you

Answer 1

From http://golang.org/ref/spec#Exported_identifiers:

An identifier may be exported to permit access to it from another package. An identifier is exported if both:

1. the first character of the identifier's name is a Unicode upper case letter (Unicode class "Lu"); and
2. the identifier is declared in the package block or it is a field name or method name.

So basically only functions / variables starting with a capital letter would be usable outside the package.

Example:

type MyStruct struct {
    id    string
}

func (m *MyStruct) Id() {
   // doing something with id here
}

//then

func foo(str *MyStruct2) {
    str.m.Id()
}

Answer 2

If you change MyStruct.Id to MyStruct.id, you'll no longer be able to access it to initialize MyStruct2, because, id will be accessible only through its own package (which is first package).

This is because MyStruct and MyStruct2 are in different packages.

---

To solve that you can do this:

Package first:

package first

type MyStruct struct {
    // `id` will be invisible outside of `first` package
    // because, it starts with a lowercase letter
    id string
}

// `Id()` is visible outside to `first` package 
// because, it starts with an uppercase letter
func (m *MyStruct) Id() string {
  return m.id
}

// Create a constructor function to return `*MyStruct`
func NewMyStruct(id string) *MyStruct {
    return &MyStruct{
        id: id,
    }
}

Package second:

package second

// Import MyStruct's package
import "first"

type MyStruct2 struct {
    // If you don't use `m` here as in your question, 
    // `first.MyStruct` will be promoted automatically.
    //
    // So, you can reach its methods directly, 
    // as if they're inside `MyStruct2`
    *first.MyStruct
}

// You can use `Id()` directly because it is promoted
// As if, inside `MyStruct2`
func foo(str *MyStruct2) {
    str.Id()
}

// You can initialize `MyStruct2` like this:
func run() {
    foo(&MyStruct2{
        MyStruct: first.NewMyStruct("3"),
    })
}