Structs. Now we’re getting to some meaty stuff!

Let’s start simple. What is a struct?

Struct types

A struct is a sequence of named elements, called fields, each of which has a name and a type. … Within a struct, non-blank field names must be unique.

Forget about blank field names for now. I’ll talk about that tomorrow.

I’m also postponing the EBNF description of structs for a while, because there are actually several subtleties to how structs work in Go, which can get into the weeds very quickly. For today, let’s just consider a few examples.

type Person struct {
	Name string
  Age  int
}

This simply defines a struct type named Person, with two fields, Name and Age, of types string and int respectively.

Notice that in this example, the type Person, as well as its fields, Name and Age, all begin with a capital letter, and therefore are exported identifiers. But unexported fields and types are also possible, and in many combinations.

type Person struct {
	Name     string
	Age      int
	password string
}

Now we’ve added the notably unexported field, password, to the same struct.

Let’s round out the day with one final example that may seem counter-intuitive, but can actually make a lot of sense:

type person struct {
	Name string
  Age  int
}

Notice now that the fields themselves, Name, and Age, are exported, but person is not. How could this ever be useful, you might be wondering.

It’s useful any time you will only ever create instances of person within the same package, but you need other packages to be able to access the fields.

When would you ever want that?

Quite frequently, actually. A very common example is when marshaling or unmarshaling JSON (or YAML, or XML, or many other formats). Because these (un)marshaling capabilities are handled by another package, the only way they can access these fields of a private struct type, is if the fields are exported.

	type person struct {
		Name string
		Age  int
	}

	input := []byte(`{"Name":"Bob","Age":29}`)

	var p person
	_ = json.Unmarshal(input, &p)
	fmt.Println(p) // Prints: {Bob 29}

See it in the playground.

Quotes from The Go Programming Language Specification Version of December 15, 2022