Structs in Go

A struct is a type that groups one or more variables under a single name. We call those variables fields, and each field can have a different type.

Structs exist because passing related values around one by one gets messy fast. A person has a name and an age. With a struct you move both together: pass them to a function, return them, store them in a slice.

Defining a struct

Here’s an example of a struct definition:

type Person struct {
	Name string
	Age  int
}

Note that I used uppercase names for the fields, otherwise those will be private to the package and when you pass the struct to a function provided by another package, like the ones we use to work with JSON or a database, those fields cannot be accessed.

Initializing a struct

Once we define a struct we can initialize a variable with that type:

flavio := Person{"Flavio", 39}

and we can access the individual fields using the dot syntax:

flavio.Age  //39
flavio.Name //"Flavio"

You can also initialize a new variable naming each field:

flavio := Person{Age: 39, Name: "Flavio"}

I prefer this form. If you later add a field to Person, every positional initialization stops compiling, because positional literals must list every field. Named fields keep working.

Naming fields also lets you set only some of them:

flavio := Person{Age: 39}

or even initialize the struct without any value:

flavio := Person{}

//or

var flavio Person

In this case every field gets its zero value: "" for strings, 0 for numbers. You can set the values later:

flavio.Name = "Flavio"
flavio.Age = 39

Structs are copied, not shared

Be careful when you pass a struct to a function. Go copies it. Changes inside the function don’t touch the original:

func birthday(p Person) {
	p.Age++
}

birthday(flavio)
flavio.Age //still 39

To modify the original, pass a pointer instead:

func birthday(p *Person) {
	p.Age++
}

birthday(&flavio)
flavio.Age //40

Nesting structs

Once defined, a struct is a type like int or string and this means you can use it inside other structs too:

type FullName struct {
	FirstName string
	LastName  string
}

type Person struct {
	Name FullName
	Age  int
}

You reach the inner fields by chaining the dot syntax:

flavio.Name.FirstName //"Flavio"